Until one is committed, there is hesitancy,
the chance to draw back -
always ineffectiveness.

Concerning all acts of initiative (and creation),
there is one elementary truth,
the ignorance of which kills countless ideas and splendid plans:

That the moment one definitely commits oneself,
then Providence moves too -
all sorts of things occur to help one that would otherwise never have occurred.

A whole stream of events issues from the decision,
raising in one's favour all manner of unforseen incidents
and meetings and material assistance,
which no man could have dreamt would have come his way.

I have learned a deep respect for one of Goeth's couplets:

"Whatever you can do, or dream you can do - begin it.
Boldness has genius, power and magic in it."

W.H. Murray
The Scottish Himalayan Expedition 1951

An opening thought:

"The human understanding, once it has adopted opinions, either because they were already accepted and believed, or because it likes them, draws everything else to support and agree with them.

And though it may meet a greater number and weight of contrary instances, it will, with great and harmful prejudice, ignore or condemn or exclude them by introducing some distinction, in order that the authority of those earlier assumptions may remain intact and unharmed.

So it was a good answer made by that man who, on being shown a picture hanging in a temple of those who, having taken their vows, had escaped shipwreck, was asked whether he did not now recognise the power of the gods. He asked in turn: "But where are the pictures of those who perished after taking their vows?"

The same reasoning can be seen in every superstition, whether in astrology, dreams, omens, nemesis and the like, in which men find such vanities pleasing, and take note of events where they are fulfilled, but where they are not (even if this happens much more often), they disregard them and pass them by.

But this evil lurks far more insidiously in philosophies and sciences, in which an opinion once adopted infects and brings under control all the rest, though the latter may be much firmer and better.

Moreover, even without this pleasure and vanity I have spoken of, the human understanding still has this peculiar and perpetual fault of being more moved and excited by affirmatives than by negatives, whereas rightly and properly it ought to give equal weight to both; rather, in fact, in every truly constituted axiom, a negative instance has the greater weight."

Francis Bacon,
"Novum Organum" - 1620

A final word of Inspiration:

"If we don't succeed, we run the risk of failure."

- George W. Bush


Welcome DIY vacuum tube audio amplifier constructors - this page is presented for your information and guidance.

It is intended to provide helpful hands-on hints to save you inevitable pain and suffering in your quest for audio excellence and to help you find joy in your chosen pathway to audiophilic pleasure!!

Please note it is not intended for the novice constructor. Basic circuit theory and construction techniques are not attempted herein because it is assumed you already know that and are competent in both.

However there are included numerous references to tube texts and manuals, or direct extracts from them, to enable design principles to be explained in the context of my own propositions.

Those propositions have evolved from more than 50 years' direct personal experience in designing and constructing audio systems combined with more than 50 years musical playing experience using tube amplifiers and various loudspeaker system configurations.

All electronic design principles and practices described are within manufacturers' published ratings. I do not advocate exceeding ratings because to so do is to guarantee a shorter component and/or tube life, as well as introducing an element of unreliability with its risk of expensive self-destruction of the amplifier.

In any event vacuum tubes are happiest when operating well below their maximum capability. (Refer to industry standards for  "Design Centre" Ratings v "Design Maximum Ratings").

Also consider that modern components have temperature rise ratings way above those of yesteryear, that translate into a higher normal operating temperature for the overall system.

In any event, you will soon see I am not a theorist, so whatever is said is based upon a combination of the reported experiences of others, helpful guidance from my electronic guru friends, reference to published texts from people who are considered to be reliable technical experts, electron tube manufacturers' manuals, data sheets, catalogues and technical publications, and my own experiments, research and experiences over more than 50 years.

In my experience, tube audio systems are just as much art as science. The opinion of the listener, no matter how ignorant of what natural sound is, remains the ultimate determinate in what is successful in the public marketplace and what is not. However because you are a DIY constructor, you are most likely designing and building for yourself.

Consequently, you want the best and you want it to sound like you think the best should sound.

I hope this overview is of help to you in designing and constructing the very best audio amplifier for your needs.


Copyright in all quoted works remains with their original owner, author and publisher, as applicable.

Please note that no warranty is expressed or implied - see footnote notice.

Intellectual property in the applied engineering concepts expressed in this paper remains exclusively with the author Dennis R. Grimwood.

The whole or part thereof of this paper and/or the designs and design concepts expressed therein may be reproduced for personal use - but not for commercial gain or reward without the express written permission of the author.

All rights reserved.



Do not attempt to design and/or construct a vacuum tube audio amplifier unless you suitably skilled, qualified and/or experienced.

The Author makes no claim whatsoever as to the validity or accuracy or otherwise of any statement, information or opinion contained in these pages and no liability will be accepted for any error or omission of any kind whatsoever.

No warranty of any kind is expressed or implied as to the workability or performance of designs, concepts or equipment described herein.

Proceed only at your own risk!!


Now that you have been suitably warned, let us proceed together to explore the world of vacuum tube audio.
















"HIGH-FIDELITY" is a term that literally means "TRUE FAITHFULNESS" - to the original.

For around one hundred years, and particularly the eighty five years since the advent of electric recording in 1925, countless scientists, researchers, engineers and DIY enthusiasts have endeavoured to attain that elusive standard of performance described as "High-Fidelity".

In modern parlance, a term such as "audio excellence" would be used to describe that sought after performance.

Others might simply say "peerless", or "the best", or "superb". The English language offers a range of superlatives that might be applied in attempting to describe what we mean - or to compare one approach with another.

In 1947, D.T.N. Williamson described his innovative new 10 Watts rms amplifier design simply and modestly as "a high-quality amplifier".

Then in 1949 McIntosh and Gow published their new 50 Watts rms design with the introductory note:

"Audio amplifiers - being one of the oldest forms of equipment built using the three or more element tube - are now one of the most difficult devices to improve, and perhaps no other field of electronic endeavour has been given more time or has been studied by more people." !!!!

In other words, way back in 1949 McIntosh and Gow inadvertently pronounced that tube audio had more or less reached its zenith and that their new design was pretty much the end of the road.

But then in 1952, the British responded via a paper by D.T.N. Williamson and P.J. Walker, entitled "Amplifiers and Superlatives - An Examination of American Claims for Linearity and Efficiency". In that paper they admonished the Americans for using British inventions and attempted to reclaim the high technical ground from the Americans.

Williamson and Walker set about defining what constitutes a "good" amplifier. They concluded their paper with the definitive statement:

"It will be appreciated from the foregoing that there are a large number of solutions to the problem of designing a first-class amplifier, and no one of these solutions can be called the best solution. Each has its advantages and disadvantages, and the individual designer must choose that which most nearly meets his needs. The "goodness" of an amplifier is not shown by its circuits diagram. Circuits have no inherent magic properties, but are merely the tools with which the designer seeks to achieve a certain result, and different designers - provided they always have the same high standards in view - may achieve the same result by different means."

However any objective laboratory or listening test must place the 1949 McIntosh Model 50W-1 transformer-coupled amplifier with unity-coupled output stage (as described in the 1949 paper by McIntosh and Gow) light-years ahead of the British technology of the time.

This amplifier had a frequency response flat to 50 kHz, THD around 0.25%, and no phase-shift across the entire audio spectrum - and that was in 1949 !!!!

It remains one of the best sounding tube amplifiers of all time, outperformed in my opinion only by the McIntosh MC-3500 and MI-350.

For a contrary view of Williamson's design see http://www.stan-white.com/a_doc_Williamson.htm

At that point long-playing microgroove records and the transistor had only just been developed - so unbeknowns to them the journey had only just begun !!!!

More recently, digital signal processing has opened up a whole new era of possibilities for wider frequency response, lower distortion, and extreme dynamic range.

Digital signal generation and processing has also created a whole new world of sounds that are un-natural, leaving the paradigm of the sine-wave as a measure of distortion back in the past with analogue sound sources and recording.

Digital home cinema has introduced an exciting new era of possibilities in audio reproduction and entertainment.

Modern recorded sound can be literally anything that we can hear or feel, so if we want our audio system to be capable of reproducing this we need to go beyond the old-school approach to tube audio amplification.

On the other hand, until domestic market digital recording and playback media match the musical quality of analogue, we still want our tube amplifier based hi-fi to sound like it always did.

So what did hi-fi analogue tube audio sound like before transistors ruled the airwaves?

Looking back to the beginning when amplifiers comprised a set of interstage transformers, output transformers, power transformers and a few tubes, coupled together with a matrix of wires with more or less nothing much else, we could turn to advertising and promotional literature for guidance. We find that source tells us that just about every product ever made was the best at the time. (That situation is still the case - caveat emptor).

One fairly obvious example is when in 1953, the British Acoustical Electric Company, described its (12 Watts rms) Quad II amplifier as "incomparable"and claimed: "For the closest approach to the original sound - that your enjoyment and appreciation of music may be unimpeded".

A hard act to follow !!!!

Then in 1954, McIntosh claimed of their MC-30 amplifier: "McIntosh brings you 99.6% perfect amplification".

But relentless technological development since has challenged and displaced these claims - but "perfection" still eludes us.

(The change to solid state amplifiers has produced its own tangential pathway, creating a myriad of new technical challenges and shortcomings. Notwithstanding progress in reducing distortion, the trend to digital encoding has in fact created a new form of distortion that replaces analogue continuity with digital "bits" of information, similar to half-wave v full-wave rectification. Nonetheless, the present situation is that the limitation on performance remains with the loudspeaker and the listener's ears).

If we review hi-fi audio electron tube based amplifier circuit designs produced over the past 100 years by the best designers and constructors from all over the world we can see much commonality between them, but most might be described as "variations on a theme".

In fact, many successful hi-fidelity amplifiers have attained their superior performance simply by taking a proven design, directly copying it, tweaking it (optimising circuit values) and using the very best components available at the time of manufacture. That approach is still pursued today by both commercial designers and enthusiasts alike.

Triodes were popular when triodes were the best option - caused by discrete component limitations - particularly output transformers and loudspeakers. Power output was limited to available B+ voltages, determined by electrolytic capacitor ratings around 450 VDCW.

Then from about the 1940's, to increase power output and reduce distortion British and European designers turned to high-gain pentodes (EL84, EL34, EL37), then high-gain beam power tubes (eg KT61, KT66, KT77, KT88) whilst US designers turned to medium gain beam power tubes (eg 42, 6F6, 6L6, 6V6, 6146, 6550). Some later US amplifers utilised colour TV sweep tubes, which offer higher gain and higher power - firebottles with grunt !!!!

The move away from triodes introduced a host of challenges in attempting to preserve their well-established and popular distinctive natural and pleasant tone.

It is important to acknowledge that many of the world's vintage audio vacuum tube amplifiers - ie pre solid state era - were designed by RF Engineers or Public Address Audio Engineers and produced by companys that primarily manufactured radio broadcasting and receiving equipment. Another important audio field was that of cinema and drive-in cinemas.

Consequently, many of the designs were based upon theoretical calculation of what should happen, rather than innovative experimentation to determine what actually happens - what it really sounded like in a live room as opposed to a test laboratory. Componentry, component layout and mechanical construction were typically produced  to conventional broadcasting equipment standards - particularly in the power supply section.

In fact, many of the better amplifiers were intended for application as broadcast studio monitors, where raw power was not so much needed as was accurate fidelity at moderate sound levels.

However all of that was set against a paradigm of influential marketing executives deciding what was good enough for the masses.

Thus the prevailing market force circumstances dictated that serious innovative hi-fi audio development would not be mainstream.

The big domestic radio and TV manufacturers in Europe and Australia were focused on producing a domestic quality audio related to AM broadcasting standards (ie 50-10kHz) that were deemed as being satisfactory to the consumer.

The 1950's consumer had been raised from childhood on low quality audio and did not know any different, so there was no real need to improve.

It was not until 1955 that FM radio broadcasting and commercial television commenced in the UK.

TV commenced in Australia shortly thereafter in 1956, but FM radio broadcasting was not until the 1970's.

Nonetheless there were serious efforts to improve hi-fi audio around the world and the results are now history.

For refreshing and honest review of the evolution of hi-fi audio see Lynn Olsen's articles at:





This paper takes a fresh look at some of the important elements essential to designing and constructing a high-fidelity amplifier.

Some of the concepts can be easily adapted to existing designs, often with very minor component or voltage changes to those chosen for an existing circuit.

What remains certain, is that notwithstanding a huge investment in the pursuit of hi-fidelity reproduction over a very long time by a very many expert scientists and engineers, there is still no singular solution to the puzzle.

Such a simple challenge should have produced a simple solution long ago.

That is not surprising because virtuoso musicians, conductors, music teachers, competition judges, recording engineers and producers - or the performers themselves - cannot agree on how a particular work should be played or how it should sound.

So what constitutes "hi-fidelity" must remain subjective.

Therefore the pursuit of high-fidelty musical reproduction is as much an art as it might be a science.

The vacuum tube is to an amplifier as the carburettor is to a motor vehicle engine.

If there is no input regulating device, an engine will simply run at full RPM and power output - usually to self-destruction, so the carburettor serves to regulate fuel/air gas mixture and flow into the engine as a means of regulating engine power output.

By means of controlling carburettor operation, the driver determines the speed at which the engine operates - to produce power for transmission through the drive train to the driving wheels.

Similarly, the vacuum tube is an electro-mechanical electronic regulating device that uses a small input effort to produce a comparatively large output power.

Just as we would not claim the butterfly valve in a carburettor produces the engine power output we would not also claim the input signal voltage to a vacuum tube produces the amplifier power output.

Importantly, the vacuum tube, like the carburettor neither produces power nor consumes it (excepting for heating requirements and efficiency losses).

In considering high-fidelity audio amplifier design principles, it is first vital to understand the vacuum tube serves two fundamental purposes or functions:

The first primary function of the vacuum tube is to electronically regulate the Direct Current (DC) flowing through it in such manner that it varies in direct proportion to the applied ALTERNATING CURRENT (AC) signal voltage. This is effected by injecting the AC signal voltage into Grid #1 of the vacuum tube. (Other Grids can be used but Grid#1 is the most common configuration).

It is the variable Direct Current flowing in this circuit that, when applied into a LOAD, produces the Alternating Current OUTPUT.

Thus the Alternating Current OUTPUT applied into a LOAD produces the useable output from the amplifier.

To simplify our understanding, we could regard the Direct Current circuit as the SOURCE of power - or INPUT POWER, and the Alternating Current circuit as the LOAD - or OUTPUT POWER.

Hence the vacuum tube could may be classed as a "TRANSDUCER" because it converts energy from one form - ie Direct Current (DC) to Alternating Current (AC). On the other hand, the complete Amplifier is not, because it uses an AC input to produce an AC output.

The resultant Alternating Current output means that a single vacuum tube provides a common current path for BOTH Direct Current and Alternating Current circuits. This is significant as will be explained below.

In the case of the DC circuit, that portion of the current path between Plate and Cathode terminals of the vacuum tube, is used to REGULATE the DIRECT CURRENT flow in the whole circuit under constant supply voltage conditions by means of the Control Grid (Grid #1). A negative DC voltage (Grid Bias) is applied to it so that it will present a negatively charged element that controls the current flow to the desired level by interfering - in a deliberately controlled manner - with the ability of the Plate to attract electrons from the Cathode.

Because the vacuum tube has negligible internal DC resistance or AC impedance (see tube rectifier characteristics for typical values) and it is a current path in the circuit supplied by the Power Supply, it is essential to insert a LOAD into the circuit to limit total current to aceptable limits - otherwise the tube would behave as a low-resistance metallic conductor.

The second primary function of the vacuum tube is to provide a return current path for the power supply source input power - which is in DIRECT CURRENT form.

Without a return path no current will flow in the power source circuit. That is to say, current can only flow in the circuit when the vacuum tube conducts.

This is demonstrated by the fact that if the power supply voltage is applied to the Cathode and Plate terminals of a cold vacuum tube, current will not flow. However if a fixed resistor - having the same resistance value as that specified for the design value of AC load impedance for the particular power output vacuum tube(s)  - is inserted into the circuit in series with the vacuum tube and the tube terminals bridged with a wire conductor, then DC current will flow to a value determined by the resistor.

In the case of a Plate loaded output transformer configuration, the output AC circuit load also is shared with the DC circuit - ie is common to both circuits. This is achieved by installing the LOAD in series with the Tube Plate and Cathode terminals.

The value of the DC power so consumed by the circuit will be equivalent to the maximum AC power output plus circuit losses.

Fundamental to hi-fi amplifier design is the principle that maximum power output of an amplifier is limited to the "prospective power" of the power supply - ie the maximum power (Volts x Amperes) the power supply can deliver in any instant of time into its load.

Actual maximum signal power output (including any distorted component) cannot exceed the "prospective power" capability less circuit losses. Circuit losses are always very significant, even at at audio frequencies, ranging from about 23% minimum to about 70% of power supply DC input power.

Thus what is of most significance to us is that assuming a constant value of load in the circuit, Actual amplifier power output is regulated by the vacuum tubes in the circuit - not by its load.

Another important concept to understand is that electron flow in vacuum power tubes does not operate in quite the same way as in the electroplating process. In that process using Direct Current, particles of a metal material are transferred from the Cathode to the solution, thence to the Anode - which is the article being electroplated. The Cathode metal is consumed by the process and eventually there is none left.

However in a vacuum tube this process does not occur - ie during normal tube operation significant amounts of Cathode material do not deposit onto the Plate. This tells us that the electron flow in a vacuum tube may be nothing more than a static stream of conductive particles that assemble in the tube to bridge the Cathode to the Anode (Plate).

Hence, the performance and "sound" of an audio amplifier should be far more dependent upon circuit design and componentry than what goes on in the vacuum tube itself.

Practical experience though tells us differently and there is provable difference in sound between different tube types.

Finally, the audio amplifier is not quite the static solid object that it appears to be.

Electricity, as a force, is fluid.

That means it neither does not, nor cannot, anchor itself to a solid object. It can only move from one solid object to another (eg a set of contacts) or move within or through a solid object to another (eg a wire or cable or X-Rays), or through a fluid such as air (electric arc) or water (electric current), or through a vacuum (eg an electron beam such as in CRT) or space (eg microwaves or light).

So long as there is a potential difference between two points in a circuit, current will flow. However for current to flow, there must ALWAYS be a circuit.

Thus if electric current passes through a conductor, there MUST be a return path back to the source so that the circuit is complete and equilibrium is restored.

This is why no current flows when a positive and negative polarity voltage is available from a pair of terminals but there is no load connected. Or why an electric lamp does not light up until the power switch is turned "on".

In audio amplifier applications, it is the LOAD that normally forms the return current path.

However, the return path MUST be electrically separate to the forward path - otherwise a short-circuit will result.

Kirchhoff's Current Law

The algebraic sum of currents entering and leaving any point in a circuit must equal zero - ie no matter how many paths
into and out of a single point in a circuit, all the current leaving that point must equal the current arriving at that point.

Thus the extremely important principle to note is that in an audio amplifier, the current returning to the source does not have to be the same current that left it. In other words, current can be drawn from an alternative source, or sources, to complete the circuit such that current back equals current forward.

Having said this, in the case of an audio amplifier, we find a whole collection of individual circuits within it - many of which SHARE current paths.

Hence the designer must pay close attention to what is transpiring separately in each individual circuit - as well as the inter-action and inter-dependence between individual circuits and sets of individual ciruits.

Note that Kirschoff's Law applies separately to BOTH AC and DC circuits - therefore each AC or DC circuit should be analysed independently to all other circuits, as well as interactively with all other circuits in the amplifier.

It is this phenomenum of inter-action and inter-dependence between circuits that causes much angst in audio amplifier design, because not a great deal of attention has traditionally been given it.

Example 1:    the concept of negative loop feedback from loudspeaker to input is a good example of how to ruin an otherwise fine set of circuits, because the loop feedback circuit bridges several individual circuits and by so doing, creates a time-delayed interaction and inter-dependence that is otherwise not there.

Example 2:    the alternating current and direct current circuits that exist about each vacuum tubes are separate but also together. Each is dependent, inter-dependent and inter-active with the other.

Example 3:    the B+ bus usually provides a common source of power to all stages of the amplifier, resulting in a situation where the current in each stage affects the voltage available to each other stage.

Example 4:    losses in the output transformer and power transformer must be offset, otherwise their circuits cannot be in equilibrium. But because what comes out is not the same as what goes in, the designer should pause a moment and think about what effect mixing the output with the input might have on performance, stability and capability.

So the message to the audiophile designer is that to effectively design and construct a high-quality audio amplifier using electron tubes, pay close attention to what is happening at each and every point in the circuit and how each individual circuit might interact with every other circuit in the amplifier.

Each individual circuit about each individual stage in the amplifier should be carefelly designed to optimise performance for that stage.


User needs for final electric power output determine the core requirements for critical components such as tube type, output transformer, power transformer, chassis size, ventilation and gross weight.

These in turn, determine needs for B+ voltage and current, grid bias voltages, rectifier and electrolytic capacitor voltage ratings etc.

Hence, a very apt starting point is to determine power needs.

It is assumed you will be using loudspeakers - which are transducers used to convert alternating current electrical energy into audio or sound pressure energy. Loudspeakers are notoriously inefficient and fragile devices and great care should be taken in their selection, mounting, installation and operation.

Generally speaking, listening to recorded music in an average room at a comfortable level requires only about one watt RMS - which is surprisingly loud with an 85-90 db SPL efficiency loudspeaker - even much more louder with 100 db SPL efficiency units.

Also, to provide for the minimum full dynamic range of recorded music - ie 20 db - a total power rating of 100 Watts RMS will be a reasonable target to enable transients to be reproduced without too much audible distortion above normal listening requirements.

In this age of digital recording, musicians have found it necessary to resort to effects to produce saleable product, hence it is common nowadays for CD's to incorporate extreme levels of low frequencies of all kinds of waveforms, to produce that "thumping" brain-deadening sound so beloved of today's teenie-boppers.

Unfortunately this means that to attain realistic sound reproduction free from audible distortion, a 100 W RMS amplifier and a 100 W RMS loudspeaker system (50 W RMS per channel - stereo) is mandatory.

Of course a lesser level of power may be usefully utilised but transients will be cut-off or truncated, with a corresponding loss of realism.

A more detailed explanation of this requirement is provided by Dave O'Brien, formerly from McIntosh Laboratories, in his description of his "Spectral Fidelity Test for Intermodulation Distortion" (IMD). IMD is what causes the sound to become "fuzzy" at high volumes.

See also an interesting overview at http://www.axiomaudio.com/archives/power.html

For a practical real-world explanation see http://www.prosoundweb.com/article/managing_power_to_properly_use_and_not_abuse_professional_loudspeakers

Note that recording technologies have always been far ahead of playback technologies. Examples of just how good recording technologies and standards have been in the past are easily heard by listening to recordings made way back in the 1940's and 1950's, or even earlier in some cases. (Some of my CD's include original recordings that go back to 1910). When transcribed to CD format and cleaned up, these recordings contain signal information not previously heard with conventional playback equipment. The information was always there on the original tape, but just not accessible. However thanks to advances in audio recording and transcription technologies, now it is.

Except as otherwise determined from the following information about loudspeaker performance, when used in a typical domestic home situation, an amplifier/loudspeaker combination of less than 100 W RMS will simply produce overload distortion in all of its forms on transients and/or intermodulation distortion on steady state heavy programme material such as sustained pipe organ music.

However this distortion may not be heard in practice because the amplifier may actually simply chop the top off the transient peaks, in which case everything else will sound just fine up to that power output level where the whole system collapses through severe overload. What will be heard is a modified sound, but the modification may not be discernible to all but the most critical listener.

On this basis, a 10 watts RMS per channel TUBE stereo will still do the job well.

In contrast, a 25 watts per channel RMS SOLID STATE amplifier will be needed to produce the same loudness through the same loudspeakers. Why? Who knows, but that's the way it is.

For those who prefer their music through headphones, then the above does not apply. Brain destroying loudness is attainable through headphones at very low power outputs - ie less than one watt.

Pop music tends to be based upon short transients comprising sound forms to which the ear is unfamiliar, so it is easy for the  average listener to tolerate very high levels of distortion - simply because the listener does not know the sound is distorting. Not so with familiar programme material such as classical music, popular programme material from yesteryear, or live recordings of musical instruments and voice well known to the listener.

However, a rational approach to hi-fi is that if you cannot hear the distortion then don't worry about it!!
WARNING: Back in days of old when everyone was honest, the unit of power was the "WATT".

This is an international standard unit and is clearly defined - see http://en.wikipedia.org/wiki/Watt

The WATT is directly related to the "HORSEPOWER" - which is a unit of "WORK".

However clever thinkers decided that a more useful unit was "PEAK WATTS", which re-expresses the rms value anywhere between 1.4 and 2.8 times the actual number - without changing anything at all.

However some people found this confusing, particularly in the case of imported amplifiers and loudspeakers competing for sales against domestic manufacturers in the same market.

So a solution was found by the good people in Japan, who straightened it all out for us.


The Electronic Industries Association of Japan (EIAJ) publishes standards for electronic equipment, including test methods for audio amplifiers.

In their wisdom they have chosen to express "watts" in terms of "peak" or "tone burst" or "intermittent" watts, which effectively doubles the rms value - for the same real power output.

I do not have the precise specification but the number is about double rms. Please send me the info if you have it. One website says 75 watts rms = 103 watts EIAJ

The Electronic Industries Association of Japan (EIAJ) and the Japan Electronic Industries Development Association merged in 2000 to form the Japan Electronics and Information Technology Industries Association (JEITA), an electronics and IT industry trade organization.

However now that we have a GLOBAL ECONOMY, where national boundaries and protection of local manufacturers mean nothing at all anymore, the entire world has moved away from rms to unspecified "WATTS"

This specification appears to be the de-facto standard for current production amplifiers and loudspeakers, which typically carry no rating markings on the actual product. Some simply say "W" for watt. This situation also applies to some well known brand names from western countries whose amplifiers carry the "CE" logo and/or DIN power cord socket - check the specs. What applied in the past may not be applicable today.

A "WATT" can be anything one wants it to be - for selling purposes the bigger the better - after all, if the consumer can purchase a 1,000 watt amplifier for the same price as a 500 watt unit then there is likely to be a consumer perception of greater value or benefit by buying the higher rated unit - notwithstanding there is no real difference.

See also "Music Power Watts" and "PMPO Watts" in Section 2.2 below.

Although not discussed here, the USA EIA Standards add yet another level of complexity.

See also:



The great attribute of these national standard systems is that they make the deception legal.

So unless the seller has specifically stated watts rms (continuous) in a written document then always assume it is an inflated number.

Watch also for local conversion by retailers who substitute "watts rms" for EIAJ Watts in local specs for imported products. Unless one can locate the original manufacturer's specs it is often difficult to verify claims made.

Remember always that watts out cannot exceed watts in. Compare the rated AC mains power input per the rating plate or fuse size ( volts x amps = watts) with the claimed audio power output.

Remember also that when current flows through a wire it will get hot as a result of internal resistance in the conductor, so an intermittent application of signal and therefore power output will heat a conductor less than a continuous application of current. This principle applies to vacuum tubes, loudspeakers and transformers. Temperature rise is a function of current squared.
I squared R = watts

Ancient Australian Proverb: "BULLSHIT BAFFLES BRAINS" !!!!

caveat emptor


2.1    SPL Rating

One useful indicator of power needs is the loudspeaker's SPL (sound pressure level) rating. This rating is determined by measuring sound pressure (acoustic) energy under standard defined conditions - usually at 1 metre from the radiating surface with an electrical input power of 1 watt RMS at 400 Hz (cps).

The SPL rating enables a quick reliable comparison to be made between different choices of loudspeaker. For example a speaker having an SPL of  87 db will require TWICE as much electrical input power (RMS Watts) as a similar size loudspeaker having an SPL of 90 db to reproduce the same sound pressure energy level (ie "loudness") in the listening room.

In other words a 10W amplifier/90 db loudspeaker combination will sound just as loud as a 20W amplifier/87 db loudspeaker combination!!! Now there is some food for thought!!

If you can afford electrically efficient loudspeakers like JBL, Altec, Electrovoice, or Eminence, having a sensitivity of 105 db SPL or more, then amplifier power needs will be hugely less - unless you like very loud music!!  Believe it or not, such a loudspeaker will need only 1/63 rd the power of that needed to drive an 87 db efficient loudspeaker. That is, your JBL, Altec, Electrovoice, or Eminence, will be effectively 63 times more powerful with the same amplifier!!

Another way of saying it is that a 105 db loudspeaker driven by a 1 watt amplifier will be just as loud as an 87 db loudspeaker driven by a 63 watt amplifier. Time to throw away those old inefficient drivers!!

For further details of the relationship between power and decibels please refer to the Decibel Chart provided.

A word of warning though - increased loudspeaker efficiency means higher output levels for background hum, noise and hiss. More attention will be needed to the amplifier's design if an acceptable result is sought. Ultimately there will be a trade-off between the competing needs. There is some practical justification then in using a low-cost amplifier to drive a low cost loudspeaker!!

When comparing SPL ratings ensure they have been measured using the same method. Some manufacturers quote SPL ratings at full rated output, which will obviously look better than the SPL produced from 1 watt at 1 metre.

NOTE: Although some specific brands of loudspeakers have been mentioned above as being (relatively) "high efficiency", not all of the models produced by these manufacturers are in that category.

SPL ratings for specific brands and types of loudspeakers vary widely so it is recommended that the manufacturer's catalogue data sheet be studied before forming conclusions.

2.2    Power Levels

It is wise to limit power output to a level that the loudspeakers can withstand comfortably, noting that loudspeaker power ratings are generally set at 400 Hz and assume the loudspeaker is installed in a properly designed enclosure that provides adequate cone loading and damping to limit system resonance and prevent cone overshoot.

As a rule of thumb, it is a wise precaution to halve the loudspeaker manufacturer's ratings, particularly if they are published as "peak" watts.

Music power, IHMF and PMP etc. ratings should be appropriately derated to RMS equivalent.

Remember, our SPL rating is nominally measured at 400 cps. The actual frequency response over the full frequency range - particularly in the lower register - needs to be evaluated.

It has been observed that some manufacturers exaggerate the power ratings of their loudspeakers - computer speakers are a current example - eg 1200 W from a 4" loudspeaker - so look for documented verification of quoted or claimed ratings.

Always compare speaker ratings on RMS rated power handling capacity. RMS is an internationally recognised unit of measurement and provides a standard way of rating and comparing.

On the other hand, the commonly used rating of Peak Power has different meanings in different places. It can be anywhere from 1.414 x RMS to 2 x RMS.
2.2.1    PMPO, or Peak Music Power, or Program Power

Means the instantaneous power the unit can handle. It refers to the current the unit can handle for a very short interval of time before it fuses or the speaker mechanically self-destructs. PMPO is totally useless and meaningless rating.

The short-time current a conductor can carry during a short-circuit - such that the conductor heats to a predetermined temperature during the duration of the short-circuit, is defined as I squared x T (current squared x time = a constant). The value of the constant is irrelevant - except to determine time.

The International Standard time for determining short-circuit ratings (which require the conductor to remain intact and not suffer visible damage) is ONE second, so for practical purposes the temperature rise of a conductor is determined by the square of the current - which is also the formula for determining power.

We can thus say that the power handling capacity of a loudspeaker (or any wire based device such as a transformer) is directly proportional to the electrical current in - up to its maximum capability, when the conductor will fuse (in a loudspeaker this is the voice coil winding)

PMPO ratings simply take an RMS value and divide it into a very short time interval.

If we use that one second nominal time as a basis for determining a PMPO rating, then for a 10 watt RMS rated loudspeaker at 400 Hz (which is the standard rating frequency), we can calculate a PMPO rating for a time interval of one half of an RMS cycle (1/800th of a second for a 400 Hz signal) by the formula:

1 second multiplied by 800 = 800 x 10 watts = 8000 Watts

This is how 10 watts increases to 8000 watts!!

Obviously the rating will vary dramatically with frequency.
2.3    Loudspeaker Design and Construction

Many modern loudspeakers have huge magnets and rock hard cones made from heavy plastic, metal or carbon fibre materials.
Unfortunately none of this indicates efficiency and it may well be that a loudspeaker that looks rugged and chunky actually has a low SPL performance, or limited frequency range, resulting in a severe efficiency loss matching the lower price tag.

Often it will be observed that in loudspeakers up to 15 inches in diameter, those having small magnets and paper cones actually outperform those with large magnets and plasticised carbon fibre cones - particularly in the lower bass region. This is because the heavy cone requires more power to drive it. More power input means more electrical current through the voice-coil. More current means larger diameter wire. Larger diameter wire is heavier and produces a lower ampere/turns ratio in the magnetic field. Heavier wire means more mass to shift. More mass to shift requires a stronger magnetic field. A stronger magnetic field requires a larger, stronger magnet. Because there are physical and cost limitations on all of these parameters, the usual approach in low-cost designs is to simply use larger wire, which translates into less electrical efficiency in the electromotive energy available to drive the cone.

Another relevant design feature with modern high-excursion cone drivers is that when the cone is driven to its extremity, the voice coil must still be within the magnetic field of the pole piece to electro-magnetically drive and/or control cone movement. It is obvious that the more the cone travels in and out, the longer the voice coil must be, which means there will be fewer turns of wire actually in the magnetic field of the driver at any instant of time. This translates into lower efficiency - unless design techniques are incorporated to maintain the electro-mechanical efficiency of the transducer.

So our modern inefficient 100 W RMS loudspeaker may need 100 W to drive it - but not to actually deliver more sound pressure output than the 20 W loudspeaker of yesteryear.

Cone suspension systems vary markedly but as a rule of thumb, modern rubber surround suspensions used in sub-woofers and car (automobile) audio speakers are not as reliable as the traditional "accordian" linen or paper suspension. Foam rubber suspensions tend to literally crumble and fall apart after a relatively short period of time - particularly if exposed to sunlight.

When selecting a loudspeaker, check to ensure the cone can move reasonably freely - but not "floppy" - and the spider suspension (usually yellow treated fabric around the voice coil at the back of the cone) is of large diameter and capable of free travel.

Note: When manually testing cone movement NEVER push on just one side of the cone. To keep the voice-coil central and round, always use two hands and carefully apply pressure evenly to opposite sides of the cone.

It will be observed by the very critical listener that the sound-qualities of paper-coned loudspeakers vary throughout the seasons. In summer, when cones dry out, they tend to give brighter, cleaner sound, and sound louder. But in winter, or wet weather, the moisture content in the cone increases, making it heavier and therefore less sensitive to transient peaks, producing a 'duller" sound.

Bass response will tend to improve slightly and cone resonance reduce from a heavier cone. This feature is incorporated into heavy duty bass speakers, which have heavy cones, heavy voice coil wire and large magnets to maintain efficiency.

Note however that a well designed loudspeaker of modest power rating - eg 25-30 W RMS - does not need a large diameter voice coil or large magnet to be an effective transducer.

For high fidelity reproduction, it is far better to have a light cone move little than a heavy cone move a lot - the further the cone travels the less linear the sound reproduction will be.

It follows that for a given level of power output, a large diameter loudspeaker will be more linear than a small diameter loudspeaker.

Note: No loudspeaker is indestructable. Solid-state amplifiers tend to produce square waves when overloaded and can quickly melt the voice coil. However tube amplifiers are not so nasty and short-time overload may be tolerable.  A good guide is audible distortion - if you can hear distortion then the system is overloading.

(Some people equate distortion with "loudness" but do not fall for that trap - clear sound is always better than distorted rubbish - especially in PA applications where listeners need to understand what is being said)

2.4    Loudspeaker Frequency Response

It has been demonstrated by reputable audio engineers and scientists from numerous tests over the past 50 years or so, that most of the audio POWER produced by a symphony orchestra occurs in the range below 300 Hz.

It can be readily deduced from this that very little of the orchestral music audio range is reproduced by mid-range (particularly fully sealed, closed-back mid-range units) or tweeter loudspeakers that are fed through a crossover network filter preventing frequencies below a predetermined numeric value from being reproduced.

Hence in a typical two or three way speaker system, it is the woofer that is doing most of the work and producing most of the audio power that is actually heard by the listener - (more obvious in two-way than three-way systems).

However in this age of cinema "effects" audio, as provided in cinema sound tracks on DVD (ie either 2, 5.1, 6.1 or 7.1 systems), and the use of physically small speaker systems, it is extremely difficult to accurately reproduce fundamental frequencies below 300 Hz - ie in the form presented by the signal source.

For example, most of the split system "sub-woofer" loudspeakers designed for use in home cinema "effects" audio, are designed for use to about 150 Hz or less. This type of loudspeaker is usually equipped with a rock hard and stiff cone that is incapable of reproducing the nuances of high-definition music. Therefore there is great loss of high-fidelity in much of the fundamental frequencies and their harmonics essential to high-fidelity reproduction.

Whilst this type of reproduction is fine for movies, where the sound-track is an adjunct to the main visual presentation, in the case of purely music reproduction the absence of capability to reproduce significant chunks of fundamental musical signal is cause for concern to the hi-fi audio enthusiast.

The solution lies in using FULL-RANGE loudspeakers having a full-range frequency response when mounted in a suitable enclosure as single or multiple drivers - of at least 40 Hz at the bottom end, to at least 15 kHz at the top end.

This frequency range is basic and is based upon the physical reality that around 40 Hz is the nominal bottom note on a double bass violin or bass guitar, and 15 kHz is the upper limit of hearing for much of the population and is also the upper limit for FM radio broadcasting. In fact, many people have difficulty in hearing beyond 8 kHz.

Note pipe organ music reproduced from CD requires an even wider range system - ie about 20-20,000 Hz flat.

In general terms, the larger the cone diameter the better the bass response and tone. In this regard the woofer should have as a wide a frequency response as is practicable so that harmonics are reproduced by the woofer and not a separate smaller unit supplied through a crossover network. This advice also applies even if a mid-range driver or tweeter is fitted.

Note that for sub-woofer applications the reverse is true because the purpose of the sub-woofer is to reproduce sound effects such as "bangs" or "thumps" rather than music. The elimination of harmonics provides a "deeper" but "colourless" bass tone.

It is suggested that it is more productive to focus on quality rather than quantity when determining the ideal frequency response for the loudspeaker system.

Loudspeaker specifications tell only a part of the story so it is always advisable to listen to a complete system in a room rather than rely upon tested performance in a laboratory. Test methods and accuracy can vary.

2.5    Frequency Response and Power Response

A frequency response of -0 db at resonance in one unit compared with a frequency response of -20 db at resonance in another (compared to sound output at 400 cps), equates to an electrical power difference of 100 times to attain the same acoustic loudness (SPL) at that resonant frequency. That will be impossible to attain equal loudness at useful power levels because the loudspeaker will not be able to handle that much power.

This rule also applies at any other frequency when comparing SPL performance.

Similarly, a frequency response difference of 3 db equates to a need for twice the electrical power to attain the same "loudness" - refer Decibel Chart for power ratios.

So a better (ie 'flatter") bass and treble frequency response curve means cost-free acoustic power output. It also means more natural sound - ie higher hi-fi.

An example of this phenomenon is shown in the following typical frequency response graph for a high-quality low-frequency reproducer - ie "Woofer" - SYSTEM - ie Woofer loudspeaker mounted in an enclosure.

IMPORTANT NOTE: These graphs do not show the frequency response of the loudspeaker by itself, and include enclosure resonances and enclosure design impact upon loudspeaker performance - which is what the listener hears in any case.

Note the average SPL of 100 db across most of the usable frequency range, indicating unusually efficient performance over a very flat response curve - by any measure this is a "top of the range" LF Woofer driver unit.

Loudspeaker System 1

In this case, the loudspeaker is mounted in a medium sized vented enclosure, resulting in two peaks at 33 Hz and 66 Hz about the natural resonance of about 43 Hz. The important feature to note with this enclosure configuration is that even when using a very high-quality transducer offering an average SPL of 100 db, the efficiency at system resonance (40 Hz) drops to 87 db - a reduction of 13 db on the average. Although this may appear to be an alarming performance, it is very typical of vented-enclosure loudspeaker systems. This system would not be suitable for bass guitar applications but may be very satisfactory on hi-fi playback. The slight increase in SPL in the mid-range is of great benefit in speech applications.

It is also noteworthy to comment that the enclosure used for the above test is relatively large by volume. Typical small volume vented enclosures will demonstrate substantially higher peaks above and below the natural resonance - see paper on Loudspeaker Enclosures design.

Patchy LF response is definitely noticeable to the musically inclined listener and should be avoided where practicable for hi-fi.

However when mounted in an infinite baffle (ie sealed box) of generous proportions, or in a wall, the response curve will change to something like the following:

Loudspeaker System 2

Note: This is not the same driver unit as per the above Loudspeaker System 1 but the effect is well demonstrated.

In this case, the low-frequency response falls away rather evenly, with no peaks or troughs in the response curve - AND - the efficiency at 40 Hz (bottom note on a bass guitar or double bass violin) drops away to only 93 db - less than half the reduction in efficiency with a vented enclosure.

Unfortunately the enclosure must be large - ie around 20 cubic feet minimum - to avoid increasing the resonant frequency of the system. For example, an enclosure of  5 to 10 cubic feet volume should increase the system resonance to about 80 to 100 Hz, causing the flat section of the response curve to start to roll-off at about 100 Hz instead of the 60-70 Hz shown.

Provided the amplifier can deliver the required power, such a system can sound quite strong at the bottom end when suitable bass boosting is used - eg tone control

It is also of interest to note the average "flat" section of the frequency response is about 3 db down on the first graph - System 1 - this system thus requiring twice the input driving power for the same loudness.

Loudspeaker System 3

This system is included to demonstrate the different low-frequency and high-frequency rolloff characteristics to System 2.

Whereas overall SPL is similar, the -3 db point is at 35 Hz in this system, the curve of Loudspeaker System 2 shows a -3 db point of 40 Hz - which is more or less the same in practice.

However at 30 Hz the SPL of System 3 is at 86 db but System 2 is at 90 db - a 4 db difference.

IMHO, for hi-fidelity reproduction, the loudspeaker shown in System 3 is a better investment, simply because the response is flatter over a wider frequency range.

Furthermore, the HF rolloff commencement at 1 kHz is ideal for 3 way systems - ie woofer, mid-range and tweeter drivers with a crossover network.

However for 2 way systems, the characteristics of Loudspeaker System 2 would be better suited because of the higher roll-off frequency, noting Tweeters do not generally commence reproducing below about 2 kHz - mostly higher.

2.6    Tweeters

It is customary in two or three way loudspeaker systems to rate the whole system and not the individual components. Hence a tweeter from a 100 W RMS system may actually only have a 20 W RMS rating on its own (within the restricted frequency response of the tweeter), because in typical music the power output is substantially more in the lower register than in the HF range.

It is common to see lower priced drivers rated in "Music Power" in an attempt to match low and high frequency power ratings when processing a "music" signal.

The tweeter and/or midrange unit in fact may have an even lower power rating if it is supplied through a system crossover network, which typically soak up 50% of power input (-3 db insertion loss).

In the case of the three examples above, for an even full-range frequency response, the Tweeter driver would need to have an efficiency matching the Woofer driver unit. In all three cases illustrated, the Tweeter will need to be of very high quality - ie SPL 95-100 db - to match the Woofer performance.

In general terms, horn shaped or "curvalinear" tweeter cones are superior than simple flat or straight tapererd cones.

Whilst paper cones deliver a soft natural tone and are the most popular form, mylar cones can also deliver a very natural neutral tone.

Ribbon tweeters are superior because of their ultra-wide range flat response, very low distortion, clarity and natural tone.

Planar tweeters with mylar diaphragms are also excellent. The "Infinity" planar tweeter is a superb unit and well worth trying - sheer magic !!!!

Then there is the electrostatic tweeter - also excellent, delivering crystal clear highs.

Note: A 100W rms system for home music requires a tweeter capable of say 20W rms - perhaps less. But for live music such as guitar and PA, to prevent destruction the tweeter should have the same power rating as the woofer.

2.7    Coaxial Loudspeakers

A number of manufacturers have produced coaxial designs wherein the tweeter is mounted coaxially inside the woofer unit.

These loudspeakers have the advantage of simplicity, compact size and no physical separation between low and high frequency sound sources (to the listener).

A typical coaxial unit is the British "Goodmans of England" model 212 hi-fi speaker, produced from about the 1950's.

This unit has a 12 inch woofer with a pressure loaded horn driven tweeter, and inbuilt L/C crossover frequency of 5,000 Hz.

It has a rated frequency response of 30-20,000 Hz, as designated on the nameplate attached to each speaker magnet housing.

The frequency response of this unit is shown below:

Loudspeaker System 4

It can be seen that notwithstanding the manufacturer's catalogue rated useable frequency response, at the limits of 30 Hz and 20,000 Hz the response is -10 db to the reference SPL  test level of 0 db.

As explained above, for a flat response across the full hi-fi audio range, a +10db power boost is required at both top and bottom ends. This equates to a power output increase of ten times the SPL at 1,000 Hz.

However it is stressed that despite this apparent shortcoming, the above loudspeaker is a "top of the range" hi-fi unit and such variances in response are both normal and standard for the loudspeaker industry.

Importantly, the crossover between woofer and tweeter at about 5,000 Hz is seamless.

What the chart does not show is "what it sounds like". That is a subjective judgement for the individual listener.

2.8    Multiple Loudspeakers

Multiple loudspeakers - preferably of identical design and construction (although satisfactory performance can be obtained with dissimilar but tonally compatible units) - connected in series or parallel as needed - are a very efficient means of adding more power handling capability.

It is commonly believed that the more loudspeakers connected the less power output will be obtained from the amplifier - ie the extra speakers will "drain" the amplifier power - however this assumption is completely wrong.

The amplifier is a power source supplying "POWER" - to a load.  Provided the correct load impedance is maintained to match the amplifier transformer terminals, there is no real limit to the number of loudspeakers that can be connected to the amplifier. To maintain the original load impedance set for the amplifier, multiples of 4, 9, 16, 25 or 36 etc drivers having the same voice-coil impedance can be arranged in a series/parallel configuration to present a load equal to just one of those drivers.

All that happens is that the total power from the amplifier is divided equally between the number of loudspeakers.

It is often cheaper to add an extra loudspeaker than to double amplifier power. Also it is usually cheaper to add an extra low-cost loudspeaker to double power handling capacity (matched to the original unit of course) than to instal a single high cost higher performing unit. Do the sums!!

However don't forget to compare SPL efficiency performance. Two loudspeakers having an SPL of 87 db will not compete very well with one unit of 104db SPL.

There is a practical cutoff point though - at about 100 W RMS continuous, where the available options for loudspeakers disappears very rapidly. Few manufacturers make loudspeakers capable of handling both the electrical and physical stressors created at this power level. Usually they come in giant sizes - ie 18 or 24 inch or larger drivers, intended for heavy duty professional PA, musical instrument or organ applications - which are beyond the bounds of practicality for most home users. Another aspect is that large speakers are often set up with stiff cones for woofer duty and sound "muddy" in the mid-range and highs.

So for home hi-fi use a wide-range corrugated paper cone is preferable to a stiff paper or carbon fibre cone or equivalent. Refer to speaker manufacturer's specs for comparison of frequncy response.

A good starting point is the US made "Eminence" range, which in my experience offers great performance at affordable cost.

However a true 100 W RMS can easily be handled continuously with low distortion by four 12 inch drivers, mounted in a single large enclosure, or in multiple enclosures.

If a quad set of 12 inch units will not fit into your room or decor, then a single high-quality 18 inch driver with 3 inch dia voice coil will do most anything required in a home situation. Note however that the enclosure volume needs to be proportional to the total power output and frequency range required. A tweeter (and maybe a mid-range driver) with LF blocking crossover will also be required.

Serious consideration must be given to the loudspeaker behaviour at the power level intended to be reproduced. The validity (genuineness) of manufacturer's ratings needs to be carefully researched because it is too late to discover that a manufacturer has exaggerated ratings after the loudspeakers have been purchased and installed.

A little headroom can save an expensive loudspeaker from self-destruction resulting from electrical or physical damage.

Note: The more loudspeakers situated side by side that more the entire system moves froma single point source towards the "planar" mode of sound generation. This has immense benefits and need not be expensive.

Note: Research has verified that the more cones included in an array of loudspeakers, the greater the low-frequency response from the system.

For an excellent explanation of this option see http://www.roger-russell.com/columns/columns.htm

My personal stereo uses 2.4 metre (8 feet) high column loudspeakers for the same reasons as Roger Russell explains. I would never go back to one, two or three way single driver enclosures because they lack the "reality" of stereo columns.

2.9    Distortion

Loudspeakers are notorious for generating distortion - of many kinds - so the more headroom the better. It follows that multiple speakers will require less cone travel for a given level of acoustic energy (loudness).

Twin-cone loudspeakers incorporating a whizzer cone, tend to change their frequency response with increasing cone travel. At low power outputs the cone and voice coil are very much controlled by the magnetic circuit, but at high cone excursions the cone may overtravel, be less damped, and lose relative drive to the high frequency whizzer cone - resulting in diminished high-frequency response and increased intermodulation distortion. However twin cone loudspeakers offer simplicity of enclosure design, high power output, predictable response and are a preferred low-cost option for public address systems, guitar amplifiers, car audio and home sound systems where full-range reproduction is sought. There is no loss of power into crossover networks.

In twin cone units for hi-fi, it is preferable for the whizzer cone to be of curvilinear, exponential or logarithmic curve shape - ie trumpet like form. Plain simple cones do not provide linearity in frequency response or ideal tonal properties.

Some researchers have suggested that mixing twin-cone loudspeakers with tweeters does not work well, however it has for me so I am converted to accepting mixed cone systems.

For a technical report on wide-range drivers see http://www.commonsenseaudio.com/fostextech.html

2.10    Cone Excursion

Remember that the more the cone excursion, the greater the cone overshoot-travel and the greater the back EMF into the amplifier.

Cone travel is a product of its kinetic energy so basically the heavier the cone the harder it is to stop it and reverse its direction - in response to the driving signal power.

If the amplifier is configured with negative loop feedback, the higher back-EMF will cause the feedback system to work harder, which in turn will cause the tonal properties of the sound to change as the amplifier tries to correct the effects of back-EMF from the loudspeaker system.

Another feature of excessive cone travel is substantially increased intermodulation distortion - not so much of a problem for bass guitar or electronic organ, but a definite challenge for hi-fi.

A simple solution to this effect is to use multiple loudspeakers in a totally enclosed or suitably loaded enclosure. The internal air pressure dampens cone travel in both directions, resulting in substantially higher efficiency and reduced back-EMF.

Research has demonstrated that the more cones included in an array of loudspeakers, the greater the low-frequency response from the system.

2.11    Crossover Networks

Another design element to consider is that commonly used crossover networks soak up power so it is an essential pre-requisite to determine how much audio power will be lost in the crossover network and add that to total needs.

Commonly seen losses in crossover networks are in the region of 3 db, which is another 50% of power gone down the drain, never to be heard of again!!

Some crossover networks are worse than that.

Crossover networks also suffer roll-off at the crossover frequency - ie roll-off in both upwards and downwards responses, so there tends to be a hole or dip at the crossover frequency.

A popular, but expensive, alternative - more viable nowadays with integrated multi-channel surround sound system amplifiers - is to use electronic crossovers at the input/pre-amplifer stage - and drive each set of LF and HF loudspeakers with a separate amplifier.

The ubiquitous solid-state powered sub-woofer solves much and is an ideal adjunct to the tube amplifier - particularly the popular modern single-ended triode variety having limited power capabilties.

2.12    Enclosures

Horn systems are significantly more acoustically efficient that direct radiator systems - in the order of 30% compared to 5% - ie up to six times more efficient. Accordingly, the amplifier drive requirements for a horn system may be significantly less.

However horn systems have different "sound" or tonal qualities to direct radiator systems and that may not be acceptable to all listeners.

Horns have practical limitations regarding physical size. Low frequency reproduction requires a very large horn (just look at the "Tuba" or "Euphonium" or a pipe organ)  - mostly impracticable to most users.

All loudspeaker systems require adequately damped cabinets to minimise resonance from the cabinet material and shape. This can occur at any audible frequency - not just in the bass register.

In the 1950's it was the norm for enclosures to be as large as was practicable. Totally enclosed loudspeaker enclosures (infinite baffle) require large volume for low resonance - 10-20 cubic feet volume was considered the minimum essential for resonance free solid bass response down to 30 Hz with a 12" speaker (it still is).

Dedicated audiophiles actually built enclosures having sand-filled walls to attain effective enclosure resonance damping.

Vented enclosures (with or without tunnel or passive radiator) offer substantial reduction in enclosure size but produce two resonance peaks - one above and one below the natural loudspeaker resonant frequency, resulting in a sometimes obnoxious peaky bass response on some kinds of program material. (This is why bass guitar speaker systems tend to use a totally enclosed (infinite baffle) design for best results). But the advent of small apartment housing for the masses has led to demand for "bookshelf" speakers. Unfortunately bookshelfs do not reproduce sound very well. Get a real loudspeaker and a real enclosure!!

Finally, in my opinion it is wiser to spend more on the loudspeaker than the amplifier, because the loudspeaker will have greater effect on the final sound. A practical way of evaluating this statement is to check out a typical automobile radio retailer or hi-fi shop display setup where a range of loudspeakers can be switched to connect to a common amplifier. The results are profound, and tell us a lot about how loudspeakers "colour" sound.

It also tells us that in the current global trading environment - price and performance are not directly connected.

2.13    Tone

Although two and three way systems have always been popular because of their wide frequency response and reduced intermodulation distortion, they do tend to suffer from tonal imbalance between units.

Some systems also suffer from varying efficiencies between units, resulting in a peaky response.

Thus in two and three way systems it is important to select the individual drivers for compatibility of tone.

Another factor is the directionality of mid-range and tweeter units.

To overcome this, which is experienced by noticing that the highest frequencies (including important harmonics) tend to be heard only on the centre-line axis directly in front of the system - a feature not always desirable, when the listener is not able to located directly in front of the speakers.

One method of overcoming this is to use two mid-range drivers, each mounted to point slightly outwards off the front centre axis.

Tweeters should also be two in number, again pointing off axis.

Unfortunately this arrangement may result in an impedance mismatch so arrangements need to be made to overcome this.

The simplest way is to use FOUR each of midrange and tweeter units to attain a matching impedance to the woofer and amplifier.

A simpler solution is to use multiple twin-cone drivers - say 4 x 8" or 4 x 12" per enclosure. Connected in series/parallel they will present the impedance of one unit to the amplifier. eg 4 x 8 ohms speakers in series/parallel connection will present an 8 ohm load to the amplifier.


The "sound" or "tone" of a vacuum tube audio amplifier may be varied at will - or better put, may be customised to suit your personal tastes and hearing capabilities.

However "sound" or "tone" are subjective terms and will vary in conceptual meaning between individuals.

The old adage "one man's trash is another man's treasure" is very apt in this situation, so a person's description, or expression, of a particular sound characteristic or quality will mean something quite different to another person hearing the same sound.

Terms like "bright" tone may be "harsh" to someone else, and "smooth" or "mellow" may be "dull" to another, so avoid (if you can) prescribing tonal qualities as far as you can.

However, the basic styles of output tube configuration do have generic tonal properties that may be fairly recognised as being significantly different to each other.

The choices include:

  • Triode (includes tetrodes, pentodes and beam tubes connected as triodes)
  • Tetrode
  • Pentode
  • Beam power tube (choice of aligned or non-aligned grids)
  • Ultra-linear connection
  • Unity coupled connection - ie McIntosh (USA) or Quad (UK) configuration
  • UltraPath™ (Bypass capacitor connected directly from B+ to cathodes)
  • Parallel Feed (Load connected between plate and earth)
  • Ultra Parafeed (Load connected between plate and cathode)
  • Output Transformerless (OTL) connection
  • Cathode Follower connection
  • Single-ended
  • Parallel single-ended
  • Push-pull
  • Parallel push-pull
  • Single-ended push-pull
  • Parallel single-ended push-pull
  • Circlotron
  • Super Triode ®
  • Super Pentode ®

  • .
    Also there is a choice of interstage coupling design:
  • Transformer-coupled
  • RC (Resistor/capacitor) coupled
  • CC (Choke/capacitor) coupled
  • Direct coupled
  • SRPP (Shunt Regulated Push-pull)

  • .
    .and yes there are also more exotic options available too!!

    Furthermore the tonal qualities will be vastly different for either single-ended or push-pull configurations, each of which offers unique tonal and distortion properties - see article.

    Important: The "sound" or "tone" of any amplifier can be dramatically changed by replacing just one component in a circuit. So before replacing your expensive entire amplifier just replace one or two selected "sound affecting" components

    The first place to start is the interstage coupling capacitors. Generally speaking, the larger the caps the more "deep or bassy " the tone will be and the smaller the caps the more "bright or trebly" the tone will be.

    Other components that can be changed for improvement are the values of plate resistor, cathode resistor, grid #1 resistor (all variable + or - ) and electrolytic filter caps (generally the bigger the better).

    Also for a cleaner but softer tone try removing cathode bypass capacitor to all driver stages (not power stage).

    Changing any of the above components may cause instability so take care.

    My policy is to ensure all Grid #1 resistors are the smallest viable value and never more than 100 k ohms. This reduces hum and noise, increases stability and enables the use of feedback free circuity.


    Negative feedback offers advantages and disadvantages - see article.

    My recommendation is avoid using it - except within the output stage in the form of ultra-linear connection.

    NEVER across multiple stages.

    Note: Uncompensated (ie no negative feedback) pentodes and beam power tubes usually deliver significant "bass boom" at the resonant frequency of the speaker system. Unless a push-pulltriode, or parallel push-pull (ie multiple pairs) tetrode or pentode, or an ultra-linear tetrode or pentode output stage is used, this boom may be intolerable.

    Other techniques include using a large heavy construction fully sealed enclosure (ie infinite baffle) having heavy cabinet damping (eg carpet underlay or fibreglass insulation glued to the timber)

    None of this is practicable with "bookshelf" sized speakers so either use a triode output stage or get a transistor amp.


    This step is quite critical because it will have great effect upon performance and sound properties.

     A core design requirement is to set the load impedance on the output tubes such that they operate efficiently and effectively across the frequency range, power output and distortion levels intended for use.

    The amplifier load impedance is produced by the effect of the output transformer multiplying or "reflecting", the loudspeaker impedance back onto the plates of the output tubes.

    The amplifier load impedance is determined by multiplying the loudspeaker load impedance by the output transformer primary to secondary impedance (not turns) ratio.

    NOTE: "Output Impedance" is not the same as "Load Impedance".

    "Output impedance" is the source impedance as seen by the loudspeaker and relates in a practical sense to the cone movement damping characteristic or "damping factor".

    Loudpeaker damping is necessary because back EMF created by mechanical cone overshoot generates power that is reflected back into the amplifier by the output transformer. That power is then consumed by the effective load on the loudspeaker. Therefore the lesser the amplifier output impedance the greater and faster the back EMF will be dissipated.

    Mathematically, "Output Impedance" does not reduce by using parallel tubes.

    "Load Impedance", in a push-pull output stage,  is the plate to plate load presented to the output tubes via the output transformer - usually the loudpeaker impedance multiplied by the transformer primary to secondary impedance ratio. In a normal amplifier/loudspeaker combo, the actual loudspeaker impedance varies dramatically across the audio frequency range, thus the actual plate to plate load will also vary accordingly in response to the varying loudspeaker impedance reflecting through the fixed transformer ratio. However for standard design convention and convenience, the loudspeaker load impedance is taken as the fixed nominal value as specified by the manufacturer.

    For a technical explanation see "Output Impedance 1" and "Output Impedance 2".

    McIntosh and others have determined that the lower the ratio of full primary to full secondary impedances in the output transformer the better the performance.

    It is also claimed by some commentators that the more turns in an output transformer the lower the high frequency performance and greater the distortion.

    It follows that a significant reduction in both output transformer load impedance and ratio can be obtained by the following options:

    5.1    Parallel operation of output tubes

    Every extra pair of tubes reduces the load. The actual transformer load impedance required is calculated by dividing the effective primary Plate to Plate load divided by the number of pairs. Thus two pairs (4 tubes) in parallel-push-pull connection will halve the required transformer load impedance and therefore halve the impedance ratio.

    Four pairs (8 tubes) will reduce the Plate to Plate output load impedance to one quarter that for one pair of tubes.

    5.2    Series operation of loudspeakers

    Every extra loudspeaker increases the effective secondary load impedance by the number of loudspeakers - ie two doubles the load, three triples, four quadruples etc.


    1. In a series connected string of Loudspeakers, if one fails by open-circuit then all will fail. For this reason, parallel connection is used for public address installations where reliability is important.
    2. In a series connected set of Loudspeakers, the total inductance (as presented to the amplifier terminals) will be multiplied by the number of loudspeakers and the total capacitance will be divided by the number of loudspeakers.
    3. In a parallel connected set of Loudspeakers, the total inductance (as presented to the amplifier terminals) will be divided by the number of loudspeakers and the total capacitance will be mutiplied by the number of loudspeakers.
    4. Provided appropriate Loudpeaker impedances are selected and provision is made for unequal power distribution in the network, Loudspeakers may also be connected in a series-parallel configuration.

    From the above it can be demonstrated that series connected Loudspeakers will sound different to parallel connected Loudspeakers, because the actual or effective values of reactive inductance and capacitance as presented to the amplifier terminals will be respectively different for each configuration.

    Electrical efficiency will also vary depending upon the power factor in the circuit.

    5.3    Combined Parallel Tube Operation and Series Loudspeakers

    Combining both techniques offers substantial advantages. Thus using eight tubes in parallel-push-pull connection under given conditions, provides four times the output power with one quarter the primary load impedance.

    When coupled with four series connected loudpeakers the transformer impedance ratio reduces to one sixteenth that required for conventional operation with two tubes and one loudspeaker.

    eg in a situation requiring 8000 ohms plate to plate primary coupled to an 8 ohm secondary, the turns ration will be the square root of 8000 divided by 8 - ie the square root of 1000 = 31.62:1

    If we use eight tubes (four pairs) in parallel push-pull and maintain the same effective load per pair of tubes, the transformer primary load impedance reduces to 8000 divided by 4 = 2000 ohms. If we then connect the four 8 ohm loudspeakers in series we get a load of 32 ohms. The transformer impedance ratio will then be 2000 divided by 32 = 62.5 and thus the turns ratio will be the square root of 62.5 = 7.9:1

    So the impedance ratio has been reduced from 1000 to 62.5 = 16 times, and the turns ratio has been reduced from 31.62 to 7.9 = 4 times.

    In many cases this improved set of operating conditions may be achieved at no extra cost.

    Benefits of parallel tube operation include:

  • significantly lower operating voltages
  • cheaper tubes and sockets
  • typically same space requirements
  • lower cabinet height (small tubes are shorter)
  • more choice of tube types
  • low cost tube types
  • more readily available components and spares
  • smaller power supply components
  • lower voltage B+ busbars and electrolytics
  • same driver stage output voltage as for one pair
  • effective output transformer primary inductance increases proportionately with each pair of tubes - translates into better output stage efficiency and improved bass response
  • output transformer primary/secondary insulation (dielectric strength) can be reduced offering bifilar and interleaved winding options
  • output transformer can be smaller and therefore less expensive for the same standard of performance as from one pair of tubes
  • output transformer leakage inductance and parallel capacitance should be less, so the bandwidth and coupling will improve significantly, resulting in decreased "bass boom"

  • .
    5.4    Disadvantages of parallel tube operation are few but include:
  • increased DC in the output transformer primary requires heavier wire - not recommended for single ended operation
  • output transformer leakage inductance effectively increases proportionately with each pair of tubes added - translates into reduced HF response and risk of instability - or need for a higher quality transformer
  • averaging of the audio signal results in less precision in the reproduction of transients (this may be noticeable to listeners changing from a single pair) - offset by more power and more headroom
  • requires adequate spacing and ventilation (but may be no more than an equivalent pair of large tubes)
  • more difficult to select a matched set of tubes (can be offset by independent individual grid bias adjusters)
  • maximum permissible input impedance to the output stage will be correspondingly less so the load presented to the preceding       driver stage is proportionately less also - may need cathode follower or transformer coupling
  • higher risk of parasitic oscillation - more care needed with component selection and wiring layout
  • requires proportionately higher values of filter capacitors, to provide adequate DC current for transients - very large filter caps need extra precautions to prevent disasters and electrocution
  • requires proportionately higher value of capacitor connecting centre tap of output transformer to ground, to provide adequate frequency response from lower value of load impedance in series with this capacitor

  • .
    5.5    Connecting Cables

    Long connecting leads between the amplifier and loudspeaker may have a DC resistance of up to say one to two Ohms (there and back). In an 8 ohm system this will soak up a significant portion of the amplifier's power output. Hence one can either use welding cable having low DC resistance - or use a speaker system having higher impedance.

    It is common in modern SOLID STATE amplifiers for power output to be quoted into a 4 Ohm load. Obviously if one or more Ohms of resistance is present in the connecting cables then not much power is actually going to find its way to the loudspeaker.

    In such situations it is true to say that the cable characteristics will be audible - in the sense that the effects of such a cable may be heard. However in a tube amplifier having an output impedance of 16 ohms or more, no deterioration of sound quality will be noticed by the average listener.

    For the technically minded, 100 watts RMS into 4 Ohms will require a current of 5 Amperes.
    (I2R) 5 Amperes x 5 Amperes x 1 Ohm cable resistance = 25 Watts loss into the connecting leads

    On the other hand, 100 Watts RMS into 16 Ohms will require only 2.5 Amperes.
    (I2R) 2.5 Amperes x 2.5 Amperes x 1 Ohm cable resistance = 6.25 Watts loss into the connecting leads

    So a 100 Watt solid state amplifier feeding a 4 Ohm loudspeaker through a connecting cable having one Ohm resistance will deliver only 75 Watts of electrical energy at the loudspeaker - a reduction of about 1.5 db (hardly audible but significant in reduction in transient response capability and sound reinforcement applications).

    On the other hand, a 100 Watt tube amplifier feeding a 16 Ohm loudspeaker through a connecting cable having one Ohm resistance will produce 93 watts of electrical power at the loudspeaker to be converted to audible energy.

    In actual practice the result will be worse for the solid state amplifier because it will actually be presented with a load of 5 Ohms and not 4, thereby reducing power output even further.

    Of course to be absolutely fair, the same operating conditions would equally apply to a 4 Ohm tube amplifier or a 16 Ohm solid state amplifier. However 16 Ohms is not a popular preferred output impedance for solid state amplifiers and as explained above, the lower the LOAD impedance of a tube amplifier the more the transformer characteristics will adversely influence operation and performance.

    The higher the load impedance the lower will be the losses in connecting leads and the better the output transformer will perform - a win/win solution!!

    The simplest way to obtain a high-quality conductor for speakers is to use copper conductor as used in house wiring systems for power circuits. In Australia this is rated at 32 amps continuous so is good for about 7 kW RMS at 8 ohms.

    Copper conductor is made from 100% IACS "high-conductivity" copper, which is a better conductor than commercially pure silver.

    Note 1: Copper water pipe is a poor conductor because of additives used to improve solderability, however that characteristic is somewhat offset by its increased cross-sectional area compared to small diameter copper wire.

    Note 2: Do not use aluminium wire because, on a size for size basis, it has substantially higher resistance than copper. It is also difficult to prevent high-resistance joints when terminating aluminium .

    Note 3: For the record, pure silver has the same conductivity as high-conductivity copper. Brass is about 27% conductivity of copper and steel about 7%.

    Note 4: In the case of Alternating Current, most of the current is carried in the outer skin of a circular, square or rectangular conductor, regardless of the material. This is why tubular busbars are used for high current applications in electric power switchyards. Unfortunately in the case of copper the manufacturing process creates a low-conductivity oxide film, that permeates into the surface about 0.1 mm deep. Thus in any given copper conductor, the outer 0.2 mm of the diameter is a high resistance oxidised material that does not carry current too well (and will get hot). Consequently it is better to use a larger diameter single strand conductor rather than a multi-stranded small diameter conductor. This comment applies to any electrical conductor, including transformers.

    Note 5: For the budget conscious, another option is to use 200-400A automotive copper cables as used for battery/starter jumper applications as Loudspeaker cables.

    5.6    Output Impedance

    "Output impedance" is the source impedance as seen by the loudspeaker and relates in a practical sense to the cone movement damping characteristic or "damping factor" of loudspeaker mechanical resonance.

    Loudpeaker damping is necessary because back EMF created by mechanical cone overshoot generates power that is reflected back into the amplifier by the output transformer which, if not diverted or consumed, results in audio acoustic power being produced by the loudspeaker acting on its own.

    If the Output Impedance is low compared to the loudspeaker impedance, any back EMF will be absorbed, or consumed, by the output stage.

    Mathematically, "Output Impedance" does not reduce by using parallel tubes because as the number of push-pull pairs increases, so the transformer ratio reduces in direct linear proportion.


    Calculation of Output Impedance gave the designer some idea as to how much of the back EMF would be consumed, thereby (by absorbing electrical energy) reducing the capability of the cone to overshoot - ie electrical "Damping".

    In the early days of radio and audio engineering when much of the electronic theory was developed, the typical tube amp power supply comprised a high-impedance high-voltage transformer feeding a high-impedance rectifier tube, feeding a high-impedance filter - typically the field coil of an electro-dynamic loudspeaker, having a DC resistance of about 500 to 1,000 Ohms.

    Filter capacitors were small, typically in the order of 8 uF - even in the recommended two stage filter, which has an even higher internal impedance. This small value was due to the capacitor manufacturing technologies of that time, the use of relatively high B+ supply voltages, AND the cathode-current limitations for available vacuum tube rectifiers - see Tube Manuals or Data Sheets for details.

    So the design convention was to ignore the characteristics of the power supply when calculating Output Impedance, resulting in the mathematical statement that "Output Impedance" does not reduce by using parallel tubes because as the number of push-pull pairs increases, the transformer ratio reduces in direct linear proportion.

    Now since - from an alternating current perspective - the power supply is in parallel with the output stage AC bypass capacitor return circuit, it follows that the characteristics of the power supply will also affect the amplifier's capability to absorb or consume back EMF to the load.

    If we look at the impedance of an 8 uF filter cap at the output transformer centre-tap, its capacitive reactance (impedance) at 30 Hz is 663 Ohms. If this component is supplied via a filter choke having an inductive reactance of 10H the impedance of the choke is 1,880 Ohms.

    It can be readily seen that these components, together with any other components in the power supply - such as another choke and more capacitors - form a parallel (shunt) network with the output stage bypass capacitor.

    So although the plate to plate output impedance has not changed in a parallel push-pull arrangement, the power supply impedance has - and the power supply forms the return AC circuit for the output stage.

    Consequently it can be said that when an extra pair of parallel push-pull tubes is added, and individual components are increased in value accordingly (ie doubled in value) the impedance of the power supply will be halved. This is necessary also to maintain power supply regulation - ie limiting change in output voltage between zero and maximum signal conditions.

    However if we increase the values of the filter capacitors to modern day available types - supplied by a full-wave low-impedance solid state rectifier system -  we can easily instal a 500 uF or 1,000 uF component in place of the old 8 uF.

    As a comparison, if we use a 500 uF capacitor in the output stage AC bypass circuit, the capacitive reactance reduces to only 10.6 Ohms - instead of 663 Ohms with an 8 uF cap

    Thus it can be said that increasing the value of the bypass capacitor the damping of the load will be similarly increased linearly.

    A rule of thumb for tube amplifiers is to use 1,000 uF per each 100 mA of B+ supply DC current - ie 10,000 UF per Ampere. Note however this is not possible with tube rectification - ie can onl;y be used with solid state rectifiers.

    Note that in solid state amps the final filter cap is likely to be in the order of 10,000 uF or more, so this design aspect is not an issue for solid state amps.


    In assessing the damping characteristics of the load we must also consider the rate of power absorption of any back EMF from the loudspeaker.

    Quite clearly, the type of capacitor used will determine its capability to charge or absorb current for a source - in this case the loudspeaker back EMF transformed back through the output transformer. Hence a high quality capacitor with fast charging (power absorption) characteristics will deliver superior results.

    If the capacitor cannot absorb or consume the back EMF within the allotted time - as determined by the rate of cone movement in the loudspeaker and energy transformed, the damping will not be complete or effective at the frequency and cone overshoot will still be evident - although diminished in amplitude and/or time.

    It is also obvious that in the case of any amplifier - using one or more pairs of output tubes - the larger the value and better the quality of the final output stage AC bypass capacitor the better the loudspeaker damping performance.

    WARNING: Excessively large capacitor values may result in electrocution if accidentally touched - even when the amplifier is switched off.

    Stored energy not consumed during the amplifier switch-off cycle can remain in the capacitor - at full voltage - for many days.

    Note: In the case of SOLID STATE amplifiers it is usual to see specified Output Impedance expressed as a very low value - eg 0.1 Ohms, with a correspondingly very high Damping Factor - ie the ratio of loudspeaker impedance to amplifier source impedance - of typically 40 or more. However if we insert a cable resistance of say 2 Ohms into the circuit between the amplifier terminals and the loudspeaker, it is obvious the specified Output Impedance and Damping Factor no longer apply. In fact, if the loudspeaker is 4 Ohms and the cable resistance is 2 Ohms (1 Ohm in each conductor leg), the Damping Factor is only 2 and the bass response may become audibly "boomy". The situation will progressively worsen as power output increases - because the conductor losses increase simultaneously whilst loudspeaker cone excursion increases, causing back EMF to increase.

    Therefore if long leads - ie more than twenty feet (6 metres) are required between the amplifier and its load, it could be a wise move to instal a pair of matching transformers in the line. To maintain fidelity and efficiency, such transformers should have a reasonably low primary to secondary turns ratio. Since they do not carry DC current they can be relatively small physically. For more information refer to "line matching transformer" catalogues available from most electronic component suppliers.


    This is also a very crucial step in the design process.

    The choice of tubes is very large, however in practice, there are only a few types that have demonstrated over time that they are worthy to be included in a list of preferred tubes for hi-fi amplification.

    In my experience most tubes of any given type number have similar characteristics - irrespective of manufacturer - however it is worth mentioning that some current production tubes have different specifications, characteristics and ratings to the original type number specs. In other words, some current manufacturers have abandoned the convention of building a tube that strictly conforms to internationally recognised, "once-only" specifications for any specific type number, permanently determined at the time of original registration.

    The standard protocol has always been that an incremental change or variant requires either a suffix or a new type number.

    Proceed with caution.

    6.1 Output Tubes

    The style of output stage will have already been made at Step 3 above, and that will determine the first main category of tube - ie triode, tetrode, pentode or beam power tube.

    Top cap or plain.

    The next step is to select a tube that will deliver the required power output.

    Tube handbooks usually provide typical operating characteristics for a range of conditions so the choice must be made to determine actual operating conditions.

    Classes of operation, fixed or cathode bias and type of interstage coupling from the driver stage need to be determined.

    This set of decisions will determine:

  • Grid #1 bias voltage
  • Grid #1 resistor or transformer impedance
  • Cathode bias resistor (if fitted) and bypass capacitor spec's
  • Output transformer load impedance
  • Simple or complex transformer

  • .
    6.2 Tube Mounting

    Tubes may be mounted:

  • vertically with base down     (the most common configuration)
  • vertically with base up         (may require a restraining device to prevent the tube from falling out of its socket)
  • horizontally                        (best for multiple tube configurations because the heat is able to dissipate more easily)

  • .
    NOTE: In the case of horizontal mounting, to prevent grid sag when hot and consequent risk of changing grid characteristics - or even creating an inter-electrode short-circuit, power tubes must be mounted with the grid wires aligned in the vertical plane. This mounting configuration also enhances heat dissipation into the surrounding air, resulting in cooler operation.


    In all cases at least 6 mm or 1/4" inch spacing must be provided between tubes to ensure their bulb temperature does not exceed rated limits.

    Adequate ventilation is essential. Do not mount tubes near components likely to ignite or to be affected by heat - eg electrolytic capacitors, transformers, cables, plastic components, wooden cabinets etc. - allow at least 2 inches (50 mm) free air clearance. Electrolytic capacitors can dry out then explode.

    In the case of rectifiers and power tubes the most common tube internal design configuration is where the Plate assembly is approximately rectangular in section - often with a join in the centre of the wide faces.

    The vertical wire posts that support the Plate assembly are nearly always on the hottest faces of the tube - often accompanied by extra cooling fins that extend out from the Plate.

    In all cases it is advisable to instal the tube such that the wide face of the Plate - which is the hottest part of the tube - is facing away from adjoining components or other tubes.

    The Cathode and Grid wires can sometimes be seen from the outside and these indicate how the tube is constructed, however in the case of Beam Power Tubes the internal wires are usually hidden inside the Plate and Beam electrode assembly.

    If a cooling fan (easy nowadays with the availability of low-cost computer fans) is installed, ensure the fan blows towards the narrow face of the Plate assembly so that heat emitted from both sides of the Plate is cooled. Do not instal the fan too close to a rectifier or power tube because the RADIATED heat will extend a significant distance - allow at least 50 mm (2") clearance between a fan and a power tube.

    6.3 Driver Tubes

    Here we have a wide choice, but essentially it is one of triodes or pentodes.

    Both styles come in low, medium or high gain varieties.

    Triodes are available in single or twin styles (in the same bottle)

    Recommended tubes for low-noise, low microphony, ultimate hi-fi are the 12AY7/6072 triode and EF86/6BK8/Z729/M8195 pentode, however very good results will be obtained from more readily available tubes.

    In general the 6AV6/12AX7/7025 provides a "bright" sound, or tone, the 12AT7, or 12AU7 or 12AV7 a "neutral" or "natural" sound, and the 6CG7/6SN7 a "gutsy" or "bassy" sound. All of course, will provide a "flat" frequency response when an amplifier is on test.

    The choice of an "industrial" grade tube does not always provide superior results to a domestic equivalent - it is wise to consult the tube specs to determine why it has an industrial number often the industrial characteristic has no impact on "sound" at all - eg long life.

    Some of the RCA Hi-Fi circuits use pentodes, such as the 6AU6, 6CB6, 6U8, and 5879. There is also the Euro EF86.

    Irrespective of the class of tube - ie receiving, industrial, RF, military or hi-fi - care should be taken in selecting actual tubes - for any given type number there are very wide variations in tube characteristics such as hum, noise, microphony and reliability. The most reliable method is to just plug a tube in and try it.

    Sometimes it will be necessary to use shielding cans around driver tubes to eliminate hum and stray RF pickup - however cans tend to increase the risk of microphony (mechanical feedback).

    Do not mount driver stage tubes near magnetic components such as transformers or loudspeakers - to prevent magnetic interaction or hum induction.


    This aspect of design is most important for any amplifier, but particularly for a hi-fidelity audio amplifier.

    The following standard classes of operation are available - as defined by RCA Tube Handbook RC-14 (1940)

    Class A: "A Class A amplifier is an amplifier in which the grid bias and alternating grid voltages are such that plate current in a specific tube flows at all times" - as defined by RCA Tube Handbook RC-14 (1940)

    "A Class A amplifier is an amplifier which operates in such a manner that the Plate output waveform is essentially the same as that of the exciting Grid (#1) voltage.

    This is accomplished by operating with a negative Grid bias such that some Plate Current flows at all times, and by applying such an alternating voltage to the Grid that the dynamic operating characteristics are essentially linear.

    The Grid must usually not go positive on excitation peaks and the Plate Current must not fall low enough at its minimum to cause distortion due to curvature of the characteristic.

    The amount of second harmonic present in the output wave which was not present in the input wave is generally taken as a measure of distortion, the usual limit being 5%." - as defined by Sylvania Technical Manual (1934).

    Class AB: "A Class AB amplifier is an amplifier in which the grid bias and alternating grid voltages are such that plate current in a specific tube flows for appreciably more than half but less than the entire electrical cycle" - as defined by RCA Tube Handbook RC-14 (1940)

    "A Class AB amplifier is one which is over-biased, operating as a Class A system for small signals, and as a Class B amplifier when the signals are large.

    The result is that Plate Current flows during appreciably more than half a cycle, yet less than 360 degrees of the cycle."  - as defined by Sylvania Technical Manual (1934).

    "Class A is a method of operating a valve so that the Grid remains always negative to the Cathode. The applied signal voltage is small enough to allow the oprating point to remain on the straight portion of the Ia/Va curve, and no Grid-current flows." - as defined by the Radio Society of Great Britain.(1948)

    Class B: "A Class B amplifier is an amplifier in which the grid bias is approximately equal to the cut-off value so that the plate current is approximately zero when no exciting grid voltage is applied, and so that plate current in a specific tube flows for approximately one half of each cycle when an alternating grid voltage is applied" - as defined by RCA Tube Handbook RC-14 (1940)

    "A Class B amplifier is an amplifier which operates in such a manner that the power output is proportional to the square of the Grid (#1) excitation voltage.

    This is accomplished by operating with a negative Grid (#1) bias such that the Plate Current is reduced to a relatively low value with no Grid excitation voltage, and by applying excitation such that pulses of Plate Current are produced on the positive half-cycle of the Grid voltage variations.

    The Grid may usually go positive on excitation peaks, the harmonics being removed from the output by suitable means." - as defined by Sylvania Technical Manual (1934).

    "Class B is a method of operating a valve, by progressively increasing both the steady bias voltage on the Grid, and the applied signal voltage. In Class B, the steady bias is such that, without any applied signal voltage, the Anode current would be reduced to zero. With the signal voltage applied, Grid-current flows and the valve is working beyond the straight portion of the Ia/Va curve. Because of the steady bias, Anode current flows only during the positiove half-cycle of the applied signal voltage." - as defined by the Radio Society of Great Britain.(1948)

    Class C: "A Class C amplifier is an amplifier in which the grid bias is appreciably greater than the cut-off value so that the plate current in each tube is zero when no alternating grid voltage is applied, and so that the plate current flows in a specific tube for appreciably less than one-half of each cycle when an alternating grid voltage is applied"

    Note 1: "To denote that grid current does not flow during any part of the cycle, the suffix 1 may be added to the letter or letters of the class identification"

    Note 2: "To denote that grid current does flow during any part of the cycle, the suffix 2 may be added to the letter or letters of the class identification"

    It has been traditional in hi-fi circles to regard Class A as the preferred mode of operation, however a moment's reflection upon this subject will demonstrate that this has been a false assumption.

    If we regard the electron flow in the output tubes as being similar to that of a fluid in a pipe then the term "valve" is relevant to controlling the electron flow.

    Class A Amplification

    A Class A amplifier works like a butterfly valve, as commonly used in a carburettor. This type of valve usually has an axis central to the pipe and when rotated in a flip-flop action, opens or closes, thereby regulating the flow in the pipe. In terms of electron flow in a vacuum tube, this valve may be described as "normally closed", such that when the grid bias is set for zero signal conditions, the plate current will be at or near its maximum permissible dissipation within the rated plate dissipation value for the specific tube type.

    When the AC signal alternates positively, the grid bias is increased more positive by the signal and plate current will increase. However the signal power out will also increase so the plate dissipation will decrease.

    When the AC signal alternates negatively, the grid bias is decreased more negatively until the plate current decreases to at, or near, its cut-off value.

    Hence the limits of operation for Class A are determined by maximum plate dissipation, positive Grid-current and by the value of negative AC grid voltage that will cause the plate current to effectively cut-off.

    It is likely the grid bias will be set at about halfway - similar to a butterfly valve - so the axis of the bias will be in between the maximum and minimum limits for grid bias/plate current.

    Cathode bias is generally suitable for Class A amplifiers because the plate current does not vary much between minimum and maximum signal conditions - so is often seen in hi-fidelity Class A amplifiers - eg the "Williamson" triode amplifier.

    It will be seen also from tube plate characteristic curves that the values of grid bias within the above range are such that most tube types cannot operate linearly in Class A - ie the positive alternation of signal will not produce the same plate current change as an equal negative alternation of signal.

    Furthermore, any transient peak signal having an amplitude several times the RMS value will not be reproduced in the output stage load unless it is in the positive alternation of the AC signal - because any signal greater than plate current cutoff cannot be reproduced - ie the negative signal alternation will be truncated.

    It is also relevant that when the AC signal drives the grid bias positive, there will be a point at which Grid-current begins to flow, causing a collapse of the drive signal and extreme distortion.

    This will occur at a grid voltage of about one volt less than the Cathode voltage - typically 0 volts or "ground" in a fixed bias amplifier, or the positive DC Cathode bias voltage in a Cathode-biased amplifier, where the Control Grid is typically at zero (ground) DC volts.

    The following example is the popular 6L6GC power tube. Analysis of the bias voltage requirements demonstrates the above characteristics.

    In this example, the linear portion of grid bias is between about -8 and -32 VDC, providing a useable signal range of just 24VAC peak - ie 17V rms.

    The problem for Class A hi-fi reproduction, is that a high-amplitude transient signal can occur in either the positive or negative direction (polarity) - depending upon chance, or probability. It is obvious from the above that the capability of the amplifier will not be the same in the positive and negative alternations, (in the case of a positive alternation the tube will draw grid current and high plate current, causing voltage drop in the B+ supply) (in the case of a negative alternation the tube will cut off and no plate current will flow - therefore no power output) so the result obtained will depend upon the polarity of the signal, the bias voltage of the mid-point axis of the AC signal and the characteristics of the specific tube installed.

    An asymmetrical peak signal will be either cut-off and distorted due to grid-current changing the bias voltage in the positive alternation, or will be truncated and distorted due to the negative excursion driving the tube to Plate-current cut-off.

    This is why a single-ended amplifier is fundamentally a waste of time and money for realistic dynamic hi-fi.

    This is also why a Class A push-pull amplifier is undesirable - unless it has sufficient headroom power to cater for all power output conditions required for high fidelity reproduction at the required listening level.

    See paper by Williamson and Walker on the above topics.

    Class B Amplification

    On the other hand, Class B operation works like a "normally closed" valve - such as a gate valve, cock, tap or fawcett. The more the valve is opened, the more fluid flows in the pipe.

    In terms of electron flow in a vacuum tube, this valve may be described as "normally closed", such that when the grid bias is set for zero signal conditions, the plate current will be at or near cut-off  for the specific tube type.

    Consequently, Class B provides the potential to reproduce any signal - limited only by the amplitude of the signal driving voltage, the ability of the driving stage to provide AC SIGNAL POWER to the output tube grids, and the capability of the power supply to maintain voltage and current into the output tubes - up to saturation of the output transformer.

    Note: As AC power output increases the plate dissipation decreases, so the power tubes will normally run cooler. However tubes do have limitations on maximum cathode current capabilities so that needs to be considered.

    In Class B, the Control-Grid (Grid #1) bias is set such that only the positive alternation of signal input from the phase-splitter/inverter driver (or centre-tapped transformer driver) is reproduced in each output tube.

    The negative alternation of the signal input from the phase-splitter/inverter driver (or centre-tapped transformer driver) is not amplified by the output tubes because each tube is biased at or near Plate current cut-off and cannot therefore be driven in the negative direction.

    In other words, the output from each Class B power tube is half-wave only. (Single-ended amplifier tube buffs note that a Class B amplifier should sound like a single-ended amplifier - but eight to sixteen times more powerful)

    However - and most importantly - the actual DC Grid voltage will be approximately twice the value as for Class A (twice as negative), which means that the AC signal input can be twice the voltage as for Class A.

    This translates into headroom and capability to handle asymmetrical peaks in the signal - a vital characteristic for reproduction of  high-amplitude digital recordings.

    A push-pull driver stage usually features "balanced" alternation, compared to the "unbalanced" alternation of front-end Class A drivers, hence push-pull Class B operation is the only viable option for true high-fidelity reproduction.

    Unfortunately Class B does have some extra challenging requirements, such as higher distortion, providing driving power to the output tube grids, a low impedance driver stage, low DC resistance output transformer, fixed Control Grid bias, accurately matched pairs of output tubes, a stable low-impedance regulated power supply with good regulation, and grid bias set exactly at the plate current cutoff value for both sides of the output stage - all at the same time (a challenge for parallel-push pull output stages) - but what the heck if it is for yourself!!

    On the other hand, Class B is more forgiving than Class A, however so long as the power supply can maintain reasonably constant plate voltage the output power and transient performance (thus "realism") is more likely to be substantially superior to Class A.

    One great benefit from Class B operation is that expensive output tube tube and component life is maximised, because most of the time they will not be working very hard at home listening levels - ie low Cathode current and low heat dissipation.

    One major challenge with Class B though is to get the Grid bias right.

    Because the output signal from the phase-splitter is balanced, the central axis of the signal (assuming it is a symmetrical sine-wave) will correspond to the AC neutral point at the centre of the grid-leak resistors, or driver transformer secondary winding centre-tap, to the output tubes.

    In theory, the central axis of a symmetrical signal voltage must align electrically with the mid-point of the balanced push-pull input circuit, or else the signal will not be equally amplified by both power tubes.

    For true high-fidelity reproduction, this requires the Control-Grid bias for Class B to be set such that Plate-Current cutoff occurs exactly at the same time as the input signal is at its mid-point axis - ie equivalent to AC 0 volts - for both sides of the output stage - not an easy task to determine and set up!!

    If the bias is set too negative, the first portion of the positive alternation of signal voltage will not cause Plate Current to increase because the tube will still be biased at Plate Current cut-off, so some of the signal will be lost. Because this part of the signal - closest to the mid-point axis - will include a significant portion of the power (area under the curve), the amplified sound will be incomplete - ie some of the signal will be missing.

    At first glance this may not seem a significant problem, but in parallel push-pull output stages it is very difficult to have all tubes exactly matched such that they all experience Plate Current cut-off at exactly the same time, so some compromise is necessary. This also applies where a single pair of output tubes is not matched.

    As has been previously explained in these pages, it is very difficult when replacing a failed tube to match a new tube with those remaining - because single tubes are not available as "matched pairs" - so an output stage requiring exactly matched tubes will be a very expensive proposition in the long run. Better to incorporate a few adjustments or design for less than optimum tube specs.

    A simple solution is to go for Class B1.

    This means that grid current is not intended to flow, thereby eliminating the need for a powerful driver stage.

    It also eliminates the switching characteristic of the power tubes at zero plate current bias.

    An easy compromise is to set the zero signal plate current to about 20 mA per tube. This ensures the tube is already conducting wehen a signal is applied.

    Important Note: Class B power tube output configurations require a low value of Control Grid (Grid #1) resistor - measured between Grid #1 and the Cathode. Too high a value will cause the Grid Bias to go positive - ie less negative - as grid current flows during the input signal cycle. Refer Tube Manuals for details of "Maximum Grid #1 Circuit Resistance." This value includes any resistances in the Grid Bias circuit, including potentiometers and wire-wound resistors, and is often exceeded in guitar amplifiers in order to gain extra drive voltage into the output stage, but generally results in excessive Cathode Current at full output - particularly in sustained overload mode - and can create instability, with self-oscillation apparent. One trick sometimes used to minimise this effect is to instal a fixed resistor in the Cathode circuit of each power tube - eg 30 to 60 Ohms - which causes positive Cathode Bias to be developed at high output currents, which in turn creates a lesser voltage differential between Grid #1 and Cathode. A bypass capacitor may also be required to each resistor.  This modification will affect the "sound" so for hi-fi proceed with caution.

    However all is not lost!!

    Class AB2 Amplification

    Class AB2 is a close second to Class B, but avoids the necessity for complex drive circuitry.

    Class AB2 also avoids the necessity to set the grid bias exactly at the plate current cutoff value in order to wholly reproduce the amplitude of all signals. In other words, by setting the bias slightly above the cutoff value, full amplitude reproduction of all signals is assured.

    Fixed Control Grid bias is essential. If the grid bias is set such that a small current flows at zero signal - eg 10-20 mA per tube - then one can be sure the tube will conduct all signals.

    Cathode-follower Driver Stage

    It is generally recommended that a cathode-follower driver stage be used for Class AB2. This has the advantage of being able to be supplied directly from a high B+ voltage, such that after voltage drop across the cathode-follower load resistor - eg 100kOhms + unbypassed conventional cathode bias resistor (with recommended grid-leak resistor - ie not direct coupled) to the cathode-follower tube, the maximum permissible plate voltage can be applied across the driver tube - eg 300 VDC actual applied Plate to Cathode for a typical triode (from a 450 VDC B+ rail). This method ensures maximum output voltage from the driver.

    The cathode-follower stage is out of phase with the power stage so no voltage drop in the B+ supply is required. A conventional filter choke is advisable though into a large filter cap, to reduce hum if present in the B+ and to provide adequate reserve energy for the phase-splitter, which will draw several milliamps.

    Cathodyne Phase-splitter

    In my experience, to maximise the driver stage performance, it is essential to drive the cathode-follower with a voltage amplifier installed AFTER the phase-splitter. The high-impedance input to the cathode-follower maximises voltage gain and voltage output of the driver. This design approach leaves the phase-splitter to do only that. GEC have a design showing this.
    See Circuits/GEC400w.gif

    If we examine the Cathodyne phase-splitter in the GEC circuit carefully (same as the Williamson), we can see that the plate and cathode resistors are of equal value. Since Kirschoff's Law tells us that whatever happens in one part of a circuit also happens in every other part of the same circuit, it follows that the current through each load resistor will be exactly equal. It also tells us that the voltage across each identical load resistor will therefore also be exactly equal. Therefore the output voltage to the following stage will be exactly equal. This is very important for push-pull operation because it is a reality that this is the only phase-splitter design that provides exactly equal and opposite output voltages from each half at any instant in time.

    However the voltage output from the Cathodyne is limited because the B+ supply is distributed over two load resistors and the tube itself, resulting in a relatively low applied plate to cathode voltage in the tube itself. This is why it is desirable to instal an extra driver stage between the cathodyne and the output tubes - preferably with the cathode-follower final stage as well - as shown in the GEC 400Watt design.

    Some critics claim this phase-splitter is not ideal because the output has different impedances, however being a triode the higher the load the better. Use of a following driver stage enables the grid resistor of that stage - being the load on the phase-splitter - to have a high value - eg 100k to 1 Megohm - thus having negligible effect on the phase-splitter output.

    Direct Coupling of V1 to V2

    In the GEC 400W circuit, the first 6SN7/6CG7 is direct-coupled a-la Williamson.

    As designed the two halves of the 6SN7 are interactive (follow the parallel circuits) - in my view a risky approach. Distortion or reduced power may result from tired or out of spec tubes. When the first tube draws plate current on the positive signal alternation, the plate voltage drops so the grid bias on the second tube (phase-splitter) decreases, thereby increasing current flow in the second tube. This causes the voltage across the tube to fall thereby reducing output. However on the negative signal alternation the plate current in the first tube reduces, causing the plate voltage to increase, causing the bias on the second tube to increase, thereby reducing current through that tube - thereby increasing the cathode voltage, thereby reducing the grid bias causing the plate current to increase. (Close to a good oscillator).

    I have undertaken extensive trials with different values of plate and cathode resistors in each tube and the end result is always the same - inadequate bias to V2 - which must always be sufficient to handle the peak signal voltage amplitude.

    There is always a parallel circuit between the two tubes because both are supplied from the same B+ point.

    To avoid these problems, it is recommended that this stage be configured as a standard R/C coupled stage with large (1 uF) coupling capacitor - adjust the value to taste. This configuration allows the grid bias on the second 6SN7 to be maximised to preset fixed voltage for maximum signal amplitude headroom - a very difficult challenge with the circuit as shown, which relies upon each half of the triode working as intended. When individual tube characteristics are added into the mix this is a very interactive configuration.

    Another advantage of a cathode-follower driver is that it better matches the grid-leak/load presented by the LOW RESISTANCE grid-leak resistors to the output tube grids - eg 100k Ohms maximum per tube - proportionately less for parallel-push-pull.

    For adequate low frequency response, very large "fast" coupling capacitors are need - in the order of 10 to 20 uF in Metallised Polypropylene or Polyester film. Oil-filled paper types are not recommended for this application.


    It has been standard practice in hi-fi/audiophile circles to regard a push-pull amplifier as cancelling out distortion when compared with a single-ended design.

    However all is not as it seems.

    One important difference is HOW the signal is handled at high amplitudes of signal input voltage, when grid current is drawn and the tube overloads into distortion.

    ALL amplifiers operate in Class A some of the time - that is the power tubes do not draw grid current some of the time.

    A pure Class A amplifier - ie Class A1, does not normally draw grid current at all.

    However a Class A2 or AB1 or AB2 amplifier will draw grid current some of the time. This typically occurs when the signal input voltage drives each power tube control grid bias into the range above about -2V to 0V. The difference in class denotes ONLY the degree of grid current that flows. This also generally coincides with differences in operating conditions.

    Note the distinction between classes is by the drawing of grid current - not configuration or performance.

    Each tube in Class A is nominally loaded with half the single tube normal load, hence the output transformer has a load impedance equal to that for one single-ended tube. Power output is about twice that for a single tube.

    Each tube in Class AB1 or AB2  is nominally loaded with a single tube normal load or more, hence the output transformer has a load impedance at least twice that for one single tube. Power output is typically 50% more than for Class A under the same operating conditions.

    On the other hand, Class B amplifiers are quite different in concept and simply comprise a pair of single-ended amplifiers that operate in consecutive sequence one after the other in real-time in a push-pull pair. This is because each tube can only conduct when its grid is driven positive above its already "cut-off" mode at zero signal. Therefore, each half-wave power output alternation occurs in the respective halves of the push-pull pair only when each power tube control grid is driven positive above its quiescent (zero signal) operating condition.

    The effect is similar to that seen in a full-wave centre-tapped rectifier - each half conducts in sequence only when its AC input is positive.

    Typically the power output of a Class B amplifier is about four to six times the power output of the same amplifier operating in Class A mode. THD will be higher.

    BUT - if we disconnect the drive to one of the tubes in the Class B pair we get only a half-wave output - ie the amplifier will deliver power only every second half-cycle.

    This is not so in Class A, where by disconnecting one half of the input drive we still get a full-wave output - but also at half power.

    The reason for this difference in performance is explained above in terms of "balanced" and "unbalanced" input driving voltage systems.

    However, what is not explained in the textbooks is that when output POWER is calculated, it is calculated on a real-time basis for a FULL CYCLE.

    This is because the "Watt" is a unit of WORK performed over a specified time interval. (In my opinion it is misleading to quote output in "rms watts" for less than one cycle).

    Consequently if we go back a step and compare Class A and Class B configurations, we can see that since Class A produces its power output from two tubes operating together at the same time, to obtain a specific power output the amplifier will be utilising BOTH halves of the output stage simultaneously - each tube therefore producing the whole of a symmetrical sine-wave output.

    Both tubes will overload at the same time - which is when the control grid bias approaches zero volts and/or when each tube reaches its negative control grid bias cutoff value - and that will normally occur at the maximum rms rated output. Thus there is no headroom in Class A for transient peaks above rated rms output.

    In other words, the amplifier's PEAK power output will be the same as its normal RMS power output.

    In the case of Class B we can see that since Class B produces its power output from one tube at a time in consecutive sequence - ie one half-cycle after the other, then each tube will need to deliver FULL power output in turn in order to equate to a Class A amplifier.


    This is not so, because the "POWER" is calculated using a standard wave-form sine-wave signal over a specific time-period that covers more than one cycle - eg "continuously".  In Class B each positive and negative half-cycle alternation produces half of the power which, when added together represents total power.

    That full-wave condition is OK for test purposes when a continuous sine-wave signal is applied however, in the real world of music, a different situation applies.

    So for hi-fi analysis the standard conventions are misleading, because our interest should focus on half-wave asymmetrical peak transients.

    Why?? Because that is where the "sound" is.

    "SOUND" in reproductive terms, is "information".

    Information in this context is continuous in real-time and in an electronically forward direction.

    So if the input signal - eg the first half-cycle of a percussive instrument or guitar - is asymmetrical, then the amplifier must reproduce this faithfully.

    Consequently we must either INCREASE the undistorted maximum power capability, or use an amplifier that will reproduce the waveform as submitted.

    The better option then, is to increase the total power capability to a prospective power output that will cater for all required signal forms, including transient peaks.

    As had been explained above, the Class B1 - or AB2 - configuration offers the best COMPROMISE option for transient power response.

    Class B1, or AB2 BIASING combined with UNBALANCED DRIVE though, still offers better transient response (short-time peak power) than in Class A or AB1 because of its symmetry and linearity in both positive and negative alternations - and its wider dynamic range.

    For best results, UNBALANCED DRIVE as explained above is essential, regardless of chosen operating Class of the power output stage.

    It is hoped the above explanation demonstrates that provided cut-off is not attained - either positive or negative, the only practical difference between the sonic qualities of the classes is that Class A with Cathode Bias always has a balanced input whereas Classes A, AB1, AB2 and B with Fixed Bias usually have an unbalanced input.

    However Class A with Cathode Bias suffers from bias interaction between each pair of push-pull power tubes (see textbook explanations of the Cathode-coupled, Schmitt, or Long-tail Pair Phase-splitters). This requires power tubes that are exactly matched and exactly linear throughout the entire power range from zero to maximum.

    But if the amplifier is to be run into overload then Class AB2 or Class B offer superior dynamic performance.
    Some notes on biasing systems:

    1.    Comparison of Fixed Bias and Cathode Bias systems

    Ravenswood, Herbert. "The Fixed-Bias Story," Radio-Electronics, February 1958, pp. 47-49. (4.2M)

    2.    Back Bias

    Randall Aiken - "What is Back-Biasing"?

    And now for an expert opinion:

    In his fascinating book "High-Power Audio Amplifier Construction Manual - 50 to 500 Watts for the Audio Perfectionist" (McGraw Hill 1999), G. Randy Slone makes the following observations to solid state amplifier constructors:

    a) On tubes v solid state amplification:

    b) On classes of operation:



    This aspect of amplifier design offers the most creative among us the latitude to do whatever one fancies.

    My money goes on installing the phase-splitter as early as possible in the circuit to enable the use of (balanced) push-pull drivers thereafter - ie to convert an "unbalanced" driver into a "balanced" driver. This approach facilitates more creativity in choosing the methods for amplification of the AC signal driving voltage to the output tubes.

    For a detailed explanation of what does what, how and why see:

    http://www.clarisonus.com/Archives/Amp Design/Williamson 1952 The Williamson Amplifier.pdf

    One simple, but effective method, is to use a driver transformer having a grounded centre-tapped secondary at the input then push-pull throughout the entire amplifier, to provide a "balanced" amplifier throughout. Installing a volume or gain control is a little tricky but not impossible.

    The choice of phase-splitter designs is also wide, and it is a case of determining which configuration is the most suitable for the attributes sought.

    It is important that the driver stage be capable of delivering adequate signal voltage to the output tubes. This is usually not difficult, however when parallel-push-pull output is used, the Grid # 1 circuit resistance must be proportionately reduced, resulting in a low value of load to the preceding driver stage.

    One way to assist this is to use "bootstrapping" techniques to increase driver stage plate voltage, however I just use a high voltage power supply (usually about 450 VDC) such that the driver stage plate resistor can be large - ie 470 k ohms - to deliver maximum rated plate to cathode voltage to the driver. This large value of plate resistor also helps to offset the reduced value of the following stage Grid #1 resistor. Of course the rated plate voltage of the driver tube must not be exceeded.

    A similar advantage is gained when using a cathode-follower driver stage


    Review of past design practice shows us that many designers had no idea of the principles essential to optimising chassis layout.

    In a wide range amplifier, the wiring and componentry act as antennae, to pick up stray and induced signals, such as ultra-sonics, RF, hum and noise, from adjacent circuitry.

    The basic rules for component layout and wiring are:

  • do not mount electrolytic capacitors close to tubes - the caps will dry out and fail
  • do not mount electrolytic capacitors close to transformers - the caps may induce hum and interference into the B+ bus
  • do not mount hot tubes near transformer windings - the windings will burn or dry out
  • do not mount driver stage tubes near transformers or mains wiring - to avoid induced interference or hum
  • do not mount exposed driver tubes near output tubes or rectifier tubes - they may pick up induced signals such as RF, hum and even audio, and trigger parasitic oscillation in the circuit
  • do not mount tubes near the edge of the chassis - they can burn the enclosure or cabinet when the chassis is installed and operating
  • do not mount input wiring near output wiring either within a stage or across stages - to avoid stray signal induction or oscillation
  • do not mount input terminals near speaker terminals or mains supply leads or plugs - to prevent instability and/or interference from induced signals
  • do not mount tubes upside down without restraining devices - they can fall out partially, with loss of one or more contact pins, or completely fall out and break
  • do not mount tubes horizontally unless their grid wires are installed in the vertical plane - to prevent grid wire sag when hot and thus uneven electron flow, or internal shorts
  • always use shielded wiring for signal pathways in the early stages - to prevent stray hum and noise pickup and induced feedback signals
  • never ground potentiometer wires to the chassis at the potentiometer - always ground directly to the relevant cathode (through a shielded co-ax lead)
  • always shield input tubes with discrete shields or a grounded metal enclosure (there are currently more than 100 million mobile telephones in the world, thousands of radio and television transmitters and significant numbers of high-powered industrial and military transmitters, and they do generate a significant amount of RF energy for your hi-fi amp to pick-up)
  • always mount tubes as close to each other as space will permit - to minimise wiring length and minimising exposure to induced signals or interference (but always leave at least a 10 mm gap between bottles for natural air cooling - more for larger tubes having more than 25 watts plate dissipation)
  • always use short leads between connection points - to minimise wiring length and minimising exposure to induced signals or interference
  • never connect pins on tube sockets directly together with straight or solid or heavy wire unless there is a loop to enable each socket pin to locate properly about its tube pin - tube pins do not bend well to match socket misalignment, so some movement in the socket terminals is essential
  • always mount tube sockets such that the heater pins are aligned with the direction of wiring - to minimise heater lead length
  • always twist heater wires together - to neutralise AC radiation to nearby wiring and components (cancel-out hum signals)
  • always ground grid and cathode resistors (and bypass caps where applicable) to the cathode pin, if grounded, or to the grounded end of the cathode resistor. One very effective technique is to instal the resistors vertically, straight up from the socket pins, then simply join the ends to be grounded together (neatly). Connect them via a wire to chassis ground. This bundle of components does not need further support as they are a strong structure mounted directly from the tube socket. Some miniature sockets have a centre tube spiggot for shielding the pins from each other - this should be also grounded. The tube shield provides a useful structural support
  • where practicable, instal interstage capacitors with the outer layer connected to the previous stage plate - to minimise hum pickup Vintage caps often had a band printed on one end to indicate the outer layer
  • try to layout the chassis following the circuit diagram (schematic) - this method actually works!!
  • ensure the amplifier is well ventilated - to ensure adequate cooling and to facilitate long-term component life
  • always insulate and isolate high voltage and mains power circuitry - death is permanent!!
  • always use an isolating transformer between the mains power source and the rectifier and heater/filament system (to prevent electrocution)
  • always insulate and isolate high voltage capacitors - particularly large capacitance units - they can store electrical energy for several days. An amplifier switched off may still be hazardous or even lethal
  • always use an enclosed METAL chassis system - if necessary fit a removable lid/cover for component access. 20 gauge galvanised steel sheet (1 mm) thick is ideal but aluminium is more suitable for large chassis to reduce weight. Note that it is not possible to solder components to aluminium using tin/lead solder so all chassis joints must be tightly screwed together.
  • never use 50/50 tin/lead solder - that is for plumbers. Use only a high quality 60 tin/40 lead solder having a cored flux. When soldering components to prevent dry joints (imperfect metallurgical joint bond) always allow time for the joint to thoroughly heat before removing the soldering iron. Printed circuit board conducting strip tends to separate from the bakelite/fibreglass base board if overheated - best to avoid them.
  • always ground or earth the chassis (to prevent electrocution)

  • the earth side (outer shielded cover) of the input shielded lead must be grounded to the amplifier chassis to prevent hum pickup, however modern television receivers do not have an isolating power transformer and use a "hot" chassis system. This means that if you connect yourself - or someone else - between the TV chassis and ground there is a high likelihood of death. If connecting audio signal leads to a TV receiver or similar device ensure there is an isolating capacitor in series with each lead - ie both live and earthed input leads, to prevent direct connection between the amplifier and the hot (ungrounded) source. If you are not qualified to do thisthen take your system to a qualified technician - you only have one life!!


    One of the most challenging aspects of an amplifier design is to select the most suitable operating voltage for the power tubes.

    Most tube handbooks publish a range of optional operating conditions for power tubes and the designer is free to choose from a wide variety of tube types and voltages.

    Is it better to run a tube well below its rated maximum limits for long life and cool operation, or to run a tube at or near its maximum ratings to increase power output and efficiency??

    Do tubes sound better when pushed?

    These are questions only the user/designer can answer, because like "sound" itself, assessment of results is very subjective.

    However, because a tube is a voltage amplifier and AC POWER output from the amplifier is calculated as the square of the output voltage divided by the load impedance, it follows that for power tubes, a small increase in voltage will increase power output dramatically.

    The limiting factor to applied DC voltage will be the maximum tube ratings.

    However a practical solution is to base the B+ voltage on available working voltage ratings of the electrolytic capacitors used in the power supply filter circuit - because these are the most difficult voltage dependent components to acquire.

    Rated working voltages for electrolytic caps tend to be available in 50V increments, however there are some popular ratings that are more readily available - particularly in the larger capacitance sizes - such as 350 VDCW, 400 VDCW, 450 VDCW and 500 VDCW.

    Not much is available at affordable cost with ratings over 500 VDC.

    Hence, for B+ voltages over 450 VDC, it is usually the case that electrolytic capacitors need to be connected in series to withstand the applied voltage. Unfortunately this has the effect of halving the effective capacitance (assuming both caps are identical). Alternatively, consider uning oil-filled paper caps, which are readily available with voltage ratings into the kilovolt (kV) range, however they are relatively physically large, heavy and cost substantially more than electrolytic types.

    Consequently, if the designer chooses to use a B+ voltage greater than about 450 VDC then considerably more chassis space and cost is required.

    It may be cheaper to consider parallel-push-pull output.

    On the other hand, if a voltage doubler power supply is used, two series caps are required in any case, so no extra componentry is needed - at least in the rectifying section. Furthermore, the power transformer size will be smaller than a full-wave style, so the final space and cost may well be less.

    IMPORTANT: Directly heated tube rectifiers - ie those having a filament - produce a surge voltage upon switch-on. This surge will be usually 1.4 x the RMS input voltage, and will definitely be substantially higher than the full-load output voltage. Hence filter capacitors MUST be rated to withstand the DC surge voltage - or else they will fail (a somewhat spectacular and messy process). Solid state full-wave or bridge rectifiers do not produce this surge because the full-load continuous output voltage is essentially the same as the switch-on voltage.

    It is well to remember that tube performance is primarily controlled by the Screen Grid - so plate voltage is not as important as it might seem. Low Screen Grid voltage translates into less Cathode current, lower operating temperature, longer tube life, reduced distortion and more faithful reproduction.


    The power supply is one of the least understood components of an amplifier, yet may well be the most important.

    The primary functions of the POWER SUPPLY are to:

    a)    isolate the amplifier from the AC mains - an essential safety requirement.

    b)    convert (rectify) the power available from the AC mains supply to the particular forms of AC and DC power required by the tube

    c)    shield the amplifier from spurious mains voltages

    d)    provide rectified uni-directional DC current - under considerably varying levels of load current ranging from zero to full-current
    requirements - at more or less CONSTANT DC voltage to discreet sections of the amplifier.

    e)    ground the amplifier chassis and metallic components to a reliable earth.

    For further detailed explanation of the above, please refer to my paper "POWER SUPPLY"  (click on this link to connect)

    For an excellent technical explanation of how power supplies work and options available see http://www.tpub.com/neets/book7/27a.htm and follow the "next" links to scroll pages

    See also a technical paper by Williamson and Walker


    It is an axiom of engineering practice that a component or complete device must look attractive.

    This rule also applies to audio amplifiers.

    If it does not look good then it is unlikely to work well, so abandon it and start again.


    These articles, courtesy of Electronics Australia, June 1983 edition, may cause you to reconsider what you are actually hearing!!

    Hearing and Hi-Fi: Page 1

    Hearing and Hi-Fi: Page 2

    Hearing and Hi-Fi: Page 3

    Hearing and Hi-Fi: Page 4


    For those who want to be different and own and use tube amplifiers for their hi-fi  - notwithstanding their shortcomings and limitations - good luck to you and enjoy what you have.

    If you feel inspired to improve what you already have then hopefully these pages will have helped in your quest.

    There is no restriction or cost imposition upon the DIY home hobbyist constructor to using these concepts - the only restriction is on commercial exploitation where copyright is applicable - so if you do not like it do not do it.

    If you want your hi-fi to improve its performance at minimal cost to you then experiment. The concepts presented here do work and cost very little to implement.

    Thank you for reading and considering my tube hi-fi ideas as expressed in these pages.

    Please let me know if you can add to this body of tube hi-fi knowledge and I will add it to this commentary, which is intended to communicate the results of my personal research and experimentation over about 50 years of my life.

    To those who consider relating these concepts to RF applications - this is an audio focused site - experiment at your own risk. It may be safer to stay with the tried and true - waste a little power and live with it.

    However RCA, in their Transmitting Tube Manual TT4, say at Page 44 -" The only restrictions on tube operating values are those imposed by the published maximum ratings." The copyrighted design concepts presented above should enable designers to withdraw operating conditions back into specification whilst improving performance.





    "All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident."     Arthur Schopenhauer

    If you feel a need to comment or need further clarification you are invited to contact me directly to discuss your comments and feedback - I am happy to respond to rational requests.

    However I see no productive purpose in responding to internet forums etc. because there is nothing on this earth that is without fault or defect and anyone can denigrate anything if that is what they want to do - particularly those who hide behind the anonymity of an alias or pseudonym.

    So the ideas and concepts expressed here are offered on a take it or leave it basis - it is up to you.

    I feel no need to justify my ideas just because they are different or you might disagree.

    There is no charge - they are absolutely free - so no-one can claim exploitation.

    To those who say that a product is only as good as what you pay for it, then these concepts are of no value to you because they are free. You would be wiser to spend a hundred grand on a commercial system and feel better. While you are so doing and being a sceptic, ask the manufacturer to justify the circuit design and component choices to you - ie why the design is what it is and not some other alternative approach. That is "why is it better?"

    Ask yourself WHY do commercial manufacturers continuously change their designs? If what they have is so good why replace it with something else?  Why do they offer so many different models based upon sometimes widely varying design concepts?

    For those who say it has all been done before, I would welcome copies of papers or articles that verify that. I will be happy to incorporate links to suitable websites, or where not available - and subject to copyright restrictions - add them to this site. I would also be happy to revise relevant text affected by that evidence from this site.

    To assist the home constructor's understanding of the technological purpose in using a particular design concept it is essential to explain and verify the cause and effect relationship behind it, so a reference or schematic without explanation of how it works is not of much value to anyone.

    To my critics I say:

    Those who revert to name-calling and labelling indicate to everyone else their own ignorance and lack of technical capability to rationally discuss unfamiliar concepts and principles.

    Remember - once upon a time planet Earth was flat and the sun revolved about it - yet most people now believe the opposite.

    Once upon a time, triodes were replaced by more efficient and less distorting tetrodes, pentodes and beam power tubes - yet most audiophiles prefer triodes.

    Once upon a time, transformers were proven to be a very undesirable component and transformerless designs were developed - yet most tube amps use transformers.

    Once upon a time, it was scientifically proven that push-pull amplifiers are superior to single-ended amplifiers, yet many audiophiles prefer single-ended.

    Once upon a time, it was scientifically proven that negative feedback dramatically improved amplifier and loudspeaker performance, yet most musicians prefer amplifiers that do not incorporate negative feedback.

    Once upon a time, it was scientifically proven that transistor amplifiers are superior to tube amplifiers in every aspect of laboratory tested performance and reliability, are substantially cheaper to produce and operate, and are substantially more compact in physical size - yet after nearly half a century of solid state design and user experience most audiophiles prefer tube amplifiers.

    So the question remains - why??

    If you know the answers then please enlighten us all.

    Perhaps nothing is perfect and everything has shortcomings - which leaves scope in a competitive society for someone else to do it better and cheaper.

    In a free society, everyone is entitled to their point of view and personal sensory perceptions.

    Unfortunately in a capitalist society, good ideas are often lost in the wash of competitive forces as the dominant players prevail in the marketplace and destroy or displace superior competing products. Just because a particular design feature has become displaced by competition or a change in fashion does not mean it was not valid or worthy of preservation - or simply "better". Therefore, just because someone makes something today does not mean there was not a better product yesterday.

    An alternative answer is that everyone likes "difference" - ie few people want to be the same as everyone else and that is translated into the things we want to buy, own, use and do.

    Finally, a tube amplifier can only and will always sound like a tube amplifier - but that does not mean we have to accept the technological status-quo.



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    This page last modified 10 October 2012

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