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



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

It is intended to provide helpful hints to save you inevitable pain and suffering in your quest for audio and musical excellence, and to help you find joy in your chosen pathway to musical pleasure and fulfilment  !!!!

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

My personal professional musical instrument playing and tube audio experience extend around fifty years, so my contribution here is based upon a solid foundation of practical application of tube electronics to musical performance.

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 and data sheets, and my own experiments, research and experiences.

I hope it is of help to you in designing and constructing the very best bass sound system 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 and copyright owner.

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.

Proceed only at your own risk!!

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

Never forget Murphy's Law:   If something can go wrong it will !!!!


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


1.             INTRODUCTION:















Throughout the history of recorded music the bass is an instrument that is there - but not there - in the musical performance.

The bass part may be performed by a bass violin, a bass guitar, a tuba, a contrabass tuba, a sousaphone, a piano, a pipe organ or an electronic organ. Other instrument variants are also possible.

The primary function of the bass is to provide a platform of depth of tone and aural balance to other instruments and/or vocalists playing further up the audio frequency range. To do this, the bass part is typically one or two octaves below the other instruments or vocals.

Popular perception of this elementary "background" or "ancillary" musical function often leads to the the Bass Player being regarded as the least important musician in the band lineup - because the usual role of the Bass is to support whatever is happening further up the musical register throughout a performance.

Except in jazz music, the bass part is subtle and generally in the background. The bass part is typically not a "tune", but a series of harmonies with the tune played by others.

It is rare in popular mainstream recorded music to hear a bass solo. It is even more rare to hear a dazzling virtuoso performance from the bass line.

However, regardless of the number of other instruments present, or the complexity of the music, or arrangement, or orchestration, the Bass Player is required to match it - but in such a way as to not disturb the delicate balance of harmonies created.

Consider the band or orchestra as being a see-saw, with the bass on one side of the pivot and all of the other instruments on the other side. Balanced aural loudness is critical.

Consequently the role of the Bass Player is extremely important - albeit subtle.

Mostly the bass part is simply "just there" with no accentuation over other instruments - but its presence is known.

When listening to a musical performance, most people would not be overtly conscious of the bass line, yet without it they would become immediately aware of its absence.

Imagine the mighty work - Bach's "Toccata and Fugue in D Minor" - without a bass line !!!!

Its absence is profound. To demonstrate this point, turn the bass tone control on the stereo to the off position and listen to the effect upon the music. Music without bass is typically lifeless and dull.

In the case of classical music it is usual for the bass line to be written down by the composer and faithfully followed by the musician, however in the case of popular music it requires an ability from the Bass Player to compose in real time "on the fly" - that is, to devise a bass line that is always in harmony with whatever else is happening in the creative performance. This is particularly true for live performances, where it is expected that the bass player will not read music whilst playing.

Another vital role of the Bass Player is to work in harmony with the drums to create a rythm section. The choice of playing style of the Drummer affects the decisions of the Bass Player in his or her creativity - because these two instruments interact. It is essential they work together for common purpose.

It follows that to be a successful Bass Player, one has to contribute in a meaningful way that is constructively inter-active with others. This requires a high standard of playing ability, timing, accentuation, accurate sound reproduction and, most importantly in a large venue, adequate amplification to attain relative loudness and balance with other instruments.

At this point it is pertinent to differentiate between recorded and live performances.

Recorded Performances

In the case of recorded performances, since the 1920's when electric recording was introduced, the bass violin was successfully recorded in classical, orchestral and popular music.

This is simply because the bass violin is an easy instrument to record. With gut strings it does not exhibit strong harmonics so presents a simple waveshape in a frequency range easily recordable and replayable with low quality equipment. Even when harmonics are absent due to limitations in recording or playback, the bass violin still sounds like a bass violin.

However challenges remained in the home reproduction from records. Early tube audio systems were deficient in bass, primarily because of low-powered single ended output stages, poor quality output transformers and small loudspeakers in open backed cabinets.

The recording characteristic for 78 rpm records typically had a more or less flat bass response - after suitable equalisation that varied from one recording to the next. Because the recorded bass was accentuated and treble attenuated, "boomy" bass was the norm for open backed mantel and console radios and radiograms of that era.

Audio system tone would be described as "mellow" or "pleasing".

However, from the early 1950's, to reduce surface noise and enable a lesser groove amplitude to extend playing time, vynil EP and LP recordings generally incorporated RIAA, NARTB or NAB bass attenuation (-20db @ 20 Hz) and treble boost (+20 db @ 20 kHz) (a completely different approach to 78 rpm recording characteristic).

The RIAA equalisation characteristic is shown below:

During the 1950's, due to improvements in playback technologies, high-end audio hi-fi systems were able to reproduce bass very well, but mainstream mantle radios, console radios and radiograms were still relatively poor. The 12 inch loudspeaker was still regarded as a "large" speaker and most home systems used only 8 or 10 inch drivers.

Console style tube radiograms usually offered a maximum of about 10 to 20 watts of audio power, thus limiting the power available to the bass spectrum.  It was common for "quality" radiograms to reproduce AM radio in a superior quality sound than that from the inbuilt record player - so there was still some way to go in technological improvement.

Throughout the period from the 50's on, it was the case that 45 rpm records commonly had a 6 db bass boost at about 50 Hz to compensate for bass attenuation from small speakers in mantel radios and portable record players. Consequently 45 rpm records play back with a different tonal balance to their 33 LP equivalents.

It was not until the 1970's, with the introduction of higher powered and lower real cost transistor based audio systems with full-frequency modular components, that reasonable bass became available in the home.  Sterophonic recordings required two discreete channels for playback so the sound stage improved dramatically - generally across the full width of the home listening room.  Stero playback generally improves bass response and presence over monophonic. 12 and 15 inch drivers, commonly configured in two or three way systems with midrange and tweeter, became available at affordable cost.

Advances in recording equipment and techniques and staging "fashion" meant that the electric bass guitar gradually displaced the bass violin as the most recorded bass instrument in popular music.

Recording studio equalisation of the master tracks was modified to take advantage of the improved performance of stereo systems and bass became a permanent feature of modern recorded music.

More recently, enter 5 and 7 channel "surround sound" home theatre systems with powered sub-woofer. These enable massive bass power to be reproduced in the home, at a level completely disproportionate to whatever was originally performed. The high-powered car stereo, pumping out up to 1,000 watts per channel, also enables over the top bass response.

Nowadays, with 5 string basses and electronic black box effects, the lower register extends down to around 30 Hz - not really a problem for hi-fi playback systems (particularly those incorporating a sub-woofer unit) but very impracticable for high-powered on-stage live performances.

An important difference between recording and playback is that recording is performed internally within electronic devices or equipment. Once the performance is captured on a recording, it is not usually subjected to external forces or influences, such as venue acoustics, microphony or extraneous magnetic fields.

In the case of bass guitars, unshielded amplifier components can readily pick up induced magnetic fields when the instrument is physically located near the amplifier or a loudspeaker - resulting in hum or microphony or instability.

Finally, digital recordings may be manipulated manually or with software, to produce a myriad of variations upon the original recording. However a live performance is as-is.

Live Performances

The second consideration is for LIVE performances. Here the situation is very different.

Notwithstanding its "warmth" of tone and commanding presence, a single bass violin has a limited audible sound level and is therefore limited as the to size of venue it can service without amplification. It is also a physically large and fragile instrument so is not quite convenient to lug around to live venues.

Once the point is reached where amplification is required for live performance, the electric bass guitar becomes the favoured instrument. It is smaller, lighter and easier to play than its bass violin counterpart - but with suitable amplification, can produce a similar bass musical effect.

Of course the bass violin can also be similarly amplified, but that option is not popular for a variety of reasons.

It is also true for live performance that the power required to reproduce bass is proportional to the power produced by the other instruments present.

Given that a typical guitar amplifier will deliver about 60 to 120 watts rms and that for effective tonal balance the audible bass power level needs to be about twice that, we have a need for a bass amplifier and loudspeaker system capable of producing at least 120 to 240 watts rms of power at 40 Hz.

In modern terms, with transistor amplifiers available that can easily deliver several thousand watts of power it would not seem to be a problem to achieve this.

However, life is not that simple.

Since the invention of the vacuum tube, LIVE electronic reproduction of bass from a musical instrument has always been a challenge for musicians, audio designers and constructors - and remains so to this day.

Even electronic organs present their fair share of challenges in reproducing the lower frequency register, so have much in common with bass amplifiers.

Over the years, various approaches have been made to adapt hi-fi technologies to electronic instrument amplification, but it is clear from practical experience and theoretical analysis, that requirements for musical instrument amplification are very different to those required for hi-fi reproduction of recorded music.

"HIGH-FIDELITY" is a term that literally means "TRUE FAITHFULNESS" - to the original. Obviously that is not necessarily an objective for musical instrument amplification and reproduction, because what we are after is that elusive "sound" that creates the particular AUDIBLE musical effect we want.

What happens to the sound quality and characteristics within the reproductive system is not of concern to the player, because it is what we actually hear that matters.

This principle is most aptly demonstrated by the "electronic keyboard" - formerly known as the "electronic organ" - where the sound reproduced is created completely artificially. The only human involvement is to depress a key or press a button !!!!

Conversely, if we know the cause and effect relationships that create and manipulate the sound within the system, we can design a system that will do what we want. That is of course the methodology used by electronic organ designers, sound processors, effects pedals and suchlike

For just on one hundred years, and particularly the eighty odd years since the advent of electric recording, countless researchers and engineers have endeavoured to attain that elusive standard of performance described as "High-Fidelity".

If we review audio amplifier circuit designs from an audio engineering perspective we can see much commonality between them, but most might be described as "variations on a theme".

In fact, many amplifiers have attained their superior performance simply by taking a proven design, 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.

If we then look at the evolution of musical instrument amplifiers, we see the same "follow-the-leader" approach at work - in this case "the leader" being the big names in commercial musical instrument amplification.

This paper takes a fresh look at some of the important elements essential to designing and constructing a vacuum tube based audio amplifier intended to reproduce BASS.

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.


The professional Audio Engineer designing for a commercial application must consider design elements and factors such as performance, construction and safety specifications and standards; tube types, characteristics and availability; component availability; continuity of supply; component cost; component quality; corporate vendor/supplier policies and preferences; corporate design policies; fashion ideologies that control appearance, shapes and finishes; component and complete device colour, machine tooling constraints; sheetmetal suppliers and materials; labour costs and assembly times; packaging and delivery requirements and costs; market/buyer preferences or trends; warranties and guarantees; after-sales service; and brand-reputation etc.

In recent years, many countries have introduced consumer protection laws that require a product to be what it is claimed to be. Consequently, all of the designer, manufacturer and retailer must consider the validity of performance claims when viewed from a statutory compliance perspective.

All of these considerations and more impose constraints upon the professional designer when approaching the design of high-fidelity and professional audio amplification equipment for commercial sale.

However the home constructor has no such constraints !!

In our quest to attain our required "sound":-

We are blessed with more or less total freedom from all or any of the above.

We can take an existing commercial BASS amplifier and modify it, optimise it, tweak it, or do anything else we want.

We can use new, used, second-hand, salvaged, hand-me-down, or recycled components.

We are not usually constrained by original component price or cost.

We can use non-ideal, oversized or approximated components, or components that would not normally be used in such a device.

We can compromise.

We are not locked into printed-circuits and can use point-to-point wiring with confidence.

We are free to use any design we want.

We are free to have any layout we choose.

We are free to instal extra shielding wherever we want.

We are free to modify the design without having to be concerned about guarantees, warranties or product.

We can use recycled industrial, military or broadcast quality components.

We can use any tube type or mix of tube types we want.





But please, before you abandon all of the hard-won knowledge developed over a century of tube audio design and application,  do follow the essential core design rules set out in STEP 9 below: "CHASSIS AND COMPONENT LAYOUT AND WIRING" - ignore them at your own peril.



Here are some basic ground rules:

Electricity normally behaves like a fluid. Hence when we observe the behavioural characteristics of water, we can see much relevance with electric current flow.

Electricity is a force. Hence it can have all manner of forms - not just regular AC (Alternating Current) or DC (Direct Current).

Electricity will flow when there is a pressure difference between the two ends of a conductor or a circuit.

Electricity will not flow unless there is a "circuit". A circuit can be created by hard-wiring - or by electro-static, inductive or electro-magnetic coupling through the air.

A "circuit" can be created by adjoining components, wires or even through the air. Never assume that because a hard-wired circuit is not evident, high-frequency AC current or electromagnetic forces cannot be present to influence circuit behaviour.

Generally speaking, the greater the current through a conductor the greater the extent of the electro-magnetic field around it.

Generally speaking, the longer a wire the greater the risk of electro-magnetic interference or coupling.

Electricity is "lazy" -  It will ALWAYS try to find the shortest path. This attribute creates problems with devices and wiring in high-voltage and/or high-current circuits. In the context of vacuum tube amplifiers, "high-voltage" means anything above about 450 VDC. Above that, life becomes more and more difficult as all manner of unexpected phenomena occur in the amplifier.

Electricity flow can be controlled electrically, electronically, electro-magnetically, electro-statically or mechanically.

Power Out = Power In minus Losses. This works like a garden hose. What comes out = what goes in less friction in the pipe.
In an amplifier, AC power out = AC power in minus conversions losses from AC to DC then DC back to AC, multiplied by device efficiency.



Unlike the situation in hi-fi systems, where each discrete modular component - eg turntable, CD player, tape deck, radio tuner, pre-amp, power amplifier and loudspeaker etc  - is usually selected for individual characteristics independent to those of the other components in the system, for BASS reproduction we need to look at a closer compatibility.

This is because in a hi-fi system, each component will typically have performance characteristics that exceed our needs or hearing capabilities. Inter-dependence or interference between modular components is not usually an issue.

On the other hand, in a live music BASS system, it is likely that each component will be struggling to meet our needs.

The primary reason for this is that whatever we do will be done at continuous HIGH-POWER - a situation not usually experienced in hi-fi listening.

One example is in frequency response specs. Long ago the commercial audio engineering fraternity decided that Frequency Response should be measured at one watt rms - not exactly a difficult standard.

They also decided that Intermodulation Distortion should be measured using just two frequencies - neither of which have much relevance to BASS amplifiers because a bass amplifier will normally be reproducing only one frequency at any time.

Finally, standard lab tests are always performed using resistive loads (which present a constant load impedance to the amplifier) at frequencies that are above the lower bass register - ie below 120 Hz - where we need maximum performance.

All of these standards were based upon the characteristic of vacuum tube amplifiers which, in any given amplifier, cause distortion to increase proportionately with power output. So quoting performance at full power output was usually avoided.

Note: Solid state amplifier manufacturers now quote amazing performance specs for their amplifiers at full power output. What they do not tell you is that over-driving the amplifier beyond its limint causes total collapse of the waveform into square-waving and often self-destruction of the amplifier through overheating of the power stages. Destruction of the loudspeakers is also a common event. The performance specs also do not tell us what the amplifier sounds like.

Now a glance at the impedance characteristics of any electrodynamic bass speaker (ie standard magnet/voice coil/cone driver unit) shows a huge impedance change up and down in the region of most use in bass.

Consequently, published hi-fi amplifier performance figures are effectively useless to tell us what we need to know for BASS reproduction.

This remains true also for current generation solid state amplifiers, where rated power output and distortion performance are usually expressed at a specified load impedance - eg 4 or 8 ohms.  This has been measured of course with a purely resistive load at a specified standard frequency. However connect to a variable load like a loudspeaker and a different result will be seen.

Note: Solid state amplifier manufacturers now quote power output at a specific load impedance but often do not tell us that under-loading the amplifier - ie presenting a load impedance below the specified value - can also cause self-destruction.

On the other hand, except for the occasional power output rating, tube bass guitar amplifier manufacturers have never published performance figures at all.

So our starting point for the BASS amplifier is a no-man's land of electronic theory, design and application.


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 BASS audio amplifier design principles, it is first vital to understand the vacuum tube serves two fundamental purposes or functions:

First Primary Function:

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.

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 acceptable limits - otherwise the tube would behave as a low-resistance metallic conductor.

Second Primary Function:

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 BASS 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" to create a circuit.

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.

Kirschhoff'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 circuits.

Note that Kirschhoff'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 phenomenon 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 carefully 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 acoustic, 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.

Note however that an SPL on its own does not convey the volume of air being pumped. The SPL rating is a measure of "loudness" determined at 1 watt electrical input (amplifier output). Power used to be measured with a single frequency rms signal, but nowadays all kinds of rating changing techniques are used by loudspeaker manufacturers to rate power input - eg music power, white noise, pink noise, peak power, programme power, etc etc - all expressed in "watts".

We may reasonably then ask the question "what is a watt"?

Or, "when is a watt not a watt"?

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. This equates to 150 watts EIA, 200 watt peak or up to 1600 watts PMPO.

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

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!!


Commercial bass amp systems for live stage work come in wide range of options.

From the humble 20 watt practice amp to the gargantuan systems used for large venues, the bass musician has a myriad of choices available.

Large PA systems are beyond the scope of DIY so are not described here.

Consider the band or orchestra as being a see-saw, with the bass on one side of the audio frequency pivot and all of the other instruments on the other side. Balanced aural loudness is critical.

A general rule of thumb is that the bass amp must have a rated power rating of at least twice but preferably four times the rated power output of the lead guitar amp.

That means at least 120 watts rms to 240 watts rms for the typical bass guitar amp.

However that is a practical minimum requirement to get by.

Above that level a tube amplifier is likely to be too heavy to cart around so the option is to either go to solid state (250-500 watts rms minimum) or to a DI (direct injection) feed into a large PA system.

Of course the size of the venue, crowd numbers (people soak up power) and indoor v outdoor conditions (outdoor offers no reflected sound) affect the minimum power needs.

IMPORTANT: It is of no use connecting a powerful amplifier to small loudspeakers or inefficient loudspeakers or connecting with undersized cables where losses will be high (potentially up to 50% or more)

For bass the more speakers the better, the bigger the speakers the better and the more cabinets the better. In my experience 12 inch drivers sound better than 10 inch, 15 inch drivers sound better than 12 inch, 18 inch drivers sound better than 15 inch. Then the more cabinets the higher the acoustic efficiency.

Consider a guitar amp driving a 4 x 12 inch quad box.

Noting the human ear is more efficient at higher frequencies, the bass guitar will need an equivalent to 8 to 16 x 12 cabinet(s) to deliver the same effective loudness.

See also Section 2.5 below re loudspeaker response and SPL efficiency.

One of the curious aspects of modern band sound systems is that the typical band will have two fifteen inch PA speakers set high on pedestals for voice, two to eight 12 inch loudspeakers for guitar, one or two fifteen inch loudspeakers for keyboards, and sometimes two eighteen inch subs for drums - but just ONE fifteen inch loudspeaker for bass guitar !!

The resultant tonal balance is obvious.

Worse, in the case of solid state bass amps, many commercial models have the LF boost frequency for BOTH tone control and graphic EQ set to 100 Hz - a full one and one half octaves above bottom E.

Others use 50 Hz.

Both options produce muddy bass and require the bassist to use a pick to deliver an acceptable sound.

Fortunately some amps can be easily modified to correct this by changing the value of the boost capacitors.

The ONLY way to achieve a solid bass sound is to use tone controls and graphic EQ having a LF range to 32 Hz. Odf course this is essential for a 5 string bass where the bottom open string is around 30 Hz.

Food for thought !!



Next we look at the loudspeaker.

In traditional hi-fi systems, the bass loudspeaker (woofer) intended to reproduce down to about 30 Hz requires a large enclosure of at least 8 cubic feet volume, made from heavy timber and suitably padded internally with damping material.

However this is just not practicable for the professional musician because the box is too big and bulky, too heavy and will not readily fit into our motor vehicle or onto a stage.

Large, heavy enclosures are not likely to fit comfortably onto a standard venue stage after the other musicians have taken their share of available space. large, heavy enclosures can also present a safety hazard if their centre of gravity is too high, or their height to base dimensions ratio is out of proportion (read tall, narrow or thin enclosures)

Graphic equalisers and tone controls are usually used to compensate for bass rolloff in the lower register - if at all

What we need for the professional BASS musician is a small enclosure having a high power handling capacity in the lower register.

Most importantly it must have an even frequency response so that as each note is played moving up or down the musical scale, the sound loudness will be constant.

That is a big ask.

However, all is not lost because buried in the annals of textbooks and papers, is a wealth of information to help us in our quest to build our ideal BASS system.

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 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 slightly 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 or test reports 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.

PMPO, or Peak Music Power Output, means the instantaneous power the unit can handle. It essentially relates 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 for BASS guitars.

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.

If we increase the frequency to 1,000 Hz the PMPO will increase to 20,000 watts.

Remember ancient Australian saying: "BULLSHIT BAFFLES BRAINS" !!

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 with huge magnet actually has a low SPL performance 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 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" suspension. Cellular foam rubber suspensions tend to literally 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.

The spider should sit flat to position the voice coil centrally in its magnetic field.

It will be observed by the very critical listener that paper-coned loudspeaker sound qualities 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 resonance reduce from a heavier cone. This feature is incorporated into heavy duty professional 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.

2.4    Loudspeaker Frequency Response

The BASS guitar has a useable frequency range of about 3 octaves.

Bottom E is at about 40 Hz and top E at the G string is about 160 Hz.

Specifically, when the instrument is tuned to Middle C pitch of 256 Hz:

    Bottom E is at 40 Hz

    Top G is at 192 Hz

    The harmonics of G occur at approximately 384 (2nd), 576 (3rd), 768 (4th), 960 (5th), 1152 (6th), 1344 (7th), 1536 (8th), 1728 (9th), 1920 (10th) Hz.

So allowing for a top fundamental frequency of say 200 Hz and harmonics of 2,000 Hz, we do not need midrange or tweeters for bass.

A good quality cone speaker will do the job well.

However there is more to bass than fundamental frequencies, so some degree of midrange is desirable for colouration.

The modern trend to multiple 10 inch drivers is a case in pont, where solid bottom end is traded off for loudness and light weight. The typical 10 inch drivers illustrated below show response at 40 Hz is -10 db down on response at 100 Hz - a difficult task to offset by equalisation.

2.5    Frequency Response and Power Response

The POWER response of a loudspeaker system is critical for bass because a frequency response of -0 db at 40 Hz in one unit compared with a frequency response of -20 db at 40 Hz in another (compared to sound output at 400 cps), equates to an ELECTRICAL POWER difference of 100 times to attain the same LOUDNESS at that resonant frequency.

That will be impossible to attain at useful power levels because the loudspeaker will not be able to handle that much power.

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 frequency response curve means cost-free power output. It also means more natural sound where individual notes across the musical scale are reproduced with more equal loudness.


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, indicates 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.

Note: Although HF response rapidly falls away above 3,500 Hz, the frequency response needs described above indicate this loudspeaker is suitable for bass guitar.

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 (43 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.

However, because of its uneven response in the LF region, this system would not be suitable for bass guitar applications - unless suitably equalised - but may be very satisfactory on hi-fi playback.

On the other hand, if the lower register is equalised to flat response, supported by adequate amplifier power, then this system shows remarkable flatness across the entire bass guitar range of fundamentals and harmonics.

The slight increase in SPL in the mid-range is also 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 musician and should be avoided where practicable.


When mounted in an infinite baffle enclosure (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 or graphic equaliser.

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.


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

Loudspeaker System 3

Whereas overall SPL is similar, in this system the SPL at 40 Hz is 94 db.

The curve of Loudspeaker System 2 shows the SPL at 40 Hz is 95 db - which is more or less the same in practice.

However the HF response of System 3 rolls off rapidly from 1,000 Hz - which will produce bass lacking in harmonics and therefore definition.

Consequently, the loudspeaker of System 2 is the better choice for bass.

But in a two or three way hi-fi system Loudspeaker System 3 will deliver a more interesting mid-range and a brighter bass tone.

However bass guitar is not hi-fi
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 hi-fi system may actually only have a 10-20 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.

WARNING:  In the case of guitar amplication, Tweeters cannot be successfully used in the same way as in hi-fi systems. The guitar pumps out high frequency signal at much the same power level as midrange or low frequency signals. This means that Tweeters must have a similar power rating as woofer units in the same system.

PIEZO tweeters usually have very high power ratings but exhibit very patchy frequency response characteristics, rendering them unsuited for high quality systems.

One way around this is to use twin-cone speakers, however these are generally intended for PA applications and not made with power ratings high enough for BASS. Of course multiple twin-cone units may be practically viable for some users.

There are however exceptions designed for musical instrument applications.

The P Audio 10 inch models HP10W and HP10T illustrate the difference between identical professional musical instrument loudspeakers with and without a "whizzer" tweeter cone. This loudspeaker spec. sheet has been chosen as an example because it directly compares identical designs,  which are typical of this class of loudspeaker.

Note the improvement in HF performance above 3,000 Hz in the case of the twin cone unit HP10T. This characteristic defines the advantage of a "whizzer" cone, which enables the loudspeaker to be driven at full power across all frequencies within its range.

IMHO twin-cone speakers are the very best option in lead/rythm guitar applications, where strong harmonics are desirable. In the case of  BASS guitar, twin-cone loudspeakers can be used satisfactorily but are not necessary - unless strong harmonics are required by the musician.

Note: As with all cone tweeters, Whizzer" cones are directional, so as the listener moves away from the frontal axis the SPL reduces, however in the case of bass reproduction this is not likely to be an issue. However if the amplifier is to be used as a backup for other instruments or voice, then twin cone loudspeakers are a good investment.
2.7    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 usually cheaper to add an extra loudspeaker than to double amplifier power. Conversely it is usually cheaper to add an extra low-cost loudspeaker to double power handling capacity than to instal a single high cost higher performing unit. Do the sums!!

There is a 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 and 24 inch drivers, which are beyond the bounds of practicality for most home users. However a true 100 W RMS can easily be handled continuously by four 12 inch drivers, mounted in a single large enclosure, or in multiple enclosures.

15 inch drivers are of course superior to 12 inch for bass.

Generally speaking, the bigger the better.

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.
2.8    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 over-travel, 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 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 or SPL.

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.
2.9    Cone Excursion

Remember that the more the cone excursion, the greater the cone over-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.

Watch cone movement - in and out. All loudspeakers have finite physical limitations.

To protect loudspeakers from damage and to extend useful service life and reliability, it is better to use multiple drivers with cones travelling less than a single driver with cone travelling more.

Damping of the cone movement can be electrical via amplifier feedback systems and acoustic via cabinet design, however since we want to pump air at low frequencies, damping can be counterproductive.
2.10    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.

So for BASS guitar we do not want a crossover network in the loudspeaker system.

If top end is wanted, then it is better to bi-amp with an electronic crossover up front.
2.11    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.

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 above and 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 apartments 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's 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.

EXTREMELY IMPORTANT: A cone loudspeaker is a simple air pump. Cone loudspeakers rely upon a tight seal between the front and rear of the cone to prevent the acoustic energy short-circuiting around the rim of the frame. The free air power handling of a loudspeaker is only a small fraction of the rated power handling capacity when properly mounted and loaded in an enclosure because the lack of acoustic loading on the cone can result in over-excursion of the cone and potential physical damage at quite lowe power levels - particularly at the resonant frequency.

In the case of bass guitar applications where power levels and cone excursions may be high, it is desirable to seal the metal rim of the frame to the cabinet with a sealant such as black mastic compound - particulalry when the speaker is mounted on the front of the baffle.
2.12    Tone

Tone controls and/or graphic equaliser are essential for a BASS guitar system.

IMPORTANT: The low frequency boost should occur at 40 Hz or less, rather than the 50 Hz popular in hi-fi systems.

If the boost frequncy is too high, the bass is likely to sound boomy or hollow. When coupled with a poor quality speaker system, the result will be hollow bass devoid of depth of tone.


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 (SRPP)
  • 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 components.

    (the trick is to figure out which ones!!)

    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. However some commentators suggest excessively large coupling capacitors produce "blocking" distortion.

    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 circuitry.


    Negative feedback offers advantages and disadvantages.

    Negative feedback is the commercial solution to reducing hum and noise in audio equipment.

    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-pull triode, 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.


    The load impedance specified for an amplifier is a nominal value only. Laboratory tests verifying frequency response, distortion and power output are used with RESISTIVE loads. However in the real world we use a loudspeaker to convert the electrical power into acoustic power.
    5.1    Effect of Loudspeaker Load

    Dynamic loudspeakers do not present resistive loads - they are capacitive and/or inductive. Therefore what is claimed by laboratoty test bears little resemblance to real world application and performance of the amplifier.

    Unfortunately, to conform to industry norms, loudspeakers also are assigned nominal specifications. So an 8 ohm branded loudspeaker may actually present an actual voice coil impedance of beteen 6 ohms and 10 ohms. This variance is reflected back into the transformer primary as the plate to plate load.

    If that was not enough to contend with, we also have the characteristic of loudspeakers which results in impedance varying with frequency. This means also that the plate to plate load will vary with frequency.

    The following graph is indicative of typical electro-dynamic loudspeakers, regardless of size or power rating. Actual impedance is shown in the right hand scale.:

    In this example the minimum impedance is 6.8 Ohms at 120 Hz and about 12 Ohms at 400 Hz.

    This graph shows us that the impedance at resonance for an 8 ohm nominal loudspeaker - in this case at about 50 Hz - is 92 Ohms.

    This represents a real change in actual impedance of 13.5 times the minimum.

    Importantly, in this example, the load change occurs between 50 Hz and 100 Hz, representing a change ratio of 13.5 times over that octave. That is to say the load on the amplifier increases 13 times per octave between 50 Hz and 100 Hz.

    Typical values for all sizes of loudspeakers range from about 60 ohms to 100 ohms. Often the HF change ratio equates to or exceeds the LF change ratio.

    4 or 16 ohm loudspeakers will exhibit the same ratio of change.

    For bass applications we need to consider the frequency range 40-2,000 Hz. The above graph shows us that the impedance rises to about 24 ohms at 2,000 Hz suggesting we need also to consider the HF region.

    The result of all of this change is that the power output of the amplifier will decrease in proportion to the load.

    So as we go down the frequency scale, power output will decrease at the very time we want it to increase.

    Importantly, the low bass octaves from 40 to 160 Hz are in the range of high impedance, thereby presenting an average load to the amplifier considerably higher than nominal.

    However, the following GEC graph for KT88 Beam Power Tubes configured in various topologies, shows us a tube amplifier will tolerate a load increase of about 50% without significant power loss, offering some headroom.

    The graph tells us that increasing load from 5k to 7 k Ohms reduces power from 55 watts to 43 watts = 22% or about -1 db. A modest reduction and nowhere near as much as the variation generated by the loudspeaker itself.

    However increasing load by a factor of 10 to 12 times will obviously reduce power output significantly.

    Given a constant plate current and/or constant output plate to plate voltage, the power delivered to the load will vary directly with the load. (Power = IsquaredR or Esquared/R), resulting in a reduction in power of -3 db for each doubling of load.

    Thus increasing load from 8 ohms to 16 will reduce power by 50%, or -3 db

    Increasing load from 16 to 32 ohms reduces power to 25%, or -6 db

    Increasing load from 32 ohms to 64 ohms reduces power to 12.5%, or -9db.

    Increasing load from 64 ohms to 128 ohms reduces power to 6.25%, or -12db.

    All of this range can be comfortably offset with suitable equalisation.

    But without some form of equalisation and power output to support it, the bass will simply roll off as the frequency reduces.

    Another approach is to set the nominal primary transformer load impedance to the lowest safe value so that the optimum load impedance will be reflected under realtime conditions. Based on the above analysis it might be practicable to set the transformer primary to one fifth of the tube handbook recommended loads. This design approach is suitable ONLY for bass guitar or double bass amplification and is not recommended for wide frequency range music signals such as tuner, tape or CD.

    An alternative but equivalent approach is to connect 4 Ohm loudspeakers to a conventional output transformer 16 Ohm secondary winding.

    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 impedance ratio can be obtained by the following options:

    5.2    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 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 output load impedance to one quarter that for one pair of tubes.

    Eight pairs (16 tubes) of tubes will reduce the output load impedance to one eighth that for one pair of tubes.

    A prominent current commercial example of this approach is demonstrated at http://www.vtl.com/pages/classic_reference.html

    More importantly, the output transformer turns ratio is reduced by the square root of the impedance ratio. Hence a single push-pull pair load of say 8000 ohms plate to plate reduces to only 1000 ohms plate to plate with eight pairs of tubes. For an 8 ohm secondary the turns ratio reduces from 32:1 to only 11:1 - producing a far more efficient and linear transformer. Increase the secondary load to 16 ohms and that ratio reduces to only 8:1

    Parallel operation also enables high power to be obtained from relatively safe low voltages - eg 300 to 450 VDC B+ supply rail - avoiding the need to produce specially designed (and physically large) power and output transformers suitable for 1 to 4kV continuous operating voltage - as is the case with some transmitting class triodes. Power output from multiple pairs is simply the product of the number of pairs x the power for a single pair. Distortion performance will be the same as for one pair.

    5.3    Series or parallel operation of loudspeakers

    Every extra loudspeaker changes the effective secondary load impedance by the number of loudspeakers. When loudspeakers of equal impedance are connected in series, two doubles the load, three triples, four quadruples etc. When connected in parallel, two halves the load, three thirds the load, four quarters the load etc.

    A quad set of speakers may be connected in series/parallel - eg 4 x 8 ohm units can be connected in series to provide 32 ohms load, in parallel to provide 2 ohms load or in series/parallel to provide 8 ohms load.

    I would recommend connecting the loudspeakers in series whenever practicable because the series capacitance is proportionately reduced whilst the series inductance is proportionately increased. Thse values are multiplied by the output transformer to the tube output stage and appear as its load.

    Higher inductance means better bass. Higher capacitance means less stability.

    Parallel connection of loudspeakers causes load inductance to decrease and load capacitance to increase - both undesirable for tube amplifiers.

    Series connection minimises current in the circuit so cable power losses (particularly over long lengths) are also minimised.

    Of course the appropriate transformer tap must be used for optimised impedance matching - eg connect an 8 ohm speaker to the 8 ohm transformer tap.

    5.4    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 ration 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 ration 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
  • decreased output impedance resulting in improved loudspeaker damping - ie less bass boom
  • same driver stage output voltage as for one pair
  • effective output transformer primary inductance increases proportionately with each pair of tubes - translates into better 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

  • .
    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 8 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 relatively poor conductor (80% IACS) 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 (60-80% IACS). It is also difficult to prevent high-resistance joints when terminating aluminium, because the surface of the conductor has a high-resistance coating resultant from oxidisation. If aluminium conductor is used it should be terminated at both ends with professionally engineered crimp lugs, fitted using aluminium jointing compound.

    Note 3: For the record, pure silver has the same electrical and heat 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 (0.004") deep. Thus in any given copper conductor, the outer 0.2 mm (0.008") 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.

    For a theoretical look at connecting cables see this discussion from


    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

  • .
    Notwithstanding any other considerations, there is no doubt that for BASS the TRIODE is best.

    Next comes the ULTRA-LINEAR option using BEAM POWER TUBES.

    TETRODE or PENTODE operation should be avoided for BASS. Unless supported by a solid power supply and suitable drive circuitry, the bass will be hollow and boomy. Because tetrodes and pentodes require output transformers having a high primary inductance, effective power output at low frequencies is likely to be inadequate.

    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, tubes must be mounted with the grid wires aligned in the vertical plane - to prevent grid sag when hot and consequent risk of changing grid characteristics - or even creating an inter-electrode short-circuit. 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)

    BASS amplifiers often operate under severe conditions of vibration - just feel the floor of a stage!!!!.

    Recommended tubes for low-noise, low hum and low microphony, 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 family provide 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. All are available in military or industrial versions.

    Note however that it is well established empirically, that the material and dimensions of the Plate element affect the sound tonal characteristics and many audiophiles have specific preferences in this regard.

    The choice of an "industrial" grade or "premium quality" 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, because often the industrial characteristic has no impact on "sound" at all - eg long life. It may be "ruggedised" or have particular selection, inspection or test characteristics directed at specific non-audio applications.

    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.

    "For reasons that will eventually become apparent, it is important to recognise that the only difference between Class A, Class AB and Class B is the quiescent bias setting"

    - as defined by High-Power Audio Amplifier Construction Manual" - C. Randy Slone - McGraw-Hill 1999.

    Control Grid (Grid #1) Bias determines the current flow in the tube(s) at quiescent (zero signal), maximum signal and all of those values in between.

    A modern theoretical analysis on Classes of Operation, Load Requirements and Plate Dissipation by Earles L. McCaul is available at http://www.pentodepress.com/tubes/vacuum-tube-archeology.html

    However, read-on:

    The following standard classes of operation are available - as defined by various authorities.

    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)

    Class A operation is the normal condition of operation for a single valve, and indicates that the plate current is not cut off for any portion of the cycle". "The numeral "1" following A indicates that no grid current flows during any part of the cycle, while "2" indicates that grid current flows at least part of the cycle" - as defined by Radiotron Designers Handbook 3rd edition 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)

    "Class AB operation indicates over-biased conditions, and is used only in push-pull to balance out the even harmonics"
    "The numeral "1" following A or AB indicates that no grid current flows during any part of the cycle, while "2" indicates that grid current flows for at least part of the cycle" - as defined by Radiotron Designers Handbook 3rd edition 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 operating 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)

    "Class B operation indicates that the valves, which are necessarily in push-pull, are biased almost to the point of plate current cutoff".  "With Class B the numeral "2" is usually omitted, since operation with grid current is the normal condition." - as defined by Radiotron Designers Handbook 3rd edition 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 positive half-cycle of the applied signal voltage." - as defined by The Radio Society of Great Britain. (1948)

    " As defined previously, Class B pertains to an OPS wherein the output devices are biased to conduct for 180 degrees of the signal cycle. In times past, Class B operation was referred to as push-pull operation (analogous to sourcing-sinking action), but this is a misnomer.
    Output devices in a Class B OPS will source current to a load for a half-cycle, but they do not sink current during the opposite half-cycle: they are cut off.
    The term push-pull should be confined to Class A type stages.
    At least 99 percent of (solid state) audio amplifiers utilise a Class B OPS."

    - as defined by High-Power Audio Amplifier Construction Manual" - C. Randy Slone - McGraw-Hill 1999

    "A Class B amplifier is an amplifier in which the grid bias is approximately equal to the cutoff value, so that the plate current is approximately zero when no exciting grid voltage is applied and so that plate current in the tube, or in each tube of a push-pull stage, flows for approximately one-half of each cycle when an alternating grid voltage is applied" - as defined by Prof H J Reich in his "Theory and Applications of Vacuum Tubes" - McGraw Hill 1944

    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" - as defined by RCA Tube Handbook RC-14 (1940)

    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"- as defined by RCA Tube Handbook RC-14 (1940)

    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"- as defined by RCA Tube Handbook RC-14 (1940)

    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 and by the value of negative grid voltage that will cause the plate current to either cut-off, or positive grid voltage that will cause grid-current to flow - whichever occurs first.

    It is likely the grid bias will be set at 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 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 average RMS or music signal 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.

    This is why a single-ended amplifier is fundamentally a waste of time and money for 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 faucet. 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, and the capability of the power supply to maintain voltage and current into the output tubes - up to saturation of the output transformer.

    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.

    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 providing driving power to the output tube grids, a low impedance driver stage,  low DC resistance wide-range driver transformer, high-quality high DC current capability output transformer, fixed Control Grid bias, accurately matched pairs of output tubes, a stable low-impedance regulated power supply, and grid bias set exactly at the plate current cutoff value for both sides of the output stage - at the same time (a challenge for parallel-push pull output stages) - but what the heck if it is for yourself!!

    Another challenge is that the grid circuit impedance drops to near zero when the output tube Grid is conducting. Grid-current is caused by the Control-grid voltage being approximately the same as or more positive than the Cathode voltage, causing the Control-grid to behave as a Cathode and become part of the Cathode to Plate circuit electron flow circuit.

    A conducting Control Grid presents a low to zero load on the driver stage - a not very comforting thought since this affects driving voltage, frequency response, distortion and absolute total power output. US transformer manufacturer UTC recommends only triodes for this application, explaining that the reflected load from the near zero impedance output tube grids through the driver transformer turns ratio back to the driver stage plates (conventional driver) or cathodes (cathode follower driver) will cause pentodes to deliver dramatically reduced power output together with seriously high distortion.

    The voltage/power delivered to the output tube grids will always be limited to the actual driver stage output multiplied by the driver transformer turns ratio.

    Better still, is the use of parallel push-pull triode drivers - the lower the Plate or Cathode-follower impedance in the driver stage the better. The non-tube purist may choose to use transistors for this application. Many commercial designs successfully use the transistor driver approach.

    On the other hand, Class B is more forgiving than Class A, thus 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 at home listening levels they will not be working very hard - ie low Cathode current and low heat dissipation.

    One major challenge 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 best BASS reproduction, this requires the Control-Grid bias 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.

    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.

    The driver transformer, if used, should have the best frequency response and lowest distortion characteristics practicable, the lowest practicable primary to secondary turns ratio, together with the lowest practicable DC resistance in the windings.

    Another option is the choke/capacitor driver stage coupling system but this has difficulties when trying to obtain high-fidelity performance.

    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.

    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 by a high B+ voltage, such that after voltage drop across the cathode-follower load resistor - eg 100k Ohms + normal 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 Plate to Cathode for a typical triode. This method ensures maximum output voltage from the driver.

    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.

    For bass amps where hi-fi is not an issue, a better solution is the tried and true push-pull driver transformer, which ensures accurate drive balance at all times..

    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:


    Tubes operating in Class A carry full current all of the time. Consequently their Cathodes wear out (by losing the electron emitting surface coating) faster. Loss of emission generally results in progressive loss of high frequency response. In the case of the output power tubes in a good wide-range hi-fi system, this will be about once every one or two years.

    So for bass amplifers, unless you can hear the difference it is more economical to run the tubes in Class B to preserve their life. In Class AB they should last for many years - and Class B for a very long time if not overstressed..


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

    8.1 General Principles

    My money goes on installing the phase-splitter as early as possible in the circuit to enable the use of 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.

    It also enables the use of a voltage amplifier directly driving the output tubes - the result of which is the availability of full output voltage from the selected driver tube (compared with usually reduced output from a phase-splitter) and a significant reduction in the input signal voltage required to drive the phase-splitter to the level of voltage needed to drive the amplifier to full output.

    Another benefit of a push-pull driver stage is that it can be supplied directly from the B+ supply to the output tubes (because the output tube plates are out of phase with their respective drivers) and thus receive the highest voltage available to its plates.

    A useful design rule of thumb is to select a driver stage capable of supplying an AC peak (crest to crest) voltage twice the negative control grid voltage applied to the output tubes. This will cater for pretty much all kinds of signal waveform shapes and reduce distortion from the driver stage by operating it a less than full output. eg if the grid bias is -40 VDC then be sure to instal a driver stage capable of supplying 80 V pp. This applies to each half of the push-pull leg.

    My personal preference is a design like the Williamson and GEC 400W, however the first voltage amplifier and phase-splitter, being direct coupled, can present problems relating to inadequate bias to both halves, so some fine tuning may be required for optimum results. The same applies to the cathode-follower driver stage. One solution is to delete it and use plate follower, as is the case in the RTVH 100W circuit.

    It will be easily seen that using a phase-splitter of a type that can deliver only half or less than half the usual output voltage for that particular tube type is not likely to produce a satisfactory end result.

    The exception to the above is pure Class A, where the peak to peak drive signal to the output tubes needs to be only equal to the bias voltage (since the quiescent operating mid-point is about halfway between the grid bias voltage and cathode).

    One simple, but effective method, is to use a driver transformer having a grounded centre-tapped secondary at the input (there can be problems of adequately loading driver transformers in Class AB or B output stages), then push-pull voltage amplifying stages 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 400-450 VDC) to deliver maximum rated plate to cathode voltage to the driver stage. Of course the rated plate voltage of the driver tube must not be exceeded.

    Note that excessively high voltages to pre-amp stages - (ie more than 250-300 VDC B+) - is not advisable because it produces no real benefit, shortens tube life and can result in excessive background hum and noise levels.

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

    8.2 Decoupling

    This is another area of audio amplifier design that is often poorly understood by the home constructor, because its design principles are buried in the tube theory textbooks.

    Design formula courtesy Radiotron Designers Handbook 3rd edition 1940 Chapter 4

    In essence it works like this:

    When two circuits operating at the same frequency have an impedance common to both, there is coupling between them, and the phase relationships may be such that the coupling is either regenerative or degenerative. In the former case, instability may result.

    If the gain from one stage to the next is more than unity, then positive feedback can occur through back EMF in the B+ supply.

    However it is also the case that in a cascade coupled amplifier the output from each stage is usually of opposite AC polarity - ie "out of phase" - to its previous driving stage. Therefore, the textbooks tell us we can have up to two stages feeding from the same point in the B+ supply because the input of the first stage is "out of phase" to the input of the second.

    But if we add a third stage, the input of the first stage is synchronised with the output from the third, so positive feedback is likely through the B+ supply.

    The usual approach is to instal a cascode coupled decoupling resistor between each stage - or two of the three - the resistor having a sufficiently high value to "decouple" the two circuits from each other.

    Another method is to use separate decoupling RC network to each stage from a common B+ source, in a similar manner to "star" earthing. But in this case more parts are required. The decoupling resistors need to also have adequate value to achieve the desired result, so the same voltage drop problem remains as with cascade coupling - only .

    F. Langford-Smith recommends the decoupling resistor - or choke having adequate equivalent impedance at the amplified frequencies - should have a value of about one fifth the value of plate resistor(s) supplied.

    Chokes offer low DC voltage drop and improved regulation in the B+ supply. Resistors cause voltage drop when current flows through the B+ supply (not really an issue with Class A drivers but becomes a problem if the push-pull drivers are driven into Class AB by high signal voltages).

    In choke decoupled circuits - like the AWA A515 and some Williamson variants - if the amplifier is intended to reproduce very low frequencies the inductance of the choke needs to be quite high. Note too that when chokes are in series their inductance is added together, so there is some benefit in series connected choke decoupling configurations. To prevent unwanted emf pulses or spikes floating about the B+ supply it is essential for a high level of stability to use chokes having identical physical and electrical characteristics.

    One disadvantage with chokes is that if a driven stage B+ voltage falls below its driving stage B+ voltage then some B+ supply current will flow back to the driven stage, thereby reducing the applied plate voltage to the first driver. This phenomena could result in spurious modulation of the signal, heard as distortion.

    It is very important to use B+ supply bypass capacitors having a value sufficiently high to ensure there is negligible impedance at the lowest frequency to be amplified AND to provide adequate current to the plates during transient peak signals.

    One way to ensure good stability in the supply is to ensure the value of the bypass capacitor - which forms the AC signal return path to the Cathode - is higher in each stage than the value of the bypass capacitor in its driving stage. In this case, although we are dealing with AC signal,  the value of the tube plate resistance and cathode resistor and plate resistor and following grid resistor do not matter because the issue is the impedance between B+ supply and ground.

    Another way of explaining this is that the supply end to each stage - usually the junction of the plate resistor and B+ supply - should theoretically be at AC earth potential - ie the negative terminal of the B+ supply. Obviously if there is an impedance inserted between the supply end (nominal AC earth) of the true AC earth (chassis or ground) then there will be a voltage developed across that impedance and the system will not operate as intended. Since in a free system or conductor voltage will appear between positive and negative points, it follows that current will flow between those points - in this case through the supply to each driver stage.

    The answer is to use adequate values of decoupling resistor or choke AND high values of bypass capacitor.

    Note however that high values of capacitor require more current to fill them with charge and take longer to recharge. Also high values of decoupling resistor may result in adequate plate voltage to the voltage amplifying driver stages.

    The solution is to think it through and proceed with care.


    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 transformers close together - eg side by side. If this is not practicable mount the transformer cores at right angles to each other to minimise magnetic interaction
  • do not mount power transformers near signal leads or unenclosed components such as a magnetic pickup cartridge - in the case of pre-amplifiers this may mean a physical separation of a couple of feet - ie half a metre. Generally speaking, the larger the transformer the further the magnetic field will extend.
  • 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. To prevent dry joints (imperfect metallurgical joint bond) when soldering components 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. Take care not to overheat plastic covered components.
  • 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 this then take your system to a qualified technician - you only have one life!!
  • always use wire having a cross-sectional area adequate for the current to be carried. To minimise voltage drop and wire heating effects, it is desirable that Heater wiring should be arranged so that each heater is wired directly to the source transformer, rather than sharing a common pair of wires for all tubes.
  • always twist Heater wiring together, to minimise the AC field around the wires.
  • always use 105 deg C rated wire. This produces a better looking result because the wire will not melt when soldering the conductor ends.
  • always try to use common wire colours for discrete circuit functions
  • when using filter capacitors in series, always bypass with equal value bleed/bypass/voltage equalising resistors. If resistors are not installed the capacitors will self-balance, which may result in one being subjected to excessive voltage leading to failure.
  • always use speaker cables having as high a cross-sectional area of conductor as possible (one Ohm of resistance in a 4 Ohm system - ie 0.5 Ohm in each conductor - produces a 25% loss of voltage and a 25% loss of power at the loudspeaker terminals)

  • Note: The above rules are not listed in any particular order.

  • .


    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 using 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.

    IMPORTANT: A solid state power supply with large filter capacitors is essential for BASS.


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

    The entire power supply is connected in parallel with the entire amplifier and in series with the output stage - ie between the Plates and Cathodes of the power tubes, so any characteristic of the power supply will be in series with the output and will therefore directly affect it.

    Second, the amplifier power output, and therefore transient response capabilities, are wholly dependent upon the power supply being able to deliver the required power at the required instant in time.

    Another way of expressing it is that the amplifier is actually a simple modulator of the power supply.

    It is easy to see that if the power supply was connected to the amplifier unconstrained, it would deliver its full power capability. Conversely, if the power supply is completely constrained, then no power will be supplied.

    What happens in practice is that the power output from the power supply is "modulated" or "regulated" by the audio signal.  Therefore an amplifier cannot perform better than the capability of its power supply.

    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 POWER,  comprising 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.

    A reduction in plate voltage from the B+ power supply will result in a reduction of power output and increase in distortion.

    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

    IMPORTANT: A solid state power supply with high value storage capacitors is essential for BASS.

    A filter choke may be omitted if hum is not audibly apparent.

    500 to 1,000 uF per 100 watts rms output is a good starting point.


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

    This rule also applies to BASS 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 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.

    However 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, ask the manufacturer to justify the circuit design parameters and component choices to you.

    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.





    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.




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