How The TMB Tone Stack Works
By Rob Robinette, edited 1/21/2017
This page is available for download as a pdf file: How the TMB Tone Stack Works
The Fender/Marshall/Vox TMB (Treble Mid Bass) tone stack has been around since the 1950's and has become a staple with Fender and Marshall amps. When Marshall copied the Fender 5F6-A Bassman almost verbatim to create the JTM45 amplifier the Fender TMB tone stack came with it and it's still being designed into new amps today.
On this page I use the 5F6-A Bassman's tone stack but all the stack's component values can vary in other amps. To see how component values affect the tone stack's output you can use the Duncan Amps Tone Stack Calculator program discussed below.
The TMB tone stack is passive, meaning it can only remove frequencies, not boost them. The TMB stack is made up of several low and high pass audio filters that work together to work their magic. It's a relatively low impedance circuit (high load) so it works best when fed by a low impedance source such as a cathode follower like in the 5F6-A, JTM45 and many other amplifiers.
The seemingly simple TMB tone circuit is surprisingly complex due to the interactive nature of its four audio filters. I will attempt to describe how it works in the most simple way possible. We'll start with an overview of the tone stack then examine the Treble, Bass and Mid filters separately in detail.
The Fender 5F6-A Bassman Tone Stack
Guitar AC signal voltage enters the tone stack at upper left and flows through all three tone caps. Note the Bass Pot is wired as a variable resistor while the Treble and Mid Pots are wired as potentiometers. In typical amp circuits all three tone caps also function as coupling caps to prevent high voltage DC from flowing downstream.
Duncan Tone Stack Calculator Graph of 5F6-A Tone Stack
All three tone controls are set to their mid point. Note the mid scoop (dip in graph) at 800Hz. Fender used the mid scoop to compensate for typical guitar pickups which over emphasize mid frequencies.
The free Duncan Tone Stack Calculator program is a free, very cool and easy to use Windows program that will help you understand how interactive the tone controls really are. Not only can you move the tone pot sliders and see how the frequency response graph changes but you can also double-click any component in the schematic and change its value. For example you can double-click on the tone slope resistor and see what changing its value does to the slope of the frequency response graph.
Download the 5F6-A .tsc Tone Stack Calculator file (you must select the "Marshall" tab after loading the file). The tone stack file is setup for a cathode follower feeding the tone stack like in the 5F6-A. If you want to simulate a tone stack fed by a normal gain stage as in the AB763 blackface then change the Zsrc (far left) from 1.3k to 38k.
Quick Audio Filter Review
An RC (resistance-capacitance) low pass filter has a resistor first followed by a capacitor connected to ground. A low pass filter allows low frequencies to pass but high frequencies are blocked. The TMB tone stack has only one low pass filter. A CR (capacitance-resistance) high pass filter has the capacitor first followed by a resistor connected to ground. A high pass filter allows high frequencies to pass but blocks low frequencies. The TMB tone stack has three high pass filters.
Tone Stack Overview
One low pass and three high pass filters make up the tone stack. Note how the bottom high pass filter formed by the Mid Cap + Mid Pot varies from 318Hz (Mid Pot at max) all the way up to infinity (Mid Pot at min). With an infinitely high cutoff frequency no audio frequencies will pass--they will all be blocked. Note the Tone Slope Resistor does not form a low pass filter with the Bass Cap because they are connected in series. The Tone Slope Resistor + Mid Cap form the only low pass filter.
The treble high pass filter made up of the Treble Cap + Treble Pot blocks frequencies below approximately 2400Hz and allow higher frequencies to pass. The high frequencies are sent to the top terminal of the Treble Pot. The bass and mid frequencies that pass through the bass and mid filters flow to the Treble Pot's lower terminal so the Treble Pot is a balance control that allows you to select between the high and low frequencies at its outside terminals.
The Tone Slope Resistor + Mid Cap low pass filter works with the bass and mid high pass filters formed by the Bass Cap + Bass Pot and the Mid Cap + Mid Pot to form band pass filters. The low pass filter removes high frequencies and the high pass filters remove bass & mid frequencies to leave a band of frequencies that are allowed to pass through the tone stack.
The Tone Slope + Mid Cap low pass filter's cutoff frequency is a very low 142Hz so how do any mid frequencies pass on to the Treble Pot? The Mid Pot's resistance (below the wiper) comes after the low pass filter, which reduces the low pass filter's effectiveness so some mid & high frequencies pass through the filter and on to the Treble Pot.
The bass control's high pass filter cutoff frequency varies between 8Hz and 318Hz. With the Bass Pot full up the bass high pass filter only blocks frequencies below 8Hz so all guitar and bass frequencies get through. With the Bass Pot full down the filter blocks frequencies below 318Hz so most of the bass is filtered out.
The mid control's high pass filter cutoff frequency varies between 318Hz to infinitely high. With the Mid Pot full up the mid high pass filter only blocks frequencies below 318Hz. With the Mid Pot full down the mid filter blocks all frequencies.
The TMB tone stack will interact with a high impedance driving stage such as the plate of a 12AX7 like in Fender blackface amps. The TMB stack's high load (low impedance) loads down the guitar signal coming off the tube plate causing attenuation. In other words the TMB stack's low impedance is in parallel with the driving tube's plate load resistor which lowers gain.
The Treble Control
Treble Cap + (Treble Pot resistance of 250k + Mid Pot resistance) = high pass filter, so high frequencies flow across the top of the tone stack. Tone Slope Resistor + Mid Cap = low pass filter, so bass and mid frequencies flow through the Tone Slope Resistor and Bass Cap around to the Treble Pot.
The Treble Cap + Treble Pot high pass filter blocks frequencies below 2313Hz (with the Mid Pot at max) to 2548Hz (with the Mid Pot at minimum).
Turning the Treble Pot does not change the Treble Cap + Treble Pot high pass filter cutoff frequency because the Treble Pot's full 250k of resistance is always in the filter's path to ground--moving the Treble Pot's wiper does not change this. Turning the Treble Pot adjusts the balance between high frequencies on the top side of the Treble Pot and Bass & Mid frequencies on the bottom side of the pot.
As mentioned above, the tone stack's low pass filter cutoff frequency is a very low 142Hz but the Mid Pot's resistance below the wiper comes after the low pass filter, which reduces the low pass filter's effectiveness so some mid and high frequencies pass through the filter to the Treble Pot.
The Mid Pot's resistance below its wiper is added to the treble high pass filter's resistance. The treble high pass filter's resistance ranges from 250k with the Mid Pot set to minimum which gives a cutoff frequency of 2548Hz, to 275k with the Mid Pot at Max and a cutoff frequency of 2313Hz.
The Bass Pot setting has no effect on the treble high pass filter cutoff frequency because the treble filter's path to ground can flow through the Bass and Mid Caps which avoids the Bass Pot.
The Tone Slope Resistor got its name because its resistance affects the balance between high frequencies that flow across the top of the tone stack and bass & mid frequencies that flow below through the rest of the tone stack. A smaller Tone Slope Resistor will tilt the frequency balance toward bass frequencies by reducing treble and increasing bass. A larger resistor will tilt it toward high freqs and less bass. A larger Tone Slope Resistor will also reduce the Tone Stack's load (raise its impedance). That's one of the reasons the Fender blackface amps use a 100k Tone Slope resistor.
The Bass Control
Slope Resistor + Mid Cap = low pass filter, so bass and mid frequencies flow on to the Bass Cap. Bass Cap + (Bass Pot + Mid Pot 25K resistance) = high pass filter, so the Bass Pot blocks low frequencies.
By filtering the guitar audio through a low pass filter followed by a variable high pass filter (Bass control) a variable band pass filter is created that only affects bass frequencies.
With the bass high pass filter's resistance of 1M Bass Pot + Mid Pot 25K = 1.025M with the Bass Pot at max gives a cutoff frequency of 8Hz.
With the bass high pass filter's resistance of 0 Bass Pot + Mid Pot 25K = 25K with the Bass Pot at minimum gives a cutoff frequency of 318Hz. Note the mid filter's lowest cutoff frequency is 318Hz so the bass and mid filters match up perfectly.
As mentioned above, the tone stack's low pass filter cutoff frequency is a very low 142Hz but the Mid Pot's resistance below the wiper comes after the low pass filter, which reduces the low pass filter's effectiveness so some mid and high frequencies pass through the filter to the Bass Cap.
The Bass Pot has no effect when the Mid and Treble Pots are full down.
The Mid Control
The Tone Slope Resistor + Mid Cap low pass filter passes bass and mid frequencies to the Mid Cap. The Mid Cap + Mid Pot resistance = high pass filter. The combination of the low pass and high pass filters creates a band pass filter so only mid freqs make it through.
The Mid Pot is a band pass filter that functions like a mid frequency volume control. At higher Mid Pot settings the control acts as a simple high pass filter which blocks low freqs but allows mid freqs through. As you turn down the Mid Pot, the mid filter's cutoff frequency increases. The cutoff frequency runs from 318Hz with the Mid Pot at max, to 637Hz with the Mid Pot at its mid setting, to infinitely high with the Mid Pot full down.
A high pass filter with an infinitely high cutoff frequency blocks all frequencies so no audio will pass.
As mentioned previously, the tone stack's low pass filter cutoff frequency is a very low 142Hz but the Mid Pot's resistance below the wiper comes after the low pass filter, which reduces the low pass filter's effectiveness so some mid and high frequencies pass through the filter. But dial the Mid Pot full down and the Tone Slope + Mid Cap's low pass filter really does cut off all frequencies above 142Hz so only low bass frequencies make it to the lower side of the Treble Pot.
In some amps the Mid Pot is replaced by a simple resistor with a typical value of 6.8k. In the AB763 blackface amps Fender used either a 6.8k resistor or a 10KA (audio or log) Mid Pot wired as a variable resistor (with input and wiper terminals jumpered). A quirk of wiring the Mid Pot as a variable resistor causes a complete dropout of the guitar signal when all three tone controls are turned full down.
Mid Pot Wired as Potentiometer or Variable Resistor?
The TMB tone stack's Mid Pot can be wired as a potentiometer as in the 5F6-A Bassman or as a variable resistor (with input and wiper terminals jumpered together) as they are in the Fender AB763 blackface amps. The difference is subtle but wiring the Mid Pot as a potentiometer shows the treble and bass high pass filters a resistance to ground that does not vary as you adjust the Mid Pot so there is less interaction between the tone controls.
With the Mid Pot wired as a variable resistor treble frequencies drop off more when the mid control is turned down. When the Bass Pot is turned down the mid control will also cut bass frequencies as it is turned down. A quirk of wiring the Mid Pot as a variable resistor causes a complete dropout of the guitar signal when all three tone controls are turned full down.
Mid Control Wired as Potentiometer
Note the Mid Pot, R4 at bottom center of the schematic wired as a potentiometer.
Mid Control Wired as Variable Resistor
With the Mid and Bass Pots both full down high frequencies are 4dB lower and bass frequencies are severely cut.
Annotated 5F6-A Schematic With Signal Flow and Component Function
Note the TMB Tone Stack at the center of the schematic. Guitar signal path shown in yellow. What do all those parts do? Click on the graphic to see the full size and readable schematic.
Point To Point TMB Tone Stack
Use a three terminal Terminal Strip to support both ends of the Tone Slope Resistor.
The Middle pot has been replaced with a 6.8K resistor.
By Rob Robinette
This page is available for download as a pdf file: How the TMB Tone Stack Works