Photo by Rob Robinette
How to Voice an Amplifier
By Rob Robinette, edited 2/13/18
There are many ways to tweak the tone or "voice" of a guitar tube amplifier. Amp designers and modders can alter the clean and overdrive tone by changing component values, adding components, removing components, adding gain stages or even adding complete circuits like negative feedback. This page also discusses how your amp's controls work which will help you get the most out of your guitar and amp. Knowing what a resonance control or cut control do to the guitar signal can make you a better player.
You can tweak the voice early in the amp circuit for the greatest overall effect, especially on the overdrive tone, or you can tweak it late for minimal side effect. It's ideal to have early and late tone controls in an amplifier circuit to offer the most versatility. This is especially true in high gain amps with lots of preamp distortion. Why cant you just turn down the tone or treble control to remove overdrive fizz and ice pick highs? Because they are typically caused by harmonic and intermodulation distortion added by the overdriven stages in the amp--after the tone controls. Using an early-in-the-circuit tone control to remove fizz and ice pick can smother the tone because you not only remove high frequencies but also all the high frequency harmonics that would have been generated as the guitar signal cascaded through the gain stages. A late-in-the-circuit tone control can usually remove the fizz and ice pick without killing the lush and layered overdrive harmonics and sparkle.
I'm a fan of adding a VOX style Cut control which trims high frequencies very late in the amp circuit--from the power tube input. It allows you to do a final tweak of the overdriven signal to darken it to taste without affecting the substance of the overdrive tone.
When working with a new amp pay close attention to what knob tweaks do to the overdrive tone because a minor tweak early in the amp circuit can have a major impact. An added bonus of many of the tone tweaks on this page is they can help suppress oscillation and boost amplifier stability.
The higher the gain of an amplifier the more critical its voicing becomes. The Fender 5E3 Deluxe is beautifully voiced with rich overtones and a full bodied if not lose bottom end. Its clean tone is exceptional and when lightly overdriven sounds wonderful but push the amp too hard and all those low frequencies running through the amp wreck havoc on the amplifier circuitry. The 5E3's big coupling and cathode bypass caps and lack of grid stopper resistors all work together to cause extreme blocking distortion known as "farting out." The 5E3's simple amplifier circuit can be changed into a gain monster with the help of a boost pedal and a few component swaps. Reducing the size of the coupling and bypass caps and the addition of some grid stopper resistors can allow you to play Van Halen covers with the best of them. It just comes down to tweaking the voice.
Why do most high gain channels sound thin and anemic when played clean? Because to get the best overdrive tone you have to filter the guitar signal and remove a lot of the bass to prevent a muddy overdrive tone and prevent blocking distortion. But drive the high gain channel into overdrive and harmonic and intermodulation distortion fill in both the bottom and top end for a nice fat overdrive tone. Keep that in mind next time you evaluate an amp with a high gain channel.
The opposite is generally true for amps designed for their clean tone. Coupling and cathode bypass caps are usually large to pass low frequencies for a lush, full bodied tone. It's difficult to design an amp to do both clean and hard core overdrive well and that's why many amps that attempt it have one channel that's "not so great."
WARNING: A tube amplifier chassis contains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. If you have not been trained to work with high voltage then have an amp technician service your amp. Never touch the amplifier chassis with one hand while probing with the other hand because a lethal shock can run between your arms through your heart. Use just one hand when working on a powered amp. See more tube amplifier safety info here.
The following voicing techniques are listed from early to late in the amplifier circuit:
Table of Contents
Don't forget about the earliest possible place to shape the guitar tone is on the guitar itself. Adjusting the guitar's tone control will affect the guitar audio signal's tone through the entire amplifier. If you are a set-and-forget guitar tone knobber try twiddling the tone control next time you're pushing the amp into heavy overdrive and notice the changes to not only the tone but the substance of the overdrive.
The guitar pickup impedance and tone cap form an RC (resistance/capacitance) low pass filter which means high frequencies don't pass, they are bled to ground.
The tone pot resistance is not part of the RC cutoff frequency equation. The tone pot's resistance simply controls the volume of high frequencies being bled to ground--it does not change the frequency of the filter--it controls the effectiveness of the filter.
A typical single coil has around 8k impedance. 8k of impedance + .047uF cap = cutoff frequency of 423Hz (see formula below) so frequencies above 423Hz are bled to ground. The "volume" of this cut is controlled by the tone pot resistance.
Standard Guitar Circuit
The Pickup on the left is a wire coil that generates the guitar signal. The Tone Pot sinks high frequencies to ground. It's wired as a variable resistor. Turning the Tone Pot changes the amount of high frequency signal sent to ground but does not change the filter cutoff frequency.
The #1 Input Hi Jack is at upper left, the #2 Low Jack (-6dB) is on the right. R1 is a 1M input resistor and set's the amp's input impedance to 1 megaohm. R2 and R3 are 68k Grid Stopper resistors that remove frequencies above human hearing and also act as mixing resistors to prevent two instruments plugged in simultaneously from interacting with one another.
For amps with Fender style "High and Low" input jacks the Low jack not only cuts the guitar signal in half (-6dB) but it also offers up a very different input impedance (136k Low vs 1M High) which can interact with the guitar pickup coils to color the tone. Humbucker and hot pickups often sound better in the Low jack. Always try both inputs with a new guitar or amp to hear which you prefer. Try both inputs during your next high octane overdrive session to hear the effect on the meat of the overdrive tone.
Power Supply Voltage Sag
Many early guitar amps from the 1950's had inadequate power supplies that would allow the amp's high voltage DC power supply to sag under high demand. Voltage sag is the dynamic voltage drop across the power transformer and rectifier that increases with current demand and creates output volume compression. Output volume compression occurs because high current demand during loud notes lowers the amps DC supply voltage and maximum volume is decreased. Conversely, low demand, quiet passages create less voltage sag and generate greater amplification. This makes loud notes quieter and quiet notes louder which equals compression.
Class A and AB amps differ in how much voltage sag is generated. Since Class A amplifiers idle near max current there are less current demand fluctuations and therefore less voltage sag. Voltage drop and sag help contribute to the warm, round, tubey sound of vintage amplifiers. For example the combination of the 5E3 Deluxe's small power transformer, low output 5Y3 tube rectifier and Class AB operation lead to metric shit tons of voltage sag.
You can simulate a tube rectifier's voltage sag by adding a "sag" resistor to a solid state rectifier's output. See Simulate a Tube Rectifier Mod for more info.
Increasing the value of the first power supply filter capacitor or adding a choke can reduce voltage sag and stiffen the power supply. They can also firm up the bottom end because it takes more power to amplify low frequencies. Keep in mind tube rectifiers have a first filter cap size limit so reference your tube rectifier's datasheet to keep from overtaxing the tube at turn-on. I'm a big fan of using a choke between the power tube plate and screen power nodes like the 5F6A Bassman and Marshall JTM45, Plexi, 2203 and JCM800 amps.
Typically high gain amps need a very clean, stiff voltage supply to prevent oscillation so voltage sag is unwanted. Many high gain amps use solid state rectifiers which do not have a filter cap size limit and their minimal voltage drop minimizes sag. It's a good idea to place no more than two triodes (one dual triode tube) per power supply node to prevent interaction and oscillation in high gain preamps.
A cathode resistor bypass cap boosts the gain of a tube amp circuit by reducing its local negative feedback by acting as an electron reservoir for the cathode. A very large bypass cap like the 250uF cap on the first cathode in the 5F6A Bassman is large enough to boost frequencies all the way down to low bass guitar frequencies. The 25uF cap on many Fender tweed (1950's) amps will boost the full spectrum of guitar frequencies. A more modest 5uF de-emphasizes the lowest guitar freqs which can tighten up the bass and prevent "farting out."
Many high gain amps use a very small .68uF bypass cap to emphasize mid and high freqs to keep the overdrive tone tight and de-emphasize low freqs that waste power and get muddy when overdriven. A smaller cap also reduces bias shift recovery time which can sweeten the overdrive tone. My "Lead Channel" mod for the 5E3, 5F6A and AB763 amps suggests the use of a .68uF bypass cap for these very reasons. You can even use a cathode bypass switch to offer up different tone shaping options.
The VHT Special 6 Ultra has a rotary "Depth" switch with 11 different selections of cathode resistor bypass capacitor for its single power tube. Changing the cathode bypass cap value of a preamp gain stage will have more of an effect on the overdrive tone than changing the bypass cap of the power tubes--of course only cathode biased power amps have power tube cathode bypass caps.
Here's an easy to use online calculator that shows the frequency response of different cathode bypass cap sizes.
The 25uF cathode bypass cap is parallel to the 1.5k cathode resistor.
Use this chart to help convert capacitor size such as: .1uF = 100nF and 1nF = 1000pF
The pot + capacitor Tone Pot is a very simple but effective way to remove some high frequencies to tame an overly bright tone. The upstream impedance and tone cap form an RC (resistance/capacitance) low pass filter which means high frequencies don't pass, they are bled to ground.
The tone pot resistance is not part of the RC cutoff frequency equation. The tone pot's resistance simply controls the volume of high frequencies being bled to ground--it does not change the frequency of the filter--it controls the effectiveness of the filter.
A typical 12AX7 gain stage with 100k plate load resistor and bypassed 1.5k cathode resistor has an output impedance of around 38k. 38k of impedance + .0047uF cap = cutoff frequency of 891Hz (see formula in the next section) so frequencies above 891Hz are bled to ground. The "volume" of this cut is controlled by the tone pot resistance.
This simple Tone Pot circuit also puts a much lower load on the guitar signal compared to the more complex Treble Mid Bass (TMB) tone stack. The Tone Pot is usually placed early in the preamp so it can have a profound effect on the overdrive tone.
Wire the pot as a variable resistor (tie terminal 2 & 3 together) so that turning the pot up (clockwise) increases resistance: Up = more resistance = less high freqs to ground = brighter tone.
The Tone Pot sinks high frequencies to ground. It's shown shaft down (pointing away from you) and is wired as a variable resistor. Turning the Tone Pot changes the amount of high frequency signal sent to ground. An upstream Coupling Cap forms an AC voltage divider with the Tone Cap so its size and location will affect the Tone Pot's function. The Coupling Cap is also needed to keep high voltage DC out of the Tone Pot.
Calculate a Filter's Frequency Cutoff
When dealing with guitar amp tone shaping circuits it's useful to be able to calculate the filter's cutoff frequency (sometimes called the corner frequency) which is defined as the frequency where the signal is cut by -3dB. The formula is simple: cutoff frequency = 1/(2πRC). R = the resistor value in ohms, C = the capacitor value in farads and π (pi) = 3.14159.
For the above tone pot the formula would be:
1 / (2 * 3.14159 * 38,000 * .0000000047) = 891Hz
So 891Hz would be attenuated -3dB and frequencies above would be attenuated even more.
Tip: Use your calculator's inverse key 1/x and the π key to quicken this calculation.
Here's an easy to use Filter Cutoff Online Calculator.
Bright Caps are placed around Volume pots, Gain pots and Mixing, Attenuator and Grid Stopper resistors. They allow high frequencies to bypass the resistance. On pots they can keep the tone from getting dark at low volume settings. On resistors a bright cap can act as a treble peaker to enhance the high end by allowing high frequencies to go around the resistance.
Bright cap on a volume pot.
Bright Caps on volume and gain pots become more noticeable as you turn down the pot but at maximum volume the bright cap does nothing because there is no resistance to bypass.
Reducing the cap value raises thefilter frequency and increasing the value lowers the frequency. Changing the cap size does not affect the amount or volume of bypass, it only changes the cutoff frequency. You can add a 100k to 330k resistor in series with a bright cap to reduce the amount or volume of high frequency signal passing through it.
To bypass a volume or gain pot with a bright cap you connect the cap's two leads to the pot's input and output terminals as shown above.
Here's a Volume Pot Bright Cap calculator.
Sometimes removing a bright cap can help you find the tone you're after. For Fender black and silver face amps try clipping the 10pF bright cap around the 3.3M Reverb Mix resistor to mellow the highs from the Vibrato Channel. Clipping the bright cap on the Vibrato Channel's volume pot is also a very common blackface amp mod.
Coupling caps(sometimes called blocking caps) are used to AC couple two amplifier stages together. They block DC voltage and current but allow the AC guitar signal voltage to pass. They also act as high pass filters so you can use them to trim unwanted low frequencies. The larger the value of the coupling cap the lower the bass floor (cutoff frequency) so the deeper the amp's voice. Reduce the value of a coupling cap and you will raise the bass floor and thin the tone. Raising the bass floor can tighten bass response and clean up the overdrive tone by removing muddy bass.
The 5E3 Deluxe is known for having flabby, loose bass and a common mod is to reduce the size of its huge .1uF coupling caps down to .047 or even .022uF.
Capacitor C1 is a coupling cap. It blocks the high voltage DC on V1's plate (pin 1) and keeps it out of the volume control.
Coupling cap size can have a profound impact on an amp's overdrive tone because a smaller coupling cap can reduce the severity of blocking distortion. The Fender 5E3 Deluxe uses massive .1uF coupling caps while the high gain Marshall Plexi uses a tiny .0022uF cap to control blocking distortion and limit very low frequencies to keep the overdrive tone tight on the Bright or Lead channel. My "Lead Channel" mod for the 5E3, 5F6A and AB763 amps suggests the use of a .0047uF coupling cap for these reasons. To learn more about how coupling cap size affects overdrive see Guitar Tube Amp Overdrive.
Here's an online Coupling Cap Calculator.
Here's another online calculator for the coupling cap between the phase inverter and power tubes that controls the Phase Inverter Bass Response.
High gain amps employ voltage dividers in the preamp to control signal level. Some of them are made up of large value resistors like the 470k resistors in the Marshall2203 Master Volume amp. If you place a Bright Cap on the first resistor in the divider pair (the Attenuator) you will brighten the tone by allowing high frequencies to bypass the resistance. If you place a cap on the second resistor (the one connected to ground) you will bleed high frequencies to ground creating a "Dull Cap."
This voltage divider is located between V1A and V2A in the Marshall 2203 (minus the Dull Cap). With two 470k resistors this divider cuts the guitar signal in half (-6dB). Note the "Dull Cap" around resistor R2. You wound never need to employ both a Bright and Dull Cap on a single voltage divider.
Note how the placement of the upper resistor is important. The circuit on the right is not a voltage divider.
You can place a Dull Cap across any grid leak resistor to remove high frequencies.
Here's an online Grid Stopper Calculator to calculate the frequency cutoff of a Bright Cap across a Grid Stopper resistor.
You can remove some high frequencies by placing a50pF to 1000pF (.001uF) "Dull Cap" around a preamp plate load resistor. This can be used in high gain amps to help control oscillation by removing unneeded very high freqs above human hearing. It can also be used to reduce ice pick highs. The B+ power supply on the hot side of the plate load resistor is seen as an AC signal ground so high frequencies on the plate pass around the plate load resistor to "ground."
Here's an online Plate Load Resistor Bypass Cap Calculator.
.001uF Load Bypass "Dull Cap" in parallel with the V1A 100k plate load resistor.
This mod is an extremely easy to add local negative feedback loop that can be used to tune the overdrive tone of any high gain preamp. The feedback is negative because the plate is 180 degrees out of phase with the grid. The mod connects a preamp's second, third or fourth gain stage plate back to its grid through a 22M to 44M resistor and .047uF (400v or higher) cap.
A 22M to 44M resistor and .047uF 400v+ cap connect the preamp grid and plate together to form a local negative feedback loop.The mod can also be used on the "A" triode pins 1 and 2.
Like most amplifier negative feedback loops this mod will reduce distortion, tighten the transition from clean to dirt and slightly reduce gain. At first blush I was going to recommend not using this mod on the first stage of amplification because if the cap fails as a short it could inject high voltage into the guitar circuitry but even if this does happen the large value feedback resistor will limit the current to 18 micro amps (.018 milliamp) or less so it's not a concern. This mod would be of relatively little use in the first gain stage anyway since it is rarely overdriven.
If this NFB loop tightens up the tone too much try a larger value resistor like a 44M. The larger the resistor the less negative feedback gets through to the grid. This is a pretty cool yet extremely simple circuit that should be experimented with in all high gain preamps.
The standard TMB (Treble Mid Bass) Tone Stack is a series of three CR (capacitance-resistance) high pass audio filters and one RC (resistance-capacitance) low pass audio filter that block three bands of audio frequencies. The Treble, Bass and Mid pots change the resistance of the audio filters to alter the amount of signal filtered out. The TMB stack is a passive filter so it cannot boost any frequency band, it can only remove parts of the guitar audio signal.
The TMB tone stack can be placed early or late in the preamp. The earlier it's placed the more effect it will have on the overdrive tone.
Note the Treble pot is wired as a variable voltage divider (potentiometer) while the Bass pot is wired as a variable resistor (the input and wiper terminals are tied together). In most amps with a Mid pot it is a 10KA (audio or log) wired as a variable resistor but it may be wired as a potentiometer (input connected to the pot wiper).
The AB763 TMB Tone Stack
A common TMB mod is to change the Tone Slope (yes, that's where he got his name) resistor to a 33k, 56k, 100k, or even 150k which changes the slope of the bass-treble frequency response graph which alters the way the tone controls operate and interact. A lower value Tone Slope resistor will give you more maximum bass and less maximum treble. A higher value will give you more maximum treble and less maximum bass. High gain amps typically use a higher value tone slope to help reduce bass freqs that can cause muddy overdrive lows and blocking distortion. A higher value tone slope resistor will also lighten the impedance load of the tone stack and therefore boost gain.
100k Tone Slope resistor on the left, 56k on the right. The effect is subtle but some prefer the 56k slope. Notice how the mid scoop is wider and the max bass and treble are balanced on the right with the 56k slope resistor. The 56k also boosts the midrange by 2.5dB. I'm a fan of the 56k tone slope resistor in low to medium gain amps.
Another common mod is changing the Tone Stack's treble tone cap. Common values are 220pF, 330pF and 470pF. A higher value will extend the treble controls reach into the mid frequencies and change the depth of the scoop.
The reissue 68 Custom Deluxe Reverb uses a .022uF Mid cap instead of the AB763 standard .047uF. You can see the difference between the .047 and .022uF Mid cap in this chart:
Tone Stack Mid Cap Change
The Custom channel's reduced .022uF middle cap shifts the "mid" frequency band higher and reduces the mid scoop by over 2dB.
The 68 Custom Deluxe Reverb also has an 18k resistor between the Bass pot and 6.8k Mid resistor. The 18k resistor works as a minimum bass setting and keeps the bass from falling off a cliff as you approach the Bass pot's minimum setting. It makes setting low bass settings easier and more precise but it does limit you to how low you can set the bass.
For more information on the TMB tone stack see How the TMB Tone Stack Works.
If you are considering modifying the tone stack I recommend you download the free Duncan Amps Tone Stack Calculator so you can see graphically what the modifications do and how the Treble and Bass pots will react to the new component values.
Duncan Amps Tone Stack Calculator
After installing the Tone Stack Calculator, click on the Tone Stack Calculator's "Fender" tab at upper left. By setting the Mid slider (bottom left) to 68% you get 6800 ohms to equal the Deluxe Reverb's fixed 6800 ohm Mid resistor. You can double-click any component in the tone stack schematic to change its value so it's easy to see what happens to the control movements when you change the 100k slope resistor to 56k or adjust the value of the Mid resistor. The frequency response graph on the right will change as you alter component values or move the Bass and Treble pot sliders (at bottom left). Just playing with the pot sliders and watching the graph will tell you a lot about the interactive nature of the TMB (treble mid bass) tone stack. The Tone Stack Calculator is a very cool tool.
If you do install the Tone Stack Calculator be sure and check out how a 25KL and 100KL Mid pot can act as a "Raw" control and boost the signal compared to a 6.8k resistor or 10KA pot.
Tone Stack Bypass or "Raw Switch"
This is a great mod for pretty much any amp with a TMB (treble mid bass) tone stack. Because this type of tone stack really loads down the guitar signal being able to eliminate the tone stack is a valuable option. Just add an SPST ON/OFF mini-switch to the Mid pot's (or 6.8k Middle tone resistor's) ground. No ground = no tone stack which gives you a very significant signal boost and pure "raw" unaltered tone. The raw tone has a non-scooped mid similar to the no-tone-stack tweed amps such as the 5E3 Deluxe. It also works great with EQ pedals because it lets the pedal do all the tone shaping.
Raw Switch On and Off
You can see how even with all three tone pots at max what a huge boost in signal you get when the Raw Switch is opened (tone stack ground disconnected). The mid scoop is also flattened out. Chart is from the Duncan Tone Stack Calculator.
If the raw signal boost is too much for you, you can tune the amount of signal boost and mid scoop by placing a resistor across the raw switch terminals. Temporarily alligator clip a 24k or 47k resistor to the switch terminals and tune the resistor value to taste. A larger resistor will increase the mid scoop and decrease the signal boost.
I placed my Raw mini-switch between the Treble and Bass pots.
The cold clippergain stage is a very useful tool for voicing an amp's overdrive tone. It is used in many high gain tube amps to generate early and smooth sounding overdrive tone.
For minimum distortion a tube should be biased halfway between cutoff (when all electron flow is stopped) and saturation (when electron flow is maxed out). A 1.5k cathode resistor for a typical tube amp 12AX7 triode gain stage is very close to center bias.
A cold clipper's very large 10k to 39k cathode resistor sets a cold bias that leaves little room on the shutoff side so the input guitar signal can easily be clipped when the signal's negative lobe on the grid reduces electron flow through the tube and electron flow is shutdown completely. This clipping is asymmetric because there's plenty of room on the saturation side of the bias point. The saturation side of the signal isn't distorted and carries the original musical content.
The Marshall JMC800 uses a cold clipper stage with an unbypassed 10k cathode resistor. Soldano liked to use a 39k cathode resistor for his cold clipper.
The cold clipper's asymmetric output signal can also be clipped in later gain stages at high volume levels. Asymmetric clipping tends to sound smoother and creamier than symmetric clipping where both the + and - signal lobes are clipped equally. With asymmetric clipping one lobe carries the clean signal while the clipped lobe carries the distortion. The cold clipper generates early, relatively low volume, smooth, musical preamp distortion that can be controlled by a master volume for high gain tone at lower volume. As the cold clipper distortion comes on it blends seamlessly into the downstream phase inverter and power tube distortion into a cacophony of delicious high gain tone. A cold clipper stage is a great way to enhance an amp's overdrive tone.
A Note About Negative Feedback
Many of the following late-in-the-circuit tone tweaks are inside the typical negative feedback loop which feeds the signal from the speaker jack back into the phase inverter (or driver in single-ended amps). The NFB circuit will "fight" tone tweaks and try to remove their effects. For amps with heavy NFB you're best off adjusting the NFB circuit itself (like the Presence and Resonance Controls and Ice Pick Cap) or moving outside the NFB loop like the Power Tube Plate-to-Plate High Cut Control.
Sometimes called a "fizz cap" the snubber is typically 47pF to 100pF. It connects the two phase inverter plates with a capacitor to remove very high "ice pick" frequencies. In a push-pull amp the guitar audio signals on the two phase inverter plates are 180 degrees out of phase with one another so mixing the two signals together nullifies the signal, kind of like mixing matter and antimatter. The snubber also aids amplifier stability. If you amp already has a phase inverter plate snubber you can increase its size to trim ice pick highs and overdrive fizz. You can temporarily alligator clip a second cap across the installed snubber to find the total value that works for you. This is a late tone tweak which doesn't affect preamp distortion.
Red arrow points the the 5F6A Bassman's 47pF phase inverter plate snubber cap.
a Late Semi-Active HF boost
The Presence Control is a variable low pass filter which removes high frequencies in the negative feedback circuit. Removing high frequencies from the NFB circuit acts to boost high frequencies at the speaker. Not only are the high freqs boosted but with the reduction of NFB they become "harrier" with added harmonic distortion. Turn down the Presence Control to reduce overdrive fizz and ice pick or to subtly tweak your tone.
Overdrive fizz and ice pick highs are typically caused by harmonic and intermodulation distortion added by the overdriven stages in the amp--after the tone controls. Using an early-in-the-circuit tone control to remove fizz and ice pick will sometimes smother the tone because so much harmonic content is lost. A late-in-the-circuit tone control like the Presence Control or Ice Pick Cap can usually remove the fizz and ice pick without killing the overdrive tone's fullness and sparkle.
You can also thicken up your clean tone by turning up the presence control to add high frequency harmonics. As a general rule the harder you push an amp the lower the presence control should be set.
The Presence Control affects only the phase inverter and power tubes.
Presence Control bleeds high frequencies from the negative feedback signal to boost high frequency output and add high freq harmonic distortion (add some hair). As you turn the control clockwise (up) the wiper moves in the direction of the arrow on the schematic.
5F6A Bassman Presence Control
Terminal 1 of the Presence Pot (upper left) is connected to ground. Terminal 3 is connected to the NFB circuit. The 4.7k resistor across the presence pot is installed in most original 5F6A Bassman amps.
The 4.7k resistor found on many 5F6A Bassman amp presence pots cuts the total presence resistance in half to about 2.4k. The added resistor also quickens the action of the presence control by changing the pot's taper from linear to slightly logarithmic. The reduced presence resistance also adds to the phase inverter's gain by reducing "tail resitance". More importantly, cutting the presence resistance in half also cuts the 5F6A's negative feedback voltage in half. The 27k NFB resistor and 5K presence pot form a voltage divider for the NFB signal so cutting the presence resistance in half also cuts the NFB signal voltage in half which would thicken the clean tone and slow the already lazy transition from clean to dirty tone. This circuit change makes the difference in negative feedback levels between the 5F6A and the Marshall JTM45 even greater. The JTM45 has 2.8 times more negative feedback than the original 5F6A but has 5.6 times more NFB than a 5F6A with the 4.7k resistor across the Presence pot.
Resonance Control is another Late Semi-Active Tone Control
The Resonance Control does for low frequencies what the Presence Control does for highs. The Resonance Control blocks the flow of low frequencies in the negative feedback circuit. Blocking low frequencies from the NFB circuit acts to boost those low frequencies at the speaker. Not only are the low freqs boosted but they become "harrier" with added harmonic distortion with the reduction of NFB and the bass response also becomes audibly "looser". Use the Resonance Control to subtly tweak your amp's low end.
You can also thicken up your clean tone by turning up the resonance control to add low frequency harmonics. As a general rule the harder you push an amp the lower the resonance control should be set.
The Resonance Control, like the Presence Control above, affects only the phase inverter and power tubes.
Resonance Control blocks low frequencies from the negative feedback signal to boost low frequency output, add low freq harmonic distortion (add some hair) and loosen the bass response. As you turn the control clockwise (up) the wiper moves in the direction of the arrow on the schematic.
This is a subtle, very late tone tweak that can be temporarily clipped onto the negative feedback resistor to tweak to taste. This is a great way to cure ice pick highs and reduce overdrive fizz. The cap allows high frequencies to go around the negative feedback resistor which reduces high freqs at the speaker. In other words more high frequency NFB is added. The cap can be anything from 47pF up to 500pF. The cap size affects the roll off frequency but not the amount or volume of highs. You can add a resistor in series with the cap to reduce the volume of high freqs that pass through the cap. The Ice Pick Cap affects only the phase inverter and power tubes.
100pF "ice pick cap" paralleling the negative feedback resistor in the 5F6A Bassman.
Howard Dumble added a phase inverter balance control to his Tweedle Dee tweed Deluxe amplifier. The control allows intentional mismatch between the two phase inverter output signals which generates sweet sounding even order harmonic distortion intermodulation distortion to fatten up the amp's tone. Anytime harmonic distortion is created intermodulation is also created. The phase inverter balance control lets you thicken your clean tone but also maintain control when pushing the amp hard into overdrive.
Howard Dumble Tweedle Dee & 5E3 Deluxe Differences
I have highlighted the differences between the Tweedle Dee and a standard Fender 5E3 Deluxe with red text. The phase inverter balance pot (PI BAL 10K linear pot) is shown at center.
Adding imbalance between the power tubes increases sweet sounding second order harmonic and intermodulation distortion.
Adding a balance pot to a fixed bias amp's bias circuit allows you to adjust the bias and balance the bias between the power tubes. This allows you to use unmatched tubes but bias them evenly. You can even run a 6V6 and a 6L6 power tube together and bias each tube properly. You can also fatten up the clean and overdrive tone by intentionally mismatching the two tubes' bias which increases sweet sounding even order harmonic distortion. An increased bias mismatch can cause an increase in hum so keep an ear out for that if you dial up an intentional power tube mismatch.
The 50k Bias Trim Pot sets the bias and the Balance Pot balances the current between the two power tubes.
The VOX Cut Control connects the power tube grids with a 250k audio pot and 4.7nF (.0047uF, 200v or higher) capacitor to allow variable high end cut. In a push-pull amp the guitar audio signals on the two power tube grids are 180 degrees out of phase with one another so mixing the two signals together nullifies the signal.
I'm a big fan of this very late tone tweak because it pairs well with an early tone control or stack. Use the early tone control to get the overdrive tone and substance you want then use the Cut Control to fine tune the tone and trim fizz and ice pick highs. The Cut Control affects only the power tubes.
Wire the cut pot as a variable resistor so that as you turn the knob up (clockwise) resistance increases. Up = more resistance = brighter tone.
The VOX AC30 that this Cut Control is taken from does not have a negative feedback loop. An amp's negative feedback loop will "fight" the cut control so this tone control works best with amps with light or no NFB. For amps with lots of NFB the Power Tube Plate-to-Plate High Cut Control below will be more effective.
220KA or 250KA pot (audio pot wired as variable resistor) and .0047uF 200v cap connect the two phase inverter outputs.
Cut Control Mod on 5F6A Bassman
The .0047uF Cut Cap can be supported by a terminal strip.
The cut control capacitor limits the cut effect to high frequencies but if you jumper around the cap the pot becomes a Trainwreck Type-3 Master Volume.
RobRob Master Volume / Cut Control with Pull Pot
You can turn a Cut Control into a Trainwreck Type 3 Master Volume by simply jumpering around the Cut Cap. I use a push-pull 250k audio (log) pot to select either the Master Volume or Cut Control. Pull the Cut Control knob up to select the Master Volume. With the knob down the Master Volume is completely removed from the circuit. This is a very cool and useful mod.
RobRob Master Volume / Cut Control Switch Mod
If you prefer a switch over a push-pull pot then use this layout. A Master / Cut switch is mounted next to the Cut Pot. It is a SPST On-Off mini-switch. Close the switch for a Master Volume and open the switch for a Cut Control.
Don't forget that a hot biased amp will sound different than a cool biased amp and a cold biased amp can sound thin and sterile. I always recommend sampling the clean and overdrive tone and different volume levels each time a bias adjustment is made. You may find you prefer a cooler bias than the standard 70% target for push-pull amps. Many Fender amps from the 50's and 60's came with a bias level between 50 and 60%. See my How to Bias a Tube Amplifier for more info and use my Bias Calculator page to calculate plate dissipation %.
This is similar to the phase inverter snubber above but goes between the power tube plates for the very latest tone tweak possible. The cap removes some very high frequencies. I suggest you try a 470pF 600v cap. This tone tweak is outside the negative feedback loop so it works well in amps with high amounts of negative feedback. It affects only the power tubes.
Power Tube Plate-to-Plate High Cut Control
The early Fender Model 26 Deluxe used a power tube plate-to-plate High Cut Control. Wire the cut pot as a variable resistor so that as you turn the knob up (clockwise) resistance increases. Up = more resistance = brighter tone.
This tone control is outside the negative feedback loop so it will be more effective than the Vox Cut Control in amps with high amounts of negative feedback. This mod affects only the power tubes.
A 2 Meg audio pot and .047uF 400v cap (or higher voltage rating, 200v is too small for many amps) connect the power tube plates to remove high frequencies very late in the amplifier circuit.
An undersized output transformer can be saturated (maxed out) by the guitar signal coming off the power tube plates. When this happens the loudest passages will be capped causing compression. This dynamic compression can add to the character of an amp by adding note bloom and sustain. This tends to make an amp sound more round and tubey and less sterile. A smaller output transformer can help a modern amp sound more vintage. Most high gain amps do not use undersized transformers.
A "right sized" or oversized output transformer will let you get the maximum volume out of your amp and enhance dynamics, especially the low end.
Doh! The Speaker
The amp's speaker has more affect on the amp's tone than any other component and a swap is usually an easy job. The problem is unless your amp is a known standard you never know quite what the amp will sound like with a new speaker until you try it. Most amps allow you to plug your amp into an external cab so take advantage of this feature and try your buddies' cabs and speakers.
A closed back speaker cab will have a firmer and deeper low end than an open back cab. Marshall amps were typically paired with closed back extension cabs while the classic Fender tone usually comes from open back cabs.
The speaker's efficiency rating given in dB can have a dramatic affect on an amp's loudness. Many 1950's guitar tube amps sound most authentic when paired with a speaker with relatively low efficiency around 95dB. If you want more breakup at lower volume a low efficiency speaker will let you turn your amp up more for the same output volume. Conversely if you're fighting your drummer and need more clean headroom a high efficiency speaker of around 100dB or higher can help you be heard.
By Rob Robinette
RCA Corporation, RCA Receiving Tube Manual, RC30.
Merlin Blencowe, Designing Tube Preamps for Guitar and Bass, 2nd Edition.
Morgan Jones, Valve Amplifiers, 4th Edition.
Richard Kuehnel, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, 3rd Edition.
Richard Kuehnel, Vacuum Tube Circuit Design: Guitar Amplifier Preamps, 2nd Edition.
Richard Kuehnel, Vacuum Tube Circuit Design: Guitar Amplifier Power Amps
Robert C. Megantz, Design and Construction of Tube Guitar Amplifiers
Neumann & Irving, Guitar Amplifier Overdrive, A Visual Tour It's fairly technical but it's the only book written specifically about guitar amplifier overdrive. It includes many graphs to help make the material easier to understand.
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