Oestex ©
GUITAR AMPS PAGE
SECTION 7: THE POWER AMPLIFIER:
DESIGNING for POWER and TONE
STEP 2: A REALITY CHECK
First we need to dispel a longstanding myth.
Musicians assume the internal workings of their amplifier are unique to other forms of amplifier, like hi-fi or PA – public address (voice)
ANY hi-fi or public address amplifier can be easily modified to suit the electric guitar.
This is exactly the situation in the 1940’s and 1950’s before dedicated commercial guitar amplifiers became available and affordable.
Amplifier manufacturers responded to calls for assistance from musicians wanting to tweak their PA so their guitar would sound better (to them).
The main differences between a hi-fi amplifier and a PA are appearance, frequency response, distortion, gain, component quality and price.
Hi-fi amplifiers tend to apply about 20 db of negative loop feedback from the loudspeaker, and PA amplifiers less feedback. However with open-backed cabinets as in guitar amps, spurious signals generated by overshoot in the loudspeaker will be fed back to the input resulting in a complex change in sound. Negative feedback also tends to dull the sharpness or crispness of the sound, as well as attenuating high-amplitude dynamic asymmetrical peaks. However it is common for commercial guitar amplifiers to apply negative feedback to attenuate hum and noise in the system.
Note: In the case of solid state amps, it is standard engineering design practice to incorporate massive levels of negative feedback throughout the system to reduce transistor noise and improve system stability.
A well designed electric guitar or bass amplifier with quality components and layout should not require any negative feedback.
Tube PA amplifiers have additional high gain stages in the preamp section to facilitate microphone input. However the input signal from an electric guitar is significantly more dynamic so a higher input headroom is required.
INTRO
To gain an understanding of the combination of art and engineering that blend together to become the electric guitar amplifier, it would be wise to begin at the beginning to discover where it all began and how the concept materialised into commercial designs.
The core premise is that sound amplification is sound amplification – irrespective of the source. It matters not if you are amplifying a microphone or a guitar or a bass or cassette tape or a CD or a vynil record, the process is the same.
What does vary is the detailed engineering design.
Guitar amplifier circuitry is conceptually based upon longstanding conventional hi-fi amplifier design approaches.
Bass guitar amplifiers are much closer to a hi-fi amplifier than might at first appear, because to reproduce the lowest octave the amplifier needs good design and good components.
Proven standard designs are then modified for guitar amplification to produce specific characteristics, such as higher power output, guitar specific tone control cut and boost frequencies, less negative feedback, reduced frequency response, lower spec transformers, higher gain, improved dynamic response, and different physical layout and presentation.
In fact the guitar amplifier is much more like the traditional tube “Public Address” or “PA” amplifiers than would first seem.
But neither a tube hi-fi amp or PA amp is suited to electric guitar without modification.
The object of this page is to describe some of the modifications that are known to be either adverse or beneficial.
For those DIY’ers who want to build your favourite existing commercial design then skip this page – you will not need it. But if you want to know what does what and why then read on.
The page is necessarily lengthy and detailed but I hope it will be of benefit to your DIY project.
For the purposes of this project and to simplify explanation, our starting point is the mid-1930’s when tube audio amplification was still fairly crude.
By studying what others have done before us we can save ourselves considerable time and energy by figuring out why they did what they did and what caused them to change their designs over time.
Important elements in the evolution of electronics are the improvement in components:
Filter Capacitors: Paper and electrolytic capacitor values, ratings and reliability limited advancement but have improved. In the old days filter capacitors were limited to expensive bulky oil-filled paper construction in metal cans.
Coupling Capacitors: Were originally made with oil or wax impregnated paper that leaked DC - but are now available in a range of materials offering quality and reliability.
Resistors: Carbon composition resistors were noisy, unreliable, had wide tolerance and limited operating temperature - but now offer low noise, reliability, close tolerance and high operating temperature.
Tube Sockets: Originally Bakelite sheet or moulded but now available in high quality micanol, ptfe or ceramic.
Transformers and Chokes: transformers and filter chokes used low quality core steel and poor quality coil impregnation so performance and reliability was poor. Coatings used to insulate winding wire offered limited temperature rise capability – but now high quality core steel is available and impregnation materials offer very high temperature rise ratings, allowing smaller transformers to be made for any given VA rating. Reliability and service life are now not a problem.
Rectifier: Solid state full-wave bridge (FWB) RECTIFIERS are now available at very low cost – less than a tube socket. Solid state RECTIFIERS offer negligible voltage drop under load so are ideal for a linear dynamic response. For any given DC voltage output, (FWB) RECTIFIERS allow a single winding transformer to be used having half the voltage required for a full-wave tube rectifier, enabling a lower cost smaller physical sized transformer. (FWB) RECTIFIERS also enable choke-input filters to be viable, offering even better voltage regulation.
Chassis: Aluminium was once extremely expensive but now is more affordable, enabling a lightweight chassis to be used. Aluminium is also essentially non-magnetic so offers protection against hum and inductive coupling. For the DIY constructor, aluminium is easy to fabricate and work with.
Note: There are always manufacturers who strive to be the best in the guitar amp industry at any one time. Their products will always excel in component quality. Note however that component quality and engineering circuit design success are not the same. Large scale amplifier manufacturers always source components directly from their manufacturer, but those components are often custom made and not available to the general public – so do your research before purchasing components.
IMPORTANT PRINCIPLES:
An amplifier uses DC (direct current) power to amplify (increase or make greater) an AC (alternating current) voltage signal then convert it to AC power to drive the loudspeaker. The DC power may be sourced from a battery or a generator or converted from the mains AC supply.
The guitar has two wires out and the loudspeaker two wires in. The signal voltage is AC and measured between the wires.
The amplifier has two input terminals. Voltage is measured between the terminals. The signal voltage is AC and measured between the two wires. If the first tube stage is not isolated via a capacitor or transformer the guitar pickup and connecting cable become part of the grid circuit. In that case the guitar pickup and cable also carry DC grid current.
The amplifier has two output terminals. Voltage is measured between the terminals. The signal voltage is AC and measured between the two wires. There is no DC in the output but, for convenience, output terminals are often marked red and black to indicate the polarity of the AC signal. The indicated polarity may not coincide with the actual signal polarity. Loudspeaker polarity is marked to assist phasing of two or more speakers and bears no relationship to the amplifier phase polarity.
Inside the amplifier chassis voltage is mostly measured between the grounded chassis and the live signal path, live components or live circuit points.
In tube amplifier technology the AC/DC system negative terminal is always ground. In most guitar amplifiers, for physical construction reasons the chassis forms a common negative terminal of the system circuit. Circuit negative should always be directly connected to both the chassis and to ground via the mains supply. However some portions of the circuit may vary from this for specific purposes.
The B+ supply to the tubes is at AC ground. Preamp, phase-splitter or phase-inverter, and driver tube load resistance/impedance is between the tube plate and the B+ rail shunted by any other circuit load such as the grid resistor of the following stage and/or bias adjusting network in a fixed bias stage. Tube AC signal output voltage is measured between the plate and ground.
WHEN IT ALL BEGAN – AUDIO AMPLIFIERS
Prior to WWII, sound amplification was crude.
Amplifiers were either for domestic home entertainment in devices such as radios and record players (gramophones), or for public address systems in applications such as halls, factories, warehouses, offices, churches, sports arenas, cinemas or concerts.
In the early years of radio and therefore audio – because we need an audio amplifier to hear it – radio receivers and audio amplifiers were transformer coupled. R/C coupling (resistor/capacitor) had not yet developed.
Typical of the time, the US Thordarson 1936 Model 12 Watt Triode Power Amplifier used transformer coupling throughout
A really useful compendium of vintage guitar amps is available at https://www.vintageaudioportal.com/vintage-guitar-amps.html. This site provides numerous examples of amplifier design and construction.
In May 1934 the UK “Wireless World” magazine published W.T. Cocking’s design for “A Push-pull Quality Amplifier”. It produced 4 Watts from a pair of push-pull triodes and required a balanced input – i.e. transformer input/phase inverter. This was state of the art in the UK at the time.
Then “Wireless World” of 23 December 1937 modified the power supply to operate at a higher B+ voltage and power output increased to 7 to 9 Watts
Then “Wireless World” of December 1943 published a redesign by Cocking that featured inclusion of an unbalanced input/phase splitter tube stage, power tubes changed to 6V6GT tetrodes and a power increase to 12 watts
This is an excellent amplifier and, as will be shown, formed the generic archetype for many other designs by others that followed.
The amplifier has a double PII filter and cathode bias.
The same generic circuit can be used with power tubes operating at up to 750V DC. Where multiple pairs of power tubes are used it is advisable to include an extra cathode-follower driver stage after the push-pull triode drivers as shown in the above circuit.
In later variants the use of fixed bias allows larger more powerful output tubes to be used in circuits delivering up to 100 Watts rms.
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Then in 1947 the Williamson design was published and quickly became the industry standard for an amplifier using triode connected KT66 tetrodes. The KT66 tube and the power and output transformers were expensive, but the results proved to be commercially extremely successful for those manufacturers who made initially components then later complete amplifiers. This original design produces twice the power of the pre-war Cocking “Quality Amplifier”.
The Williamson has an extra voltage amplifier stage at the input and uses direct coupling to the phase-splitter. (not recommended for reasons explained later)
This amplifier remains one of the world’s most highly regarded hi-fi amps and has spawned numerous variants.
Later in 1947 the new concept was tested in the AWA Australia Laboratories, under the supervision of F. Langford-Smith, the editor of the Radiotron Designers Handbook.
AWA/AWV made RCA Radiotron tubes in Australia under license from RCA.
The amplifier used Radiotron 807 tubes instead of KT66 and was designated the AWA A515 Amplifier.
This amplifier proved the 807 tube was a suitable replacement for the KT66.
The basic Williamson design was extrapolated later by MOV/GEC UK to use beam power tubes in ultra-linear and pentode mode for power outputs up to 50 Watts rms.
For further details of the evolution of UK amplifiers see https://oestex.com/tubes/williamson.htm and https://dalmura.com.au/static/The%20Williamson%20Amplifier%20History.pdf
November 1949 saw the US version of the Williamson in the “Musician’s Amplifier” by Sarser and Sprinkle, using 807 tubes instead of KT66.
Then in 1949 the amazing US McIntosh “Unity Coupled” design was released using 6L6GB/5881 tubes This amplifier offered super performance for audio but was expensive to manufacture because of the stringent requirements for the output transformer
This amplifier was further developed in various models up to 350W rms. The mighty MC3500 and MI350 tube amplifiers remain one of the world’s most sought after amplifiers.
For further details of the evolution of McIntosh tube amplifiers see https://oestex.com/tubes/Mac/Macintosh.html
In 1952, Sarser and Sprinkle upgraded the “Musician’s Amplifier” to use 6146 tubes to deliver 90 Watts rms with low distortion. It was renamed “The Maestro Amplifier”.
The 6146 was one of RCA’s flagship tube types of all time and has been widely used in audio and transmitting applications. It is extremely reliable when used within its ratings.
Also in 1952, Acrosound in the USA developed the “ultra-linear” output transformer. The Acrosound/Dynaco amplifier offered excellent sound quality at an affordable price, delivering triode tone with pentode’s power. The Acro design was another great leap forward in tube amplifier design The rest is history. This version uses cathode bias.
s.
1955 and back in the UK, Mullard introduced the Mullard 5/20 ultra-linear amplifier using EL34/6CA7 tubes. The rated power of 20 Watts was well under its ultimate practical capability of around 35 Watts.
In guitar amps it would be rated at 40 Watts.
The Mullard design differed from those of Cocking, Williamson, GEC and Dynaco insofaras it used the Schmitt/long-tailed pair phase-inverter, later adopted by Marshall and Fender guitar amplifiers as their standard configuration and Gibson/Epiphone in some models.. The Mullard 5/20 also used the EF86 Pentode, another Philips/Mullard tube of choice for audio. See also https://www.aikenamps.com/index.php/designing-long-tail-pairs-the-load-line-approach
The EL34 tube was Philips/Mullard’s flagship audio tube, so naturally it and its little brother, the EL84/6BQ5 were the promoted tubes of choice for UK customers. The EL34 had been around a long time and had proven itself to be reliable when used within its limits. Note the KT66 and KT88 were GEC tubes so competed with the EL34 in the European audio market.
The EL34 is a small bottle tube, similar to but taller than a 6V6GT, and takes up much less space on a chassis than the KT66, KT88 or 6550.
This amplifier can be used in straight pentode mode by supplying the screen grids from the B+ supply instead of taps in the output transformer..
1957 saw the British LEAK TL50 PLUS with KT88 or 6550 tubes for 50 Watts output. This amp is obviously a clone of the Mullard 5/20 but with larger power tubes.
In 1960 the Australian Magazine Radio Television and Hobbies published a variant design using a pair of KT88 tubes operating in Ultra-Linear mode to deliver 100 Watts rms.
This amplifier is a design like the Williamson and GEC 400W that feature a separate voltage-amplifier drive tube immediately before the power tubes. However the first stage voltage amplifier and phase-splitter, being direct coupled, can present problems relating to inadequate grid bias to both halves, so some fine tuning may be required for optimum results.
Some manufacturers wisely construct this stage as an RC coupled stage to prevent interaction between the two half tubes. This shown in the RTVH 100W design below.
The power supply in this amplifier is a low-cost option. For quality guitar or bass performance the BOTH B+ and bias power supplies should be a full-wave bridge with considerable supporting capacitance to prevent B+ sag under load.
(Note: In this schematic the bias rectifier filter caps are incorrectly drawn – the output voltage is negative volts DC).
This circuit can be used without the Ultra-linear transformer but reliability the Screen-grid voltage should be reduced to 300 VDC or less.
The power tubes can be KT88 or 6550 or any other equivalent.
For guitar or bass work the EF86 should be replaced with a triode – preferably 12AY7 for low noise and microphony.
AWA AUSTRALIA - AWA was Australia’s largest radio and electronic manufacturer until its demise in the 1980’s.
For more than 50 years from the 1930’s, AWA made many fine amplifiers for PA public address applications and broadcast studio monitors. The introduction of transistors saw tube amps superseded by the new technology.
The AWA PA100B 100W rms unit used a single pair of KT88 tubes in ultra-linear configuration. Note the cathodyne phase-splitter.
The AWA PA1002 50W rms unit used a single pair of 7027A tubes in unity coupled output configuration.
The PA1001 series, another example of an AWA 100W rms design, is shown here from the early 1960’s.
The 2 x KT88 were replaced by 4 x 7027A
This model pushed the 7027A’s to the limit with 520VDC Plate voltage and 510VDC Screen Grid voltage – similar to 100W guitar amps. Note the cathodyne phase-splitter.
This design uses a unity coupled output transformer – similar to but different to the McIntosh design - but can easily be modified to use a standard output stage.
Click here for a better quality schematic.
PHILIPS AUSTRALIA produced a range of 30 to 100W PA amplifiers during the 1960’s. These amps used either one, two or three pairs the EL36/6CM5 power tube - a low cost tube used in B and W TV’s. These amps were very reliable due to the low Screen Grid voltage – in this case 50% of the Plate voltage..
The amps are simple and low cost.
AMPEG SVT AND V4B
In 1969 Ampeg produced the SVT and V9 amplifiers.
The first models used six 6146B/8298 tubes but issues were experienced with instability.
The second model SVT and V9 released in 1970 used 6 x 6550 tubes.
IMHO this amplifier was designed by an experienced RF Engineer.
The main power amp section and power supply carry the hallmarks of that genre.
Where multiple pairs of power tubes are used it is advisable to include an extra cathode-follower driver stage after the push-pull triode drivers as shown in the GEC 400W circuit, the 1969 Ampeg SVT and V9 with 6146 tubes and the 1970 Ampeg SVT and V9 with 6550 tubes.
Note: Some good advice re the 6146 tubes in the SVT – DO NOT use 6146 or 6146A – only 6146B or 6146W –
The cathode-follower driver stage to the power tubes offers less than unity voltage gain from the preceding stage. One solution is to delete it and use a standard plate follower, as is the case in the RTVH 100W circuit below. But this too has its disadvantages because the power tube grid circuit presents a low impedance load, the voltage output from a plate follower will reduce proportionately to the reduction in load – just when you need more volts.
It is important to note that inserting cathode-follower drive tubes to the power tubes takes us right back to the early years when this method was proven to be the best way to go.
In the current era it is also the method widely used for high-powered solid state amplifiers.
Having worked on these amps I believe the primary issue causing instability was layout and wiring.
Another issue is that the filtering to the Screen Grid of the power tubes is not adequate. The actual filter cap supplies an early stage 12BH7 tube at points E and H, so exposes that tube to the PS full ripple voltage, causing hum.
Note, output power collected by the power tube Screen Grids is also directly back into the cathode follower driver and the 12BH7 – all hanging off a single 40uF filter capacitor..
Like other big amps, the amplifier has the power and output transformers at opposite ends of the chassis to balance the weight evenly. This cause the plate leads to be very long. They also pass in close proximity to sensitive sockets on the rear panel and preamp tubes.
The smart approach is to rewire the amp using shielded coax where possible and to minimise lead length.
THE ELECTRIC GUITAR AMPLIFIER
IMPORTANT:
Before reading further please watch these You Tube videos to gain an understanding of what we are trying to achieve from the amplifier.
Fender Vox Marshall The SOUND and STORY
The Best 11 Fender Amplifiers of All Time
History of Marshall | Play It Loud
History of Dumble Amplifiers – Part One About Dumbles Dumble on Tubes v Transistors
What these and similar presentations reveal to us is that the “magic” sound that’s in our head - and we all never quite attain - is actually illusional because it will vary depending upon venue characteristics, outdoors or indoors, loudness, category of music, loudspeaker and cabinet, the actual guitar used for the test, playing style, and of course personal preference.
The video reports and schematics above describe how both Fender and Marshall experimented for many years until they obtained the sound that guitarists of the time were looking for.
GIBSON
During the early 1930’s various designs of the electric guitar pickup were developed. For simplicity and convenience in this story we can assume the Gibson electric guitar pickup to be the first generally acknowledged commercial application.
In 1935 the Gibson Guitar Corporation introduced the “bar pickup” for its new line of Hawaiian steel guitars.
The electric pickup opened the door for the development of amplifiers dedicated to amplifying the electric pickup signal.
In the same year Gibson released the E-150 amplifier. This basic unit had 2 inputs but no volume or tone controls. This amp had just 4 tubes and offered 8 Watts output from a push-pull pair of 6N6 (metal can) twin triode tubes operating in Class B. The output transformer was mounted on the field-coil speaker. Transformer coupled with zero bias. The 6N6 was released mid 1936. The 6N6G was released 1938.
E-150 Schematic
1935-1936 introduced the revised EH-150 model, which was a minor revision.
In 1936 Gibson introduced the ES-150, its first electric Spanish style electric guitar.
https://www.youtube.com/watch?v=IKcmKTd5eM0
https://en.wikipedia.org/wiki/Gibson_ES-150
Next, in mid 1937, Gibson introduced the EH-150 electric Hawaiian steel guitar.
The 1937 EH-150 amplifier saw a complete redesign.
This unit had one microphone input, two guitar inputs, separate volume controls, an EQ selector switch and a double PII filter in the power supply. It also featured a 12 inch field-coil speaker. The cabinet was restyled to have rounded corners at the top.
This amp had six tubes and offered 8 Watts output from a push-pull pair of 6N6 twin triode tubes operating in Class B. Transformer coupled with zero bias. The 6N6 (metal can) was released mid 1936. The 6N6G (glass bottle) was released 1938.
EH-150 Schematic – 6N6 Model 1936 (This non-original schematic shows an output transformer but the photos show a field-coil unit)
Gibson’s 1937 Catalogue introduced the completely redesigned and upgraded third series EH-150. This model continued until 1941.
This amp had seven tubes and offered 20 Watts output from a push-pull pair of 6L6 (metal can) beam power tubes operating in Class AB with cathode bias, driven by a 6N7 twin power triode with RC coupling.
EH-150 Schematic – 6L6 Model 1937
Significantly, the EH-150 changed the cathodyne phase splitter to the Schmitt phase-inverter. The Epiphone Bass Amp range also used the Schmitt system.
However over the ensuing years Gibson used both systems from time to time across their range.
It may be the success of Gibson amps that led to Fender adopting the same system about ten years later.
All the EH-150 amps used field-coil speakers with a PII filter for low hum.
Epiphone Bass amps also used the cathodyne phase-splitter circuit throughout their range.
FENDER USA
For a comprehensive history see https://gitec-forum.de/wp-content/uploads/bassman_history_en.pdf and
https://www.fender.com/articles/parts-and-accessories/going-low-the-history-of-the-bassman
The Fender amplifier business was established in about 1946 and by 1950 was doing well.
The first generation of amps are referenced hereunder to show the rapid changes in design as the company developed field experience and refined the products.
The Fender Bassman 5A6 was introduced in 1952.
The next version was the Bassman 5B6 introduced in 1953.
The 6SJ7 Pentodes were replaced with 12AX7 triodes because the 6SJ7 is noisy and microphonic, so being right next to the speaker tended to rattle a bit. A twin miniature triode is cheaper and requires only one socket and wiring instead of two octal sockets.
The 6N7 twin triode is a power tube, enabling the 6L6’s to be pushed more into the Class B region.
On the other hand the 5A6 had a small open backed cabinet so would not have been a particularly efficient bass reproducer – but they were early days in the story.
Next came the Pro Amp 5C5
The next incremental design change occurred with the Pro Amp 5D5. This design still used cathode bias.
Rock and roll evolved during the early 1950’s, creating demand for a different sound and more volume.
Next came the Bassman 5D6 introduced in 1954. The “presence” control was first used in this model, then all Fender amps.
Fox Vintage Amps say (quote) “No schematic for the 5D6 circuit have ever been found, and only 5 of these early examples have been found to have survived. The earliest serial number known to still exist is 0013, but 0032, 0075, 0077, and 0780 are also known to exist”.
The 5D6 model introduced the self-balancing paraphrase phase-inverter into the Bassman range.
The Fender Pro 5E5A amp appears to be the first model that used fixed bias to the power tubes, dramatically increasing power output and dynamic response but still used the cathodyne phase-splitter to directly drive the power tubes. It had one rectifier tube and 40 uF total of filter caps.
Next came the Bassman 5E6 introduced in 1956.
This model introduced the cathodyne phase splitter and fixed bias into the Bassman range in a similar configuration to the Fender Pro 5E5A amp. It also stayed with 12AY7 low noise preamp tubes. It also used dual rectifier tubes and 80 uF total of filter caps for more grunt.
Next came the Bassman 5F6 Tweed introduced in 1957.
Only five years had passed for the Fender Bassman models 5F6 and 5F6A to have evolved from the first simple Bassman – the 5A6 from 1952.
This model represented a great leap forward in guitar amp technology. It used 12AY7 low-noise tubes for the preamp stage and 5881 power tubes. The 5881 was developed as a military version of the 6L6G for use in B52 bombers, so was ideal for rugged use in bass amps. This amp had one 83 rectifier and 68 uF total of filter caps.
This model introduced the Schmitt Phase-invertor, well established in European electronic engineering. This design has remained with Fender amps to this day as their standard configuration.
Fender say that (quote) “It was the circuitry of this amp that was copied in London in 1962 by Jim Marshall and Ken Bran as the basis for the first Marshall guitar amplifiers”.
However the Schmitt phase-inverter was definitely a European invention and had been used in the Mullard 20W and Leak TL25A and TL50 and Western Electric and Westrex UK amplifiers (among others) since 1947. Mullard had been owned by Philips Holland since the 1920’s.
Next came the Bassman 5F6A Tweed introduced 1958-1960.
Fender say: “many consider the 5F6-A to be one of the greatest guitar amplifiers of all time”
This amp had one GZ34/5AR4 rectifier and 88 uF total of filter caps – a little more grunt than the 5F6. The power transformer and B+ voltage remained unchanged.
We can reasonably assume that the 5F6A Bassman and the 5E7 Bandmaster resulted from many years of design evolution and practical field experience.
These amplifiers represent state of the art tube based electric guitar amplifiers, so their design is worth studying.
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The Fender 5E7 Bandmaster is a great example of the art.
The Fender Twin 5F8 and 5F8A models simply added another pair of power tubes to double power output.
Later variants added Reverb and Vibrato.
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For more information on evolution of the Fender Bassman range see https://www.fender.com/articles/parts-and-accessories/going-low-the-history-of-the-bassman
Then by the late fifties surf music emerged, leading to demand for the reverb sound.
MARSHALL UK
In the linked video above, History of Marshall | Play It Loud , three aspects are relevant and important – first the test guitarists were British guitarists playing their own forms of music, and second the style of music was changing rapidly in England and third, US made tubes, amplifiers and speakers were horrendously expensive to import.
Mullard, GEC, MO Valve and Brimar made tubes locally, Celestion was an established loudspeaker manufacturer and there were several transformer manufacturers who produced high quality transformers – and the UK needed the business.
Fender say that (quote) “It was the circuitry of this amp that was copied in London in 1962 by Jim Marshall and Ken Bran as the basis for the first Marshall guitar amplifiers”.
It is now historical fact that the Marshall’s first commercial amplifier - the JTM45 - was modeled on the 1959 Fender Bassman Model 5F6-A.
MARSHALL V FENDER GENERIC DESIGNS
The schematics for both the 5F6-A and JTM45 are reproduced below for comparison.
These two amplifier represent the fork in the road for guitar amplifiers – each approach is similar but different.
The difference in “sound” or “tone” occurs from the choice of tubes and components – because the electrical design is identical.
However bear in mind that Gibson and Epiphone had long beforehand used the basic standard generic hi-fi amplifier design that both Fender and Marshall cloned.
All guitar amplifier manufacturers have striven to produce amplifiers that satisfy their customers, but commercial realities often prevent innovative ideas from being expressed in the final product.
However in the modern era with transistors and integrated circuits a whole new era of innovative gadgetry is now available as supplementary add-ons for guitarists to enhance their tube
amplifier with a wide range of tone and effects.
Check this out: The Strangers – Rockin Rebel - Here’s a surf music track in compressed MP3 from March 1959 played on a Fender Strat demonstrating distorted sound before Marshall existed. The original recording has substantially more top end. Crank it up !!
A detailed design analysis of the 5F6-A is described at Ampbooks
The classic 60’s “Fender sound” is demonstrated here. Cliff Parker playing lead guitar and solo.
Another example is The Atlantics from 1963. Bombora and The Crusher.
DUMBLE
Since Dumble amps are well covered in You Tube videos and are based on the generic Fender/Marshall designs, for our purpose we do not need to try to analyse them.
For some Dumble schematics see https://el34world.com/charts/Schematics/files/dumble/Dumble_Schematics.htm and https://schematicheaven.net/dumble.html
Importantly when making comparisons and drawing conclusions, we need to consider that Dumble used the Fender/Marshall basic power amp design with the Schmitt phase-splitter in these units, so to discover why Dumble amps are what they are claimed to be we need to look deeper into the componentry.
The above references form our starting point for electric guitar amp design.
CONCLUSIONS:
We can conclude from all the above examples from the evolution of amplifiers that there are two basic pathways to follow.
The first is designs using the Cathodyne phase inverter.
The second is designs using the Schmitt/Long-tail pair phase-splitter.
Other phase-splitter/phase-inverting systems have been used over the years but these two are by far the most common.
If we go back to the hi-fi amp origins it is clear the Cathodyne phase inverter is superior in sound quality, so one can draw one’s own conclusions as to why the Schmitt design has been used so widely in guitar amps.
The Cathodyne offers permanent exact equality of signal to each leg of the push-pull pair of power tubes. It is adequate to drive miniature 9 pin tubes like the EL84/6BQ5, 6CZ5/6973, etc. or octal tubes like the 6V6GT and 6L6G etc. when B+ voltage does not exceed about 300 VDC. But at higher B+ or with large power tubes or when operating in Class AB2 or B, an extra amplifying stage twin triode tube is required between the cathodyne and the power tubes to drive the power tubes - but that is easy and at low cost. A balancing circuit can be added after that stage to ensure any imbalance in drive signal can be easily adjusted back to signal balance. More importantly with tubes like EL34/6CA7 that draw grid current when pushed, the Schmitt phase-inverter will not provide adequate AC signal current to offset a positive change in grid bias. The consequence of that is that the power tubes will draw current from the fixed bias supply and lose bias, which in turn will cause the power tubes to draw excessive current until they self-destruct. That can in turn take out the output transformer as well as the power tubes. Power stages with multiple pairs suffer even faster and worse.
This video of AC/DC’s system proves the point.
The Australian “Goldentone” 1756/1760 models successfully used the cathodyne phase splitter system to directly drive a pair of 6DQ6A tubes (receiving tube version of the 6146). The 6DQ6A is a TV sweep tube. That class of tube requires a low voltage screen grid supply, so it would be cheaper to add the extra driver stage as recommended above.
Note: Critics of the cathodyne system claim the difference between the plate output impedance and cathode output impedance produce unequal output signals due to the different circuit capacitance and power tube grid current if the amp is driven into Class AB2 or Class B, but this possibility is obviously overcome by the insertion of a driver tube between it and the power tubes.
POWER AMPLIFIER DESIGN
This section uses a systems approach to the design of a guitar amplifier.
The reason for this approach is simple – each stage of an amplifier should be considered as a separate stand-alone discrete building block in the chain of serially connected (cascading) single stage processes from input to output.
That is to say the output of each stage is supplied to the next stage in the sequence.
The starting point is to design each stage as an isolated self-contained step in the amplification process.
Commence with the power output stage (power tubes and output transformer)
Then the phase-splitter/phase inverter stage(s)
Then the voltage amplifier stage(s)
Then, if required, the preamp stage(s)
When all of that is done and AC and DC power requirements calculated or estimated, the power supply unit can be designed and finalised.
But first some core decisions – these determine the grand plan.
STEP 1: DETERMINE WHAT KIND OF "SOUND" YOU WANT.
First, recall Section 3: THE LIVE ELECTRIC GUITAR REPRODUCTION AND AMPLIFICATION SYSTEM
It is obvious from that, that if we change any one of the individual system components the sound will change.
Jim Lill has produced some interesting videos that describe the sound of an electric guitar.
Where Does The Tone Come From In An Electric Guitar? https://www.youtube.com/watch?v=n02tImce3AE
The One Thing Every Influential Guitar Tone Has In Common https://www.youtube.com/watch?v=y8GiF-GVLgg
Where Does The Sustain Come From In An Electric Guitar? https://www.youtube.com/watch?v=muVzwbkUUnM
Where Does The Tone Come From In An Electric Guitar String? https://www.youtube.com/watch?v=yiFcw-H5DN8
Fifteen Great Country Guitarists in 1 Minute https://www.youtube.com/watch?v=zY01V0m5lgM
My Twangey Guitar offers about 250 demos of electric guitar sound https://www.mytwangyguitar.com/twc/tubepress/
These demonstrations show how the amplifier itself is just one link in a chain of processes that produce the sound we want to hear.
If we start with the amplifier as our core building block then all other system elements become after the event and need to be selected or modified to match the sound the amplifier produces.
If we leave the amplifier choice to last then the system is in place excepting for the amplifier. In that case we need to modify the amplifier to produce the sound we want from the system.
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, or to enhance or offset characteristics of individual system components.
However "sound" or "tone" are subjective terms and will vary in conceptual meaning between individuals. The sound you hear is not the same sound as another listener hears.
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 basic generic choices typically used for guitar amplifiers include:
POWER TUBE TYPES 10 to 40 Watts rms Class A or AB push-pull pair:
. Triode (includes tetrodes, pentodes and beam tubes connected as triodes)
POWER TUBE TYPES 10 to 100 Watts rms Class A or AB push-pull pair:
. Tetrode
· Pentode
· Beam power tube (choice of aligned or non-aligned grids)
· Ultra-linear / Distributed Load connection using Tetrode, Pentode or Beam Power Tubes.
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VOLTAGE AMPLIFIER TUBES:
.
. Single Triode
. Twin Triode
. Pentode
. Pentode + Triode
.
PHASE SPLITTER/INVERTER TUBES:
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. Single Triode
. Twin Triodes
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CONFIGURATION:
.
· Single-ended
· Parallel single-ended
· Push-pull
· Parallel push-pull
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CLASSES OF OPERATION
. Class A
. Class AB
. Class B
TUBE RECTIFIERS:
.
. Directly heated
. Indirectly heated
· Single or parallel RECTIFIERS
. Half-wave
. Full-wave
. Full-wave Bridge (Requires four half-wave tubes so is not really practicable)
SOLID STATE RECTIFIERS:
. Half-wave
. Full-wave
. Full-wave Bridge
. Half-wave Voltage Doubler
INTERSTAGE COUPLING CONFIGURATION:
· Transformer-coupled
· RC (Resistor/capacitor) coupled
· Direct coupled
· SRPP (Shunt Regulated Push-pull)
. Cathode-follower
.
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and yes there are also more exotic options available too!!
Tonal qualities will be vastly different for either single-ended or push-pull configurations, each of which offers unique tonal and distortion properties.
For guitar amplifier purposes we only need consider the most common tube classes – TRIODE, TETRODE, PENTODE OR BEAM POWER TUBE – and of course RECTIFIERS.
Note: There is a thriving industry for tube suppliers who serve those guitarists who believe they can readily discern the difference between this tube or that, or even tubes made in a particular batch or year by a particular manufacturer. The internet will give you numerous comparisons to assist your choice.
Tubes do sound different to one another for a range of reasons, including construction of the internal elements and materials used but ultimately it is what it is. It’s your money.
But at 100 db loudness difference disappears into the ethers.
For guitar amplifier design configurations we only need consider the most common – single ended or push-pull power stages and RC or direct coupled voltage amplifier stages.
Phase-inverters or phase-splitters also have their own variances in tone v configuration. Several common design configurations have been used in commercial guitar amps.
Note: My personal configuration preference is to use a twin triode to drive each power tube, with the phase-splitter stage located before it. This method ensures equal drive voltage to the power tubes and allows heavier triodes like the 9 pin miniature 6CG7 or octal 6SN7GT to be used to provide adequate drive power for the power tube grids. The 6CG7 is internally and electrically identical to the 6SN7GT.
When pushed into Class B, power tubes can draw significant Grid Current. Unless the preceding driver can deliver adequate AC signal driving current, their grid bias system will collapse, resulting in excessive Plate Current and extreme distortion. As the Plate voltage decreases under load, the Screen Grid may take over as the primary anode, overheat then fuse.
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.
For further reading see https://robrobinette.com/Voicing_an_Amp.htm
STEP 2. DETERMINE IF NEGATIVE FEEDBACK IS REQUIRED OR NOT REQUIRED.
Negative feedback offers advantages and disadvantages – see https://en.wikipedia.org/wiki/Ne.g.ative_feedback.
Its primary role in a guitar amplifier is to reduce harmonic distortion and intermodulation distortion, to reduce background hum and noise and to increase the damping factor for the output stage.
The primary benefit of not using negative feedback is that the dynamic performance of the system is at its best. In my experience, negative feedback takes the edge or sharpness off sound – i.e. dulls it.
If you examine the schematic circuit of a commercial guitar amp you will see some of the output voltage supplied to the speaker is taken from the loudspeaker is wired back to the front end of the amplifier as negative feedback. The idea is that any distortion in the loudspeaker circuit – i.e. through the speaker and output transformer secondary winding and interconnecting cables, will be cancelled out when offset against the opposite phase input signal.
The problem with this longstanding design concept, scientifically proven by countless audio engineers over the past 80 or so years to be beneficial for hi-fi systems, is that is also introduces a very slight time delay into the signal, such that half-cycle asymmetrical peaks are very slightly out of step. So we experience two notes (or if you prefer – one and a bit) instead of one. Although the delay may be in microseconds, the musician can hear it.
The practical effect is that a single high-amplitude asymmetrical peak becomes spread over a slightly longer time period so becomes a less peaky or sharp sound.
In addition, any unwanted effects that appear in the output circuit, such as back-EMF from the speaker, transformer waveform variances, transformer instability or ringing, will be fed back into the input and continuously recycled through the amplifier like a fading echo.
Another disadvantage is that although the transformer does not reproduce all frequencies evenly, the feedback network applies a fixed voltage feedback ratio to all frequencies. However because the loudspeaker presents a varying load impedance to the transformer it follows that the reflected impedance back to the power tubes will also vary in direct proportion. Consequently, because the power output is not linear across the frequency range it follows that when playing a chord of six notes simultaneously, the negative feedback applied to each note will be different. – some more, some less. This detracts from the sound.
One method of dealing with this issue is to use a separate feedback winding on the output transformer secondary to provide a more linear feedback voltage. However any back EMF from the loudspeaker will be transformed to that winding because the speaker winding and feedback winding are on the same transformer core. Some manufacturers do this.
Adverse effects from negative feedback are worse when playing a chord.
DAMPING FACTOR:
In the case of open-backed cabinet speaker systems, such as in combo amps, the speaker is not constrained from vibrating in and out to do its work. This creates a very responsive sound system.
When negative feedback is applied the speaker is constrained or inhibited electronically by the Damping Factor, so its excursions are reduced, producing a reduction in dynamic responsiveness.
Speakers designed for open-back cabinets have tight suspensions to limit cone excursion, thereby offsetting the loss of damping factor.
My recommendation is avoid using it - except within the output stage in the form of ultra-linear connection.
Excessive negative feedback causes instability. To remove the risk of instability, NEVER apply negative feedback across multiple stages.
Negative feedback also robs the amplifier of gain.
UNCONVINCED ?
Take a typical guitar amp, play a hard chord with and without negative feedback. Hear the difference.
Negative feedback is the commercial solution to reducing hum and noise in audio equipment.
If the negative feedback loop is disconnected to improve the sound, the residual background hum and noise may be intolerably loud.
In this case simply increase the value of the feedback resistor until the hum and noise becomes tolerable.
If that approach does not solve the issue then it will be necessary to discover the source of the hum and noise. It may be emanating from multiple sources. They will need to be eliminated one by one.
On the other hand, if you are designing and building your own amplifier then design for low hum and noise
Solid state guitar amplifiers inherently incorporate high levels of negative feedback. This is why they sound sterile.
For further reading see “Output Transformer and Voice Coil Flyback Voltage Spikes”
STEP 3: MATCH THE AMPLIFIER OUTPUT LOAD IMPEDANCE TO THE LOUDSPEAKER IMPEDANCE
This step is quite critical because it will have great effect upon performance and sound properties.
IMPORTANT NOTE:
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 reactive loudspeaker to convert the electrical power into acoustic power.
It does not matter if you build you amplifier before you buy your loudspeakers or the other way around, so long as they match for load impedance and adequate power rating.
Fortunately some tube amplifier output transformers offer multi-tap secondary (speaker) windings to give us more options.
Transformers are expensive components so it is better to opt for a multi-tapped unit where possible to avoid problems in the future is you want to change speakers.
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3.1 EFFECT OF LOUDSPEAKER LOAD ON AMPLIFIER POWER OUTPUT AND SPL (SOUND PRESSURE LEVEL = LOUDNESS)
Dynamic loudspeakers do not present only resistive loads - they are also reactive - capacitive and/or inductive. Therefore what is claimed by laboratory test bears little resemblance to real world application and performance of the amplifier – especially with your 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 between 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. The thick line is the frequency response and the thin line is the corresponding actual impedance. Actual impedance is shown in the right hand scale. The left hand scale shows the SPL – Sound Pressure Level or Loudness at 1 metre from the driver face.
In this example of a typical 8 Ohm speaker designed for guitar, the minimum impedance is 7 Ohms at 200-350 Hz and about 12 Ohms at 120 and 700 Hz.
The graph shows us that the impedance at resonance for an 8 ohm nominal typical guitar style loudspeaker - in this case at about 50 Hz is about 150 Ohms.
This represents a real change in actual impedance of more than 20 times the minimum –with a corresponding inverse power loss of about twenty times.
Fortunately in this example with guitar, the bottom E at 82 Hz impedance coincides approximately at 18 Ohms rating and continues within 5 db relatively constant through to top E at 7-800Hz. That equates to a flat response to the first harmonic across the first 12 frets of the guitar
Typical real impedance values for all sizes of loudspeakers across the guitar frequency 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 guitar applications we need to consider the frequency range 40-2,000 Hz.
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 and up 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 = I squared R or E squared/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 real-time 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.
.
.
OPTIONS:
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:
3.2 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 - e.g. 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.
3.3 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 - e.g. 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 s inductance is proportionately increased. These 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 - e.g. connect an 8 ohm speaker to the 8 ohm transformer tap.
3.4 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 loudspeakers the transformer impedance ratio reduces to one sixteenth that required for conventional operation with two tubes and one loudspeaker.
e.g. 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 - i.e. 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 - i.e. 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
.
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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
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3.5 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 poor conductor because of additives used to improve solderability, however that characteristic is somewhat offset by its increased cross-sectional area compared to small diameter copper wire.
Note 2: Do not use aluminium wire because, on a size for size basis, it has substantially higher resistance than copper. It is also difficult to prevent high-resistance joints when terminating aluminium.
Note 3: For the record, pure silver has the same conductivity as high-conductivity copper. Brass is about 27% conductivity of copper and steel about 7%.
Note 4: In the case of Alternating Current, most of the current is carried in the outer skin of a circular, square or rectangular conductor, regardless of the material. This is why tubular busbars are used for high current applications in electric power switchyards. Unfortunately in the case of copper the manufacturing process creates a low-conductivity oxide film, that permeates into the surface about 0.1 mm deep. Thus in any given copper conductor, the outer 0.2 mm of the diameter is a high resistance oxidised material that does not carry current too well (and will get hot). Consequently it is better to use a larger diameter single strand conductor rather than a multi-stranded small diameter conductor. This comment applies to any electrical conductor, including transformers.
Note 5: For the budget conscious, another option is to use 200-400A automotive copper cables as used for battery/starter jumper applications as Loudspeaker cables.
For a theoretical look at connecting cables see this discussion from
STEP 4: SELECT THE TUBE COMPLEMENT
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.
4.1 Output (Power) 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 - i.e. 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
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Notwithstanding any other considerations, there is no doubt that for a "natural musical sound" the TRIODE is best for ALL stages in a guitar amplifier, including the power amplifier output stage.
Next comes the ULTRA-LINEAR option using TETRODES or BEAM POWER TUBES. Ultra-linear (Distributed Load) operation offers a tone midway between triode and BPT and is best for tube amplifier bass.
Note 1: Ultra-linear mode requires the screen grid to operate at the same voltage as the Plate. Check the tube ratings before committing because some power tubes have a low rated Screen Grid voltage so do not permit this configuration.
BEAM POWER TUBES offer the best performance for guitar. e.g. 6CM5/EL36, 6CZ5, 6V6GT, 6L6GC/5932, 5881/6098, 7027A, 7581A, 7868, 807
TETRODE operation is less effective for bass but is excellent for CLEAN guitar. e.g. KT66, KT88,
PENTODE operation is less effective for bass but is excellent for DIRTY guitar. e.g. 6M5, EL34/6CA7, EL37, 6BQ5/EL84/7189, 6550
Note: Pentodes are made with aligned or non-aligned Screen Grids. Aligned Screen Grids provide more efficient operation, less heating of the Screen Grid, improved reliability and better sound. Check before you buy.
Note 2: Unless supported by a solid power supply and suitable drive circuitry, the bass will be hollow, boomy and lack sustain. Because tetrodes and pentodes require output transformers having a high primary inductance, effective power output at low frequencies is likely to be inadequate.
A comprehensive list of tubes suitable for guitar amplifiers is shown here. Select and click to display
4.2 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)
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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.
WARNING: SOME TUBE TYPES ARE NOT MANUFACTURED WITH CONSISTENT TUBE ELEMENT ORIENTATION IN RELATION TO THEIR BASE PIN CONFIGURATION - THESE TUBES ARE NOT DESIRABLE FOR HORIZONTAL MOUNTING CONFIGURATIONS BECAUSE REPLACEMENT TUBES MAY HAVE A DIFFERENT ORIENTATION THAN THE ORIGINAL USED TO DETERMINE CHASSIS LAYOUT.
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 - e.g. 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.
4.3 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)
Guitar and Bass amplifiers often operate under severe conditions of vibration - just feel the floor of a stage!!!!.
TWIN TRIODES are common in Guitar Amps because you get two tubes in one bottle and one tube socket, thus saving space and cost. The most common Twin Triodes for voltage amplifier duty are 6CG7, 6SL7, 6SN7, 12AT7, 12AU7, 12AX7, 12AY7, 12AZ7. Most of these are available in industrial, military, SQ special quality and computer grades. Types with a “W” suffix in the type number are “ruggedised” for rough handling applications – like Guitar Amps.
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 6C4/EC90, 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 – e.g. 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.
Pentodes are used because they have significantly higher gain than triodes, however pentodes often suffer “microphony”, rattling, hiss and random noise.
This effect can be offset to some extent by fitting substantial rubber buffer feet between the amplifier case and the floor to absorb vibration.
Irrespective of the class of tube – i.e. 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 (grounded) 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.
STEP 5: CLASS OF OPERATION - OUTPUT TUBES
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 increased more negatively until the Plate Current decreases to, 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 - e.g. 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 - i.e. 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 - i.e. 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 satisfactory 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 and component life is maximised, because most of the time at home listening levels they will not be working very hard - i.e. 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 power tube 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 – usually ground - or else the signal will not be equally amplified by both power tubes.
In a fixed bias system the grid bias voltage is set to a negative value such that at zero signal the tube is close to Plate-Current cutoff. Maximum power output is achieved when the positive signal swing is close to but less than 0 V. The central, axis of the drive signal is grounded via the bias supply capacitor.
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 - i.e. some of the signal will be missing. This called “crossover distortion”.
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 AB or 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 - i.e. 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 - e.g. 30 to 60 Ohms 10W WW - 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 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 close to 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 - e.g. 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 - e.g. 100k Ohms + normal cathode bias resistor (with recommended grid-leak resistor - i.e. not direct coupled) to the cathode-follower tube, the maximum permissible plate voltage can be applied across the driver tube - e.g. 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 - e.g. 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:
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b) On classes of operation:
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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 amplifiers, 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.
STEP 6: DRIVER STAGE CONFIGURATION
This aspect of amplifier design offers the most creative among us the latitude to do whatever one fancies.
6.1 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 – i.e. 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 - e.g. if the grid bias is -40 VDC then be sure to instal a driver stage capable of supplying at least 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 that feature a separate voltage-amplifier drive tube immediately before the power tubes. However the first stage voltage amplifier and phase-splitter, being direct coupled, can present problems relating to inadequate grid bias to both halves, so some fine tuning may be required for optimum results. The same applies to the cathode-follower driver stage to the power tubes that offers negative voltage gain from the preceding stage. One solution is to delete it and use a standard plate follower, as is the case in the RTVH 100W circuit. (Note: In this schematic the bias rectifier filter capacitor is incorrectly drawn – the output should be shown negative volts DC)
Noting Class AB or B power stages must be driven into positive Grid bias, whatever phase-splitter or phase-inverter design is chosen it is essential that it deliver at least the nominal Grid 1 driving voltage for the particular power tube type – if not is not likely to produce a satisfactory end result.
The exception to the above is when the power tubes are operated in pure Class A. In that case 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 first stage input (there can be problems of adequately loading driver transformers in Class AB or B output stages), to provide push-pull voltage amplifying stages throughout the entire amplifier - i.e. 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 - (i.e. 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
6.2 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 - i.e. "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 - i.e. 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.
STEP 7: B+ OPERATING VOLTAGE
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 - i.e. 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 coupled with a high inductance Output Transformer are essential for strong deep BASS.
STEP 8: POWER SUPPLY
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 - i.e. 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 easiest way to understand this is to consider your motor vehicle on a cold winter’s morning. If the battery has adequate capacity and is in good shape it will deliver the current needed to start the vehicle. If the battery is too small, discharged or low it cannot do its job and may not have sufficient power to start the vehicle.
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 amplifier.
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
Recommend you study this illuminating video tutorial from xraytonyb - Increasing the Main Filter Cap in an Amplifier - What Happens?
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 but the ripple voltage will still modulate the output signal.
200 to 1,000 uF per 100 watts rms output is a good starting point.
Capacitors in series having equal capacitance values deliver an effective half of their total e.g 2 x 100uF = 50uF
If capacitors are connected in series they should each be shunted with a bleed resistor of sufficient value and wattage to handle about 10 mA. Mount the resistors away from the capacitors to prevent heat transfer.
To reduce temperature rise – i.e. cooler operation - I recommend the resistor wattage be at least twice the calculated heat dissipation.
STEP 9: PHASING
It is well known that multiple loudspeakers must be “phased”, which means that two or more loudspeakers must operate with the same polarity.
This simply means that all cones must move in and out simultaneously in unison - i.e. in at the same time and out at the same time.
Phasing is easily accomplished by connecting the speaker wires to a 9 volt battery and observing the cone direction.
A lesser voltage than 9 may be used but to prevent damage do not exceed 9 volts.
The lower the voltage the harder it is to see the cone s moving.
For engineering and commercial convenience and general ignorance on the subject, discussion of system phasing has not been discussed.
When a guitar string is plucked, it travels sideways backwards and forwards over the pickup to produce an alternating voltage (AC) signal.
The amplitude of the signal is dependent upon the amplitude of the string travel.
Also the voltage output from the bottom string is higher than the voltage output from the top string, with progressive increments in between.
Here’s the important bit:
There are two conventional ways to play a guitar – one is to pluck the strings with the fingers – classical guitar or (finger pickin’ style)
This method entails PULLING the string towards the player.
The other method is to play with a pick or the thumb.
This method entails PUSHING the string away from the player.
Does this matter ?
Yes it does – and the reason is this.
A guitar pickup is an electromagnetic transducer. It operates exactly the same way as a loudspeaker magnet and coil. In the case of the pickup the string equates to the speaker cone, while the pickup equates to the speaker voice coil and magnet. In fact small loudspeakers have often been used as microphones.
When the string moves across the magnetic field of the pickup it generates a voltage that has either a positive or negative alternation.
As the string vibrates across the magnetic field the voltage alternation changes polarity at rate determined by its frequency. The frequency is determined by the note or notes being played.
So what we are doing electronically is to create a signal voltage with a pickup and reproduce that same voltage with the loudspeaker. The amplifier is in between and is used solely to amplify the signal to create power to drive the loudspeaker.
Now a loudspeaker is conical shaped, which means that it is easier for it to travel backwards/inwards rather than forward/outwards.
Imagine a cone being pulled through water – it is easy. But try to push the cone forwards and it is difficult, requiring much more energy. The same applies to a loudspeaker.
When the speaker is mounted in an open-backed cabinet, there is little resistance to cone movement in either direction because the air pressure to both internal and external cone surfaces is roughly equal.
When the speaker is mounted in a sealed cabinet, the air pressure outside resists forward cone movement – but at the same time the forward cone movement increases suction or a reduction of air pressure internally.
On the other hand when the speaker cone moves inwards it is aided by the external air pressure. In other words, in an open-backed cabinet it is easier for the cone to move backward than forward.
A speaker mounted in an open-backed cabinet will sound more “alive” than when it is in a sealed or partially sealed cabinet.
This phenomenon is frequency dependent because as the frequency increases the sound waves radiate across the speaker cone rather than the push-pull pump action at lower frequencies.
Now if we look at the waveform of a guitar signal we see that the first half-cycle of the wave is asymmetrical - .i.e. has a high transient peak.
Our goal is to reproduce the asymmetrical peak waveform exactly.
If the first half cycle asymmetrical peak has a positive polarity, then the first stage will be driven positively. When the grid is driven positively the grid bias reduces (because the cathode is DC fixed positive to the grid) so the plate current increases so the plate to cathode voltage decreases so the output voltage decreases – that is how amplification works.
If the first half cycle asymmetrical peak has a negative polarity, then the first stage will be driven negatively. When the grid is driven negatively the grid bias increase so the plate current reduces so the plate to cathode voltage increases so the output voltage increases – that is how amplification works.
In a conventional voltage amplifying stage the grid bias of each tube is set about to halfway between the minimum plate current cutoff value and the maximum permissible plate current value for that tube type.
In the case of a cathode-bias system the grid will be set at 0 volts (ground) when there is zero signal. Zero signal DC grid bias is set by the DC cathode voltage being positive to the grid. When the signal voltage reaches 0 volts or more the grid will lose control and plate current can increase out of control, causing severe distortion. It follows that because the asymmetrical peak signal will be randomly supplied to the positive or negative half cycle, the value of the cathode bias should be as high as practicable.
The amplitude of the asymmetrical peak signal is limited to the cathode bias voltage. The tube will respond faster to a peak signal if there is no bypass capacitor on the cathode bias resistor.
Unfortunately when the grid bias reduces too far the tube will cut off and not amplify –i.e. it will chop off the top portion of the peak input voltage. Nowadays this is called “compression”.
So obviously to get the best result we need to polarise the pickup to suit the playing style – positive hot wire input to the tube grid for conventional plucking or finger pickin’ playing, or negative wire to the tube grid for standard pick playing.
So why can’t you detect this?
Because when playing at reasonable volume levels the grid will not be driven to cutoff and/or the portion of the asymmetrical peak removed will not be noticed.
In the case of very high sound levels when playing venues, the amplifier is likely to be pushed into Class AB2 or Class B, where the power tubes operate with positive grid bias. Class B operation means that each power tube responds ONLY to the positive signal cycle that causes positive grid bias change in response.
Now because each tube only reproduces one half cycle at a time, it follows that if the asymmetrical peak is out of synch with the power tube then that first asymmetrical peak will be lost.
That means we will not hear that vital first dynamic aspect to the note.
The conclusion is that to obtain best results, the power tube drive stage output needs to be synchronised or phased with the input signal as it appears at the guitar pickup terminals.
The polarity of the pickup needs to be set BEFORE any volume or tone controls in the guitar itself, because they are wired such that one side is deliberately grounded back to the amplifier via the shielded coax cable to prevent hum –i.e. it is not just a matter of reversing the input terminals at the phono socket in the amp or preamp.
Once the pickup polarity is correctly set then the power tube drive polarity may be set more easily.
Having said all that, it becomes apparent that the loudspeaker can still be out of synch with the pickup signal. So for best result the speaker should be phased also with the pickup. Phasing cannot be achieved just by reversing the speaker connections – phasing must align with the pickup itself.
In the case of the amplifier, some stages may be out of phase with the signal. The solution there is to ensure every stage can handle the highest level of asymmetrical peak signal that is possible within that stage.
To sum up, for best result the pickup output hot wire needs to be in positive polarity and the Class AB power driver stage output in positive polarity.
It becomes apparent from the above that if the pickup polarity is set for one style of playing it may still be unsuitable for the other. However all is not lost because finger picking style is not as aggressive as pick style so by setting the system to pick style the system will cope with the other.
The obvious easy solution to all of the above is to instal a changeover switch on the guitar such that pickup polarity can be switched at will.
In the standard system the player has a one in four chance of getting it right when phasing the pickup output with the loudspeaker – i.e. accurately reproducing the asymmetrical peaks.
Another method is to use a balanced input transformer with centre-tapped secondary at the amplifier input, supplied directly from the guitar pickup with a two core shielded coax cable and stereo plug and socket.
The system then becomes push-pull all the way through.
This system is common for microphone preamps and mixers.
The amplifier will then amplify the peak signal of either polarity. In that case the polarity of the loudspeaker does not matter so much. To be sure of the best arrangement the speaker can be tested either polarity termination using a guitar signal through the amplifier.
Volume and tone controls will need to be dual ganged potentiometers
The Westrex 2040-A amplifier shows how it is done. In this amplifier, gain (volume) is controlled by an external preamplifier.
Note the absence of a phase-splitter or phase-inverting stage. This stage is not needed because phase inversion is performed by centre-tapped secondary winding of the input transformer.
A simpler method is to feed the output from the centre-tapped input transformer to a twin triode first stage with their outputs mixed together. There are various methods of doing this simply.
The plates and cathodes of the tube can each be joined as pairs to form a single triode having two separate inputs.
The signal input triode should not be a 12AX7 but one that will tolerate a high grid bias voltage – at least minus two to three volts DC. The higher the better,
The reason is that an asymmetrical peak can be easily as high as 2 or 3 volts peak (half wave), or 4 to 6 volts peak to peak full wave.
One method of addressing the need for a higher than usual grid bias is to instal a positively grounded FWB rectifier and filter cap to the 6.3VAC heater winding of the power transformer – or if not used the 5VAC rectifier heater winding. The tube cathodes are grounded. Negative fixed bias can then be provided to the first stage tube and set to whatever bias voltage works best. A voltage dropping resistor and second filter cap are required. All other tube heaters are still supplied with AC as usual.
Pickup output voltage will vary depending upon the pickup output and any volume or mixing circuits attached to it.
A typical wave is shown at http://www.muzique.com/lab/pick.htm
For a detailed analysis of what happens see Rob Robinette’s excellent page at https://robrobinette.com/Tube_Guitar_Amp_Overdrive.htm#Asymmetric_Clipping
STEP 10: APPEARANCE
It is an axiom of good engineering practice that a component or complete device must look attractive.
A guitar amplifier must look attractive and professional.
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.
STEP 11: LISTEN TO THAT SWEET, CLEAR TUBE SOUND!!
These articles, courtesy of Electronics Australia, June 1983 edition, may cause you to reconsider what you are actually hearing!!
A NOTE OF INSPIRATION:
For those who want to be different and own and use tube amplifiers for their guitar - 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.
HAPPY CONSTRUCTING!!
MAY YOUR PROJECT BE A SUCCESS!!
REMEMBER - ALWAYS TAKE CARE WHEN WORKING WITH HIGH-VOLTAGE - DEATH IS PERMANENT!!
Contact: "electron"
Email: contact
This page is located at http://www.oestex.com/tubes/guitaramps/7_ampdesign.html
This page last modified 31 July 2023
