How to Voice an Amp
By Rob Robinette
This webpage is available for download as a pdf
There are many ways to tweak the tone or "voice" of a guitar tube amplifier. 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. I'm a fan of adding a Vox style Cut control which trims high frequencies from the power tube input grids. 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. It's a great way to cut "ice pick" highs with minimum tone alteration.
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 these tone tweaks 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 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.
Standard Guitar Circuit
The Pickup on the left is a wire coil that generates the guitar signal. The Tone Control bleeds high frequencies to ground. Turning the Tone Pot changes the resistance in the RC (Resistance-Capacitance) filter which changes it's 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, 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 demphasize 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 or remove ice pick highs. The tone pot and cap form an RC (resistance/capacitance) low pass filter which means high frequencies don't pass, they are bled to ground. The Tone Pot is also a much lower load on the guitar signal compared to the much 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 = 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. The Tone Control bleeds high frequencies to ground. Turning the Tone Pot changes the resistance in the RC (Resistance-Capacitance) filter which changes it's cutoff frequency. The Coupling Cap is not part of the Tone Pot circuit but is shown to emphasize the need 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 attenuated 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 set to 100k the formula would be:
1 / (2 * 3.14159 * 100,000 * .0000000047) = 3386Hz
So 3386Hz 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.
Placed around Volume and 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 Caps on volume and gain pots become more noticeable as you turn down the pot and at maximum volume the bright cap does nothing because there is no resistance to bypass.
Reducing the cap value raises the frequency cutoff. You can add a 100k to 330k resistor in series witha bright cap to reduce the amount 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.
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 .047uF.
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 used 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.
Here's an online Plate Load Resistor Bypass Cap Calculator.
.001uF Load Bypass "Dull Cap" in parallel with the V1A 100k plate load resistor.
The standard TMB (Treble Mid Bass) Tone Stack is a series of three RC (resistance capacitance) audio filters that bleed three bands of audio frequencies to ground. The Treble, Bass and Mid pots change the resistance of the RC 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.
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.
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.
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
Once you get the Tone Stack Calculator running 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 see 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 engaged (tone stack ground disconnected). The mid scoop is also flattened out. Chart is from the Duncan Tone Stack Calculator.
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 signal can easily be clipped when the input 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.
Sometimes called a "fizz cap," 47pF to 100pF. It connects the two phase inverter plates with a capacitor to remove very high 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. Also aids amplifier stability. 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 those frequencies at the amp's output. Use the Presence Control to subtly tweak your tone. The Presence Control affects only the phase inverter and power tubes.
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. The cap can be anything from 47pF up to 500pF. 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.
The Vox Cut Control connects the two power tube grids with a 250k audio pot and 4.7nF (.0047uF) 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 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.
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.
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.
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 mini-switch. Close the switch for a Master Volume and open the switch for a Cut Control. If you don't want the Master / Cut Switch simply delete it and you'll have a normal 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. 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. 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.
A 2 Meg audio pot and .047uF 600v cap (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.
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 103dB 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
This webpage is available for download as a pdf
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