Oestex ©
GUITAR AMPS PAGE
SECTION 12: VOICING THE AMPLIFIER
Note: Links are underlined – select and click to open
One of the driving market forces in the music industry is “difference” or “uniqueness”. That is the demand for guitarists (and bands) to have their own unique sound compared to everyone else.
Young aspiring guitarists might like to emulate their performing idol so want the amplifier make and model used by their favourite performers on stage in live shows – price does not matter.
Some of these personal preferences may be rational, some subjective and some irrational.
Experienced guitarists may wish to upgrade to a “better” amplifier, or perhaps just more power or a different sound.
But after acquiring the amplifier of choice, many musicians find it is not quite what they want.
Perhaps it is simply a matter of insufficient funds to purchase the amplifier of choice.
So we can say that VOICING an AMPLIFIER is the art of modifying or adjusting the pre-existing amplifier circuitry to produce the best sound possible for our purpose.
Now that we have studied and implemented Section 11: Voicing the Guitar – and the guitar setup is right and we are happy with its sound, we can now focus on the voicing the amplifier.
This object of this page is to show you how to modify your amplifier to be more powerful and better sounding than ever before.
This page focuses on how to modify an existing amplifier to change tone.
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A WORD OF ADVICE:
Before proceeding to tear your favorite amplifier apart and rebuild it, it is recommended you watch the video presentation “How To Get a Good Guitar Tone” from guitar maestro Clint Curtis, who has some advice regarding “sound” and “tone”.
https://www.youtube.com/watch?v=cx8S80GWUJg
Fender Strat Tones: - in this video if you look carefully he uses all five settings of the pickup switch throughout the track as a technique for changing tone. https://www.youtube.com/watch?v=dT5RNnSJoFk
This example of the Leonard Cohen hit “Allelujah” demonstrates a blend of choice of instrument, pickup switching selection, tone settings, playing style, use of key change, manual vibrato, timing, arrangement, “effects”, the backing band and creativity. https://www.youtube.com/watch?v=mVNDNwVqipw
Examples of his work are displayed at https://www.youtube.com/@ClintCurtis/videos
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VOICING THE AMPLIFIER
REFERENCES:
Dr. Z AMA: "Voicing an amplifier" https://www.youtube.com/watch?v=3G2rq8JxqVc
Rob Robinette: “How to Voice a Tube Amplifier” https://robrobinette.com/Voicing_an_Amp.htm
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The reason for starting the “voicing” process with the guitar, is because it is the guitar – not the amplifier - that creates the sound we want to hear.
As has been shown in Section 3: The Live Electric Guitar Reproduction and Amplification System and the links above, the guitar itself is purposely built in an immensely wide range of tonal characteristics.
It is true modern solid state amplifiers fitted with inbuilt preamp effects, or independent effects pedals, can turn a low cost guitar into an effective instrument, but this website site is about tube amps – not hybrids or solid state models.
The design rationale for the reproduction SYSTEM is that the amplifier should deliver clean undistorted sound at normal playing loudness (SPL) levels, because the object is to faithfully reproduce the input signal at the required loudness level.
As has been shown in Section 3: The Live Electric Guitar Reproduction and Amplification System, the amplifier is the second last step in the chain of processes that create the sound we hear, so is not an effective or efficient way to modify the tone of the system.
Whatever signal the amplifier delivers to the loudspeaker fixes the electronic output from the reproduction system.
In essence, the amplifier is too late in the system to try to effect major tonal changes – unless you want to push it into “over-drive”, an inherently risky mode of operation.
Tubes are electro-mechanical devices, which means they have physical mechanical construction that is designed to permit voltages to be applied and manipulated to produce electrical “voltage gain” or electrical power output (watts). A loudspeaker is the usual electro-mechanical device used to convert the electrical power output from the amplifier into physical acoustic energy – i.e. sound waves.
However since the earliest guitar amplifiers, amplifiers have progressively evolved over time to include some standard voicing components and circuitry
These will be discussed below.
THE POWER SUPPLY
Some manufacturers deliberately design for “distortion”, however typical conventional tube amp circuitry produces in a natural increase in distortion as the power output increases. The difficulty with relying on this approach is that the distortion v power output ratio is not linear. Consequently to hear significant distortion the amplifier must be driven close to or exceeding its maximum output. That, unfortunately, is a recipe for self-destruction - or at least, excessive wearing or failure of one or more power tubes and rectifier tube.
This is well explained at https://www.youtube.com/watch?v=j5C7GKGxICg
To understand what happens refer to the following tube data sheet from Tung-Sol USA in 1960 for the 6L6GC Beam Power Tube – the tube of choice for many reputable guitar amp brands.
The graph shows the relationship between the important limiting values and performance.
In this example the Plate voltage and Screen Grid voltage are typical of 40-60W guitar amps. Note that when the Screen Grid voltage is the same or less than the Plate voltage the currents shown will be greater.
The first limiting factor is the Maximum Permissible Plate Dissipation and Screen Grid Dissipation – both coincide at 60 W power output.
Plate Dissipation is the difference in heat energy dissipated by the Plate electrode and is the difference between DC power in and AC audio power out. (AC Watts = DC Watts). When the heat dissipated exceeds the ratings you can expect to see the plate glow red hot and maybe melt through and the Screen Grid fuse.
Now at the 60 Watts output level the total distortion is around 2% - an acceptable level for guitar.
BUT – if we push the amp to 70 Watts output the whole situation changes.
The amplifier shifts into Class AB2, drawing some Grid current and changing the output wave shape. Due to the relatively high value of Grid No 1 resistor used the Grid bias will shift more positive, increasing Plate current and Grid current even further.
Distortion increases to about 8%.
Plate Dissipation decreases but Screen Dissipation INCREASES to around 8 Watts per tube, which is 60% over maximum permissible ratings – a sure recipe for disaster.
However despite the safe operating limit of 40 Watts power output, the guitarist seeking distortion will try to operate the amp in the range 40 to 70 Watts because tolerable distortion increases linearly – becoming more apparent as the amp shifts into Class AB2.
Not really a smart move.
One of the strategies that amplifier manufacturers introduced to offset this reduction in reliability and therefore increasing warranty claims for burned-out power and output transformers, was to increase the current handling capacity of power and output transformers – i.e. larger diameter wire - but unless transformer physical size is increased proportionately their efficiency reduces, resulting in actually less power output under load.
Some manufacturers used higher performing iron laminations in their transformers to minimise physical size but this approach increases manufacturing costs.
Some reduced the physical size of their output transformers by reducing primary inductance, which means a smaller transformer with less iron and less wire but still delivering the required frequency response and power output.
THE POWER SUPPLY IS THE MOST IMPORTANT SECTION OF AN AMPLIFIER.
This is because the power supply is the source of ALL power to the tubes, which regulate or modulate that power to suit the signal and loudness requirements.
SINCE THE ONLY FUNCTION OF THE AMPLIFIER IS TO REGULATE THE POWER SUPPLY INPUT AND CONVERT IT INTO AUDIO POWER CORRESPONDING TO THE SIGNAL INPUT, IT FOLLOWS THAT WHAT WE ACTUALLY HEAR THROUGH THE LOUDSPEAKER IS THE SOUND OF THE POWER SUPPLY.
LOUDNESS or VOLUME is controlled by varying the GAIN within the amplifier. System gain is measured as the RATIO of output voltage divided by input voltage = e.g. 30 volts AC to the speaker divided by 1 volt AC input = 30 times.
Since vacuum tubes are DIRECT CURRENT (DC) devices and mains power is supplied in ALTERNATING CURRENT (AC), a method of converting the power source from AC to DC is essential. This is the function of the power supply.
The conversion process is called RECTIFICATION.
“Rectification” requires devices designed for the purpose. Rectifiers are available in tube or solid state format.
Tube rectifiers are available in indirectly heated or directly heated formats.
Each class of device has its advantages and disadvantages.
History tells us that the golden era of tube guitar amps occurred in about the mid fifties to mid sixties – before affordable solid state rectifiers were available. Consequently all of the famous brands and models were produced with tube rectifiers.
However when silicon rectifiers became available in the mid sixties, coinciding with Japan’s entry into large scale electronics for radio and TV, the new power supply technologies and components naturally migrated to guitar amplifiers.
The change was not welcomed by all guitarists and many felt the solid state technology would not do what they were used to – particularly the way an amp would “break-up” when over-driven.
The current era introduction of Class D solid state amplifiers, particularly for bass guitar – has seen a resurgence of similar conflicting perceptions.
POWER SUPPLY MODIFICATIONS FOR AMPLIFIER VOICING
Before proceeding with that subject I recommend you study my paper on Power Supply design at https://www.oestex.com/tubes/power.html
What this tells us is that a strong power supply will deliver better quality sound and higher power, whereas a weak power supply will deliver poor quality sound and lower power.
What makes a power supply strong?
· Full-wave rectification
· Large filter capacitors – Minimum 50 uF per 50 watts of power output (Note: Check rectifier tube data sheet for limiting values)
· PII filter with one filter choke
· Two stage PII filter with two filter chokes
· Separate screen-grid supply
· Direct rectification – not half-wave voltage doubler system
· Adequate B+ decoupling between cascade stages
· Separate full-wave bias supply for power tube bias (fixed bias system), ideally with PII filter.
· Individual bias adjustment to each power tube
· If Cathode Bias – separate cathode resistors and bypass capacitors to even plate current
· Power transformer having headroom – say 20% over total maximum load condition over all windings
· High grade EI transformer iron with double leg construction
· Good regulation Note: A solid state rectifier will always deliver less voltage drop than a tube rectifier in the same circuit, because the tube has a significantly higher internal impedance
· Tube heaters may be supplied with DC or AC. DC offers low hum in preamp stages but does not affect tube noise or microphonics.
POWER SUPPLY VOLTAGE SAG
It is assumed the above performance graph for the 6L6GC is either a theoretical calculation or is performed using a constant voltage stable power supply having a higher power output than required for the tests.
A real-world commercial amplifier will not have a stable regulated power supply, resulting in a progressive reduction in plate and screen grid voltages as plate and screen current draw increase.
Reduced B+ DC power supply voltage and current produce a reduction in power output and increase in distortion.
This is because of power supply voltage sag, which occurs in both solid state and tube amplifiers and is caused by resistance/impedance in the power supply circuit. This is explained below.
Transient power requirement for peak signals is also affected by the size of electrolytic filter capacitors.
Capacitor size is limited to relatively low values for tube rectifiers – refer specific type data sheets for limiting values.
A tube rectifier of choice through the 1960’s was the directly heated 5U4G/5AS4A.
Other directly heated tube rectifiers offer similar performance characteristics.
To understand what happens refer to the following tube data sheet from RCA USA in 1950 for the 5U4G Full Wave Rectifier with Capacitor Filter – the tube of choice for many reputable guitar amp brands.
In this case the capacitor is 10 uF however 40 uF is common to reduce ripple (hum).
Referring to the 6L6GC graph above we see the Plate Volts are 450 VDC at zero signal where total DC current drawn by the amp will be about 130 mA. Transformer input will be about 450 VAC per Plate (900 VAC centre-tapped).
At 55 Watts rms output the total amplifier DC current will be about 250 mA. Distortion is about 2%. (Barely noticeable)
BUT the maximum rated current for a 5U4G rectifier is 275 mA at 450 VDC output, so at 55 Watts rms there is not much headroom left in the power supply.
Referring again to the 6L6GC graph above we can see that when the Grid Bias is the manufacturer’s specified -37VDC, the amplifier will shift into Class AB2 at 74Vpp signal drive voltage, equivalent to 62 Watts rms output power.
The graph tells us that this occurs only when the Plate Voltage is 450 VDC and Screen Grid Voltage 400 VDC.
However to maintain 450 VDC at 250 mA the input transformer supply voltage to the rectifier must increase to 630 VAC.
But this does not happen because the starting point is nowhere near that. So what happens is the DC voltage will drop from its starting point of at either zero input voltage – i.e. when the standby switch is “off”, or alternatively, at zero signal.
This characteristic is common to all tube rectifier supplied amplifiers power supplies.
Industrial power supplies used for high-powered PA tube amplifiers used regulated power supplies but that approach is costly and required considerable chassis space.
BUT there is more.
Common practice is to use the same B+ supply for both Plates and Screen Grids. The result is that both the Plate and Screen Grid will suffer voltage drop as they draw more current corresponding to increasing power output.
That situation reduces potential power output and increases distortion.
Consequently commercial guitar amplifiers do not usually specify power output ratings as a guaranteed performance standard.
What you get is what you get.
The conclusion is that for a clean undistorted sound a conservative power supply with headroom is needed. The power tube screen grids should be supplied from a choke input filter. This can be achieved by installing a choke after the plate supply filter capacitor an extra capacitor to create a PII filter to the screen grids. Better still, use a solid state full-wave-bridge rectifier.
Note that good quality amplifiers always incorporate a separate power supply for the screen grid to maintain a constant screen grid voltage, regardless of plate voltage fluctuations. Ideally the screen-grid voltage should not exceed 50% of the plate voltage with 40% being the preferred value.
For a distorted sound use a tube rectifier, capacitor input filter, minimise the filter capacitor value (subject to tolerable hum levels) and use a power transformer having a rated power output equivalent to or less than the maximum amplifier DC requirement.
Note the iron core of the output transformer can “saturate”, resulting in a reduction of power transfer to the secondary and increase in distortion. Excessive DC current through the primary may cause it to fuse and open circuit. A replacement transformer is then required.
For a real world explanation of power and distortion see https://www.youtube.com/watch?v=j5C7GKGxICg and https://www.youtube.com/watch?v=rNvOUmwPiUQ
The first principle is that the guitar amplifier is an “Audio Equaliser” – more specifically a “High Pass Shelving Filter”
This characteristic is required to attenuate (reduce) the electric guitar pickup voltage output from the low strings compared with the high strings.
Thus can be easily demonstrated by playing a guitar through a home stereo – it will sound very full but bassy and dull and not like you want it to be at all.
Various methods have been used to modify guitar amps over time but if we go back to the very beginning with the Gibson, it is obvious their amp designers knew about the importance of this feature.
The 1936 EH-100 unit is shown here: Gibson_EH_100_42_1936.pdf (The 42 tube is the predecessor of the 6V6)
This amplifier uses transformer interstage coupling and has only one capacitor (C2) in the signal path. This is a 0.02 uF unit. R7 and R10 are conventional values so the stage will be relatively full-range.
But then Gibson made an amazing discovery – shown in the EH-150 amplifier from the same year. Gibson_ EH-150_6N6_1936.pdf
This amplifier was one of the great milestones in amplifier evolution by introducing the concept of LF signal attenuation to offset the natural deep tone of the guitar, allowing the rich harmonics of the guitar to be heard clearly. In 1936, loudspeakers had very limited high frequency response so tended to produce a dull muffled sound.
This amplifier has an inbuilt automatic constant and non-adjustable EQ. The design is attributed to the joint efforts of Bandleader and steel guitarist Alvino Rey and Electrical Engineer John Kutilek of Lyon & Healy (renown Harp Makers)
The EQ effect is achieved by just one 0.0007 uF coupling capacitor, installed between the 6C7 and 6C5 tubes. In an earlier version of this model there is just this one capacitor, but this model added a switch to select “bright” (lead) and “normal” (rhythm) tone settings. In the “normal” mode both capacitors were in circuit to provide a full sound.
This was a profound advancement for guitar reproduction.
Although designed to be part of the set of EH-150 Steel Guitar and Amplifier, the EH-150 Amplifier could be used with Spanish style electrified guitars.
However Gibson continued to make amplifiers for those who preferred a full range sound
Note: For those wanting to try out this mod, the capacitor may be of paper, silvered mica, polyester or ceramic construction. IMHO ceramic is the brightest and silvered mica the sweetest.
The Fender Vibralux 5G11 and Bandmaster Model 6G7 appear to be their first models to incorporate the idea of inbuilt constant and non-adjustable EQ.
Some later Fender models such as the Showman AA763, used an even smaller interstage coupling capacitor of 500 pF (0.0005 uF).
This brilliant but simple, effective and low cost mod is the secret to great guitar tone.
As a side note, the famous guitarist Les Paul was just 21 years of age in 1936. His time with Gibson was to come 16 years later in 1952, when the Les Paul Gibson Guitar was released.
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Now while Gibson and Fender were doing their thing in the USA, over in the UK a company named SELMER were making great amps from low power to 200W.
They chose to incorporate an adjustable EQ system using a selector switch.
A clear example of this approach is shown in their 1960’s Zenith 100 amplifier. This amplifier has a mix of boost and cut options across the guitar frequency range. In this amp the six option EQ selector was a push-button style for fast changeover.
When using this amp with a Fender Strat and its 5 position pickup switch, the guitarist is offered 30 different tonal combinations
Other Selmer amplifiers used a variety of EQ configurations. More detail is at https://www.vintagehofner.co.uk/selmer/early/sel7.html. Schematics are available on line.
If you are building your own amplifier this is the way to go.
The number of settings and capacitor values can be whatever you prefer. The more options the more versatile the choice of tone.
12.2: VOLUME (GAIN) CONTROL
Electric guitars usually, but not always, incorporate an on-board volume control.
The term “volume” is a misnomer. The control is more correctly termed a “gain” control, because it determines the input to output gain of the amplifier.
“Gain” is a measure of the ratio of input to output voltage. The greater the gain the less input voltage is required to drive the amplifier to full power output.
The gain setting is independent of signal so when there is no signal there is no gain. But the selected gain ratio remains unchanged.
However from the early days of radio the term “volume” has been used because when there is signal the control affects the volume of the sound.
The traditional volume/gain control is in the form of a variable voltage divider network comprising a “potentiometer” or “pot” made with a variable gain ratio such that the rate of change of rotation of the shaft to change of resistance is not linear.
This characteristic is vital to amplifier/preamplifier operation.
Traditionally, the rate of change or “taper” was called “audio taper” because it provides accurate control at low volume levels, where fine adjustment is needed.
The “linear” taper is used for applications like bias adjustment, tone controls, graphic EQ controls and balancing grid drive circuits.
In the 1960’s some smart operators changed the taper for volume controls to linear taper because the volume increases rapidly with a small rotation of the control shaft/knob.
This is a very successful strategy for the ignorant masses who rock into a music store and discover the amp is extremely loud at No.2 on the dial. Their assumption is that if the amp is that loud at No. 2 then it must be deafening at No. 10. This is of course nonsense, because as explained above the volume control is actually a variable gain control.
Some manufacturers even used reverse audio taper for even greater sensitivity change.
Important Note: Once the potentiometer has reached full rotation it is at maximum output, regardless of the resistance taper used
It will be noticed that regardless of the output SPL setting, at maximum rotation the sound changes.
This characteristic also applies to tone control potentiometers.
Potentiometers used in audio amps and preamps are usually the carbon track type. The carbon track wears over time and can become noisy or scratchy. A quick squirt of terminal solvent/lube will usually cure the problem.
If the track becomes an open circuit there will be no circuit for part or all of the shaft rotation. If the potentiometer is used for grid bias – as in an input circuit – the tube will not have any bias. Sometimes the carbon track will open-circuit, resulting in failure of the amplifier because bias is lost. This is a good reason to avoid using potentiometers for grid leak (grid) resistors.
Poor quality potentiometers used for grid bias may exhibit a random "crackling" noise that will not go away. The only solution is to replace with a better quality component.
Potentiometers used for power tube grid bias adjustment should be as heavy duty as possible to prevent grid current burning the track and destroying not only the potentiometer but a pair of power tubes. 6CA7/EL34 and EL84/6BQ5 tubes draw more grid current than their beam power tube counterparts such as 6L6 and KT series so be aware.
Note however that most power tubes will draw grid current when pushed.
“Tab” style potentiometers – i.e. those that do not have a mounting bush – should not be used for grid bias.
Potentiometers used for preamp tubes should be of a low noise construction to prevent hiss.
Note: Potentiometers used as a volume control in the first stage of an amplifier or preamp will present a shunt (parallel) circuit with the input device – usually a guitar or microphone. This has a couple of effects.
1. the input impedance to the source will be reduced.
2. any DC grid current will be shared with the input device.
3. If the input device is a preamplifier – including an active guitar pickup – the device should be isolated from the tube grid via a suitable capacitor. The value will vary with the effective resistance but 0.1 uF would be a good starting value. Adjust for tone. The capacitor should be installed under the amplifier/preamp chassis to prevent stay hum pickup.
12.3 TONE CONTROL
The Tone Control is an important contributor to tone.
In simplest from it comprises a treble cut capacitor in some part of the signal path. It can be installed across (shunted across) the input terminals or at any other convenient point in the preamplifier or drive stage of circuit.
DO NOT connect a capacitor across the loudspeaker terminals.
The next step up in treble control is to instal a potentiometer in series with the capacitor to enable adjustment. One terminal is connected to the capacitor from the signal wire and the other two terminals grounded.
More functionally versatile treble controls comprise a boost and cut option.
Same applies to bass boost and cut controls.
Most amplifiers will include either a single treble cut control or a pair of bass and treble boost and cut controls.
More exotic amplifiers provide a midrange tone control as well.
The boost/cut frequencies are important for tone. Over time it has been found that values different to the standard hi-fi circuits provide a better guitar tone.
A suitable starting point for experimentation would be to halve the bass boost capacitor, double the bass cut capacitor, halve the treble cut capacitor and double the treble boost capacitor.
12.4 PRESENCE CONTROL
A typical presence control comprises a potentiometer connected in series with a preamp or driver tube cathode circuit. The potentiometer may be used as the cathode resistor itself. Ensure the wattage rating of the potentiometer is adequate to safely handle the cathode current.
Connect the hot potentiometer terminal to the cathode and the other to ground – the ground connection should be to the same point as the tube grid resistor and the B+ bypass capacitor to which the same tube’s plate resistor is connected. A small capacitor – say 0.1uF to 0.22 uF - may then be connected between the grounded terminal and the centre slider terminal of the potentiometer. Other higher or lower values may be tried to taste.
A more exotic configuration would be to instal a switch with various selectable capacitor values.
Wiring layout is important because this wiring is in the signal path. If the presence control is mounted on the chassis front panel or remotely to the tube, shielded coax cable should be used to prevent hum pickup and to avoid earth loop hum. Connect the inner flexible wire between the hot terminal of the potentiometer and the tube cathode. Connect the coax shielding between the negative (floating) terminal of the potentiometer and the cathode ground as described above. The outer shield should be grounded only at the tube cathode end.
The capacitor may be mounted on the potentiometer between the slider and ground terminals. If the capacitor is mounted remotely, such as on a terminal board or on the potentiometer, shielded coax cable is preferable.
Note that cathode signal current passes through the shielded coax insulated wire core.
Note: The outer foil of a capacitor is a large conducting object likely to attract stray fields and create hum if not grounded effectively. It is preferable for the outer foil to be the grounded capacitor pigtail lead. In sensitive situations, if the outer foil pigtail wire cannot be identified then instal the cap either way and test for best effect.
12.6 CATHODE BYPASS CAPACITORS
It is common practice to instal a bypass capacitor to shunt (in parallel) preamp stage cathode resistors.
The capacitor value is typically 25 uF.
The purpose of the capacitor is to bypass the cathode resistor so that the AC signal is not attenuated by the resistor – heard as a reduction in gain.
However the capacitor can be removed altogether safely to produce current feedback in the cathode circuit. This produces a flatter frequency response with a more natural sound.
Conversely, the size of the bypass capacitor can be progressively reduced or increased in value until the desired sound is acquired
Note: Increasing the capacitor value increases gain.
A switch may be added to switch the bypass cap in or out. A more exotic configuration would be to instal a switch with various selectable capacitor values.
The outer case of an electrolytic capacitor is a large conducting object likely to attract stray fields and create hum if not grounded effectively. The shorter the grounding wire the better. Refer wiring recommendations as set out in 12.5 above.
12.7 TREBLE BOOST
Many guitarists want a bright ear-piercing tone.
Option 1: One common method of achieving treble boost is to bypass the volume control with a small capacitor - say 470 pF - connected between the input terminal and the centre-terminal (slider) of the volume control potentiometer.
The degree of boost is entirely dependent on the gain in the following stage because the maximum boost occurs when the slider is nearly at zero volume. In that condition, the capacitor is shunting the full value of the potentiometer resistance.
The ideal value of capacitor needs to be determined by trial and error listening tests.
As the volume knob is turned and the slider moves towards the input terminal, the effect progressively reduces until at full potentiometer rotation there is no boost at all.
Most people will not hear this change because the sensitivity of the human ear to discern the HF/LF ratio decreases as the volume (SPL) increases.
The downside of this system is that the high frequency signal – including tube hiss and noise – is transmitted through the system unchecked.
Option 2: An alternative is to instal a fixed boost system into the signal path – ideally immediately before the volume control for that channel. This method requires a resistor and capacitor to be wired in parallel and then the pair wired in series with the signal path immediately before the volume control.
This method results in a loss of gain overall – the level of attenuation being the relative values of the boost resistor and the volume control. Because the two resistors are connected in series the input voltage to the volume control will be determined by the relative values of the series string.
Option 3: Select a value of cathode bypass resistor for the first stage that will deliver the required tone. For treble boost start with a 0.1 uF then progressively reduce the value.
12.8 BASS BOOST
Bass boost can be also obtained simply.
Option 1: Increase the capacitance value of the power supply coupling capacitors (They are in series with the power stage signal path)
Option 2: Increase the capacitance value of interstage coupling capacitors (Their value determines stage gain and frequency response)
Interstage frequency response is determined by the mix of plate load resistor of the stage, the interstage coupling capacitor, the following grid circuit resistance AND the capacitance value of the stage B+ supply. (Because the stage B+ bypass/filter capacitor is in series with the signal path)
Option 3: Negative feedback loop. Most guitar amplifiers use negative loop feedback to reduce hum, distortion and stability – but not to improve tone.
The level of feedback is determined by the circuit design so will hopefully be the right mix in the system.
As noted elsewhere on this website, negative feedback wiring must be shielded to prevent inductive coupling with other circuits.
Assuming it is, bass boost may be obtained by wiring a resistor and capacitor in parallel then installing the pair in series with the feedback loop. The values need to be determined by experiment but a good starting point is a 10k resistor and a 0.1 uF capacitor\\
12.9 GRAPHIC EQUALISER
An alternative to tone controls is a GRAPHIC EQUALISER.
Some manufacturers provide a Graphic EQ instead of tone controls, whereas others provide a Graphic EQ as well as tone controls.
The latter system is preferable because it enables the guitarist to set up the amp for its best natural sound before experimenting with tone control settings.
One important attribute of Graphic EQ's is their ability to attenuate speaker resonance at the low end of the range and to boost high frequencies to offset loss of hearing in the guitarist.
Graphic EQ's come in usually four to thirty two bands spread across the design frequency range. The choice of bands is important because, unlike normal gentle logarithmic form tone controls, Graphic EQ's use a sharp-peaked pyramid shaped boost/cut format, which sounds very different to the logarithmic pattern standard tone control system.
The best way to use a Graphic EQ is to set it to the desired tone pattern then leave it as-is for all work, adjusting tone via the guitar settings, tone controls and pedals etc.
12.10 POWER SUPPLY TONE
The power supply is an important influencer of tone.
Option 1: Electrolytic Filter Capacitors
Electrolytic filter capacitors are normally of the "polarised" type, exhibiting a different tone in the forward and reverse direction of the AC signal.
Non-polarised capacitors are used throughout an amplifier because they present and identical characteristic to both positive and negative alternations.
They are also used in speaker crossover networks for the same reason.
I would try installing a 25 uF or 30 uF 440 VAC (600VDC) non-polarised polypropylene motor start capacitor to either shunt the existing capacitor or replace it altogether.
In the case of larger amps – say more than 50 W output with solid state rectifiers, the standard filter caps should be left in place and the motor start caps added alongside. Will still improve the tone.
Option 2: Silicon Rectifier Diode
Instal a forward biased silicon rectifier diode (3A 1kvp) in the B+ supply, between the filter capacitor and the B+ bus supply to driver and preamp stages
The output transformer will be connected directly to the main rectifier output and filter cap.
In amps having a filter choke supplying the driver and preamp stages insert the diode after the choke AND screen grid tee-off point.
The effect of this mod is to better separate or isolate the driver stage from the output stage, particularly if the driver stage has its own filter capacitor in the plate supply.
The way it works is that when the main B+ supply voltage sags under peak loads the voltage is likely to fall below the driver stage B+ point. When this happens the driver stage supply will feed back into the power stage B+ supply, resulting in a reduction in drive voltage and a delay to recover until the driver B+ capacitor recharges to full voltage.
Note: It is preferable for the power tube screen grids to be supplied from the Plate B+ plate supply before the choke or screen-grid output signal may be injected back into the driver and preamp stages via the B+ supply. However this could lead to other issues in the power stage so detailed testing is advisable.
Option 3: Preamp and Driver Stage B+ Filter Capacitors
Increase the value of preamp and driver stage B+ filter capacitors to the largest value that will fit into the chassis. Start with 100 uF but more is better - e.g 470 uF,
Noting the effect described in Option2 above, it follows that every stage should be separated from its supply stage by a diode as described (the arrow pointing away from the B+ supply).
The concept is that when a signal passes through the amplifier from input to output, each stage draws current simultaneously – the value of current is determined by each stage individually but all add up to a significant total value.
It follows that when a peak signal is transmitted through the amplifier, all of the stages will suffer voltage drop simultaneously because all stages are supplied in cascade (i.e. series connected) from a common source, resulting in a loss of linearity, gain and dynamic peak power.
However if each stage has its own power supply – created artificially by a large capacitor, the diode and the cascade B+ decoupling voltage dropping resistor – each stage can operate independently for a short period of time, so should hold voltage for a few seconds.
This mod solves the challenge of short-time voltage drop on transient signals and will amaze with its improved dynamic response
IMPORTANT MOD: Bypass EVERY electrolytic cap in the B+ supply with a 0.1 uF to 0.22 uF polyester capacitor of suitable rated voltage (e.g. 630VDCW) to ensure spike voltages from the rectifier and any stray RF are bypassed to ground. This mod improves circuit stability and therefore sound clarity.
12.11 BIASING FOR TONE
Electron tubes are designed to fulfil a specific purpose. The three main classes of tube for guitar amps are preamp tubes, driver stage tubes, power tubes and rectifier tubes.
Each discrete design is assigned a type number which stays with it forever and does not change. Minor changes are usually denoted by a suffix letter such as G, GT, GTA, GTB, W, and WA. Each suffix is specific to the tube type to which it is attached – i.e. the suffix A may mean different characteristics for different type numbers.
Military and industrial types are usually just four digits but rarely have a suffix.
Many military and industrial tubes have similar but not identical specification to their original domestic type – so be sure to check the specs.
To guarantee longer tube life and better performance many military and industrial tubes are assigned lower ratings than their domestic equivalents.
The three primary Classes of operation for tube amplifiers are A, AB and B
Each of these classes has its own sound character, caused by the way in which the tube reproduces the input signal.
We need not concern ourselves with the detail because there is audible difference.
Furthermore there is distinct difference between single-ended amplifiers and push-pull amplifiers.
Then there is difference between triode, pentode and beam power tube tone.
Difference will also occur according to the load impedance on the power tubes. This can be demonstrated by changing the speaker impedance relative to the trabnsformer rated load impedance – e.g. connecting a 16 Ohm speaker top the 8 Ohm tap.
An important feature of tubes is their specified design bogey values. See https://en.wikipedia.org/wiki/Bogey_value as published in data sheets and manuals.
Those familiar with the art of building and servicing guitar amps know that the voltages applied to tubes in popular commercial amplifiers are well beyond the upper limit specified by tube manufacturers, resulting in operating conditions designed to maximise power output.
Some modern production of popular tube types has been modified in an attempt to improve reliability, however this cannot wholly resolve the issue so some degree of unreliability will still remain.
Furthermore the design values for gain and cathode current related to grid bias are subject to wide tolerances – maybe + or – 20%
This feature was widely publicised by the Groove Tubes enterprise.
The Effect of Output Tube Bias on Tone, Gain, and Headroom.....A Side-by-Side Comparison
An important contribution to the art of biasing for tone is presented by Uncle Doug in his You Tube video: https://www.youtube.com/watch?v=hgyQ2scnLJY
There are many lessons hidden in this presentation.
12.12 GUITAR POWER TUBE COMPARISONS
The attributes of different types of tube construction, classes of operation and loading configurations was discussed in Section 7.
Here is a You Tube presentation that attempts to provide a side by side comparison between tube types and makes.
https://www.youtube.com/watch?v=HWV_Pctf_MA
This comment from a respondent to the above video sums up my thoughts
This was interesting, BUT the quality of you tube audio across internet really isn’t good enough to be able to judge the differences properly. What I took from this video is that listening in this way suggests the differences are subtle, but might not be if you were in the room with the amp. I was also interested why you chose to run the tubes at the same idle current. I think this would put an el84 and a 6l6 (for example) at different levels of dissipation, so despite best intentions it’s not really comparing like with like. I appreciate you are probably trying to help someone who is thinking “how would different tubes sound in my amp”, but that in itself is not a simple thing. It seems to me that in building an amp that there are many things that you would want or need to change to suit particular tubes.
Comment: Whilst this form of comparison offers a quick guide to deciding which tube to use it should not be taken as gospel or absolute.
Three reasons:
1. As @ali2ndmail noted, the tubes are set to the same idle current of 32 mA. Idle current is a function of grid bias so it follows that the CLASS of operation will vary depending upon the Plate Current curves of each tube type. They may all sound similar at low volume but at full power the characteristics will be very different.
2. The amplifier design is not fully shown but it appears to be a single-ended Class A unit with either Cathode Bias or Fixed Bias. Because Cathode Current will increase to a significantly higher value for the larger tubes than the smaller units, the Plate Current through the Output Transformer will be greater and therefore the transformer characteristics will be different.
3. Transformer Plate Load is different for each tube type so whatever value was used for the tests it will be less than ideal for some tube types.
There is one conclusion that can be made and that is that there is very little audible difference between tube types.
This because tubes are NOT designed for sound quality but to amplify and produce a specified power output with minimal distortion.
Despite some tubes being designated "audio" types, that descriptor is intended to differentiate between transmitting tubes and domestic audio tubes. Some tubes designated "audio" or "hi-fidelity" are usually selected from standard bogey domestic types and rebranded – e.g 12AX7/7025, 6CZ5/6973, 6L6GC/7027A. Note some of this class of tubes has differing pinouts.
So despite different tubes types across different manufacturers from different plants at different times, different materials of differing purity, different batches and tolerances, and different electrode designs and vacuum levels, it is provable by test that the main differences in sound occur due to primary class of tube design, being triode, pentode, tetrode or beam power tube AND Class of Operation – A, B or AB.
Consider also the natural sound characteristic of each type of tube that attenuate or accentuate frequencies across the audio band. This effect will interact with both the input signal and the speaker response.
TO BE CONTINUED.
REMEMBER - ALWAYS TAKE CARE WHEN WORKING WITH HIGH-VOLTAGE - DEATH IS PERMANENT!!
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This page last modified 20 March 2024