Tone Controls
Troubleshooting Case Studies
Symptom:
Tone control doesn't work. Output sounds dark like the tone control is permanently rolled down, or at "0."
There was a memorable time when I was in the guitar repair shop and a tech encountered this problem. I told him to disconnect the tone control input. The full, bright signal returned. He immediately went for the container full of capacitors. I asked him why he would do that. Of the two components involved, namely a capacitor and a potentiometer, which would you guess is bad, given the above clues?
The cap is not to blame because the cap's only job is to cause the dark output. The fact that the output was stuck at "dark" meant the cap was definitely doing its job. It was the potentiometer that was broken. It was shorted from the CCW terminal to wiper. A multimeter can verify this with a resistance measurement between the CCW and wiper terminals as the control is rotated. If it says at near 0Ωs (or stays stuck at any Ωs), then there is a problem.
Most guitar pots (CTS type) can be opened up and fixed, or simply sprayed out with a good contact cleaner like Deoxit. If it had been my job, I probably would have attempted this instead of just installing a replacement. I prefer repairing pots for guitars mainly for the aesthetics (all the pots match), or to keep a vintage guitar "straight." But the replacement is a good and reliable repair choice.
Cure:
Replace or repair the shorted potentiometer.
Symptom:
The tone control is acting like a volume control. It kills all the sound when turned down.
When the tone control acts like a volume control your pickup is probably bad. You can verify this by measuring the pickup's dc resistance. Disconnect one of the pickup's wires for an accurate test (can be "hot" or ground, doesn't matter). The measurement should be in the ball park of 2k to 12k, maybe more for a hot humbucker or large vintage Gibson bass pickups. If you're getting some reading above 50k then the wire connection is breaking or broken and your pickup has an output impedance that is unusable. The math on the filter capacitor changes so that it will now pass most of the guitar signal to ground (instead of just the high frequencies).
Cure:
Replace or repair the bad pickup.
Tone Control Theory
99% of the time, a tone control on a guitar consists of just a capacitor and a pot. To understand how it works, try just alligator clipping a capacitor (try 100nF, aka 0.1µF, aka '104', to start) across the output jack (one end to the 'tip' and one end to the 'sleeve' or ground). Try this with all other controls turned up. Does it sound dark and bassy? It should. Try a bigger value cap if it doesn't.
This is all that is happening in a tone control. A cap is simply connected from your signal to ground. The cap looks like a high resistance to low frequencies and a low resistance to high frequencies. This results in high frequency attenuation.
Using a pot to control the effect
You could just put the cap on an off/on switch, and some guitars do it that way.
Placing a variable resistor (potentiometer) between the signal and ground allows some control over this filter effect. The potentiometer value has to be relatively large to avoid affecting the signal at all times. It is usually the same value as the volume pot. It can have a very mild "sweep" kind of effect at best. Nothing like the kind of sweep obtainable with an active circuit.
Many permutations of the same control. All of these circuits will work similarly.
There are quite a few connection permutations for a cap and pot that will work as a tone control. This makes it appear when staring a wiring diagrams that each one is different. They are not. Do not be fooled that it matters whether the cap is directly grounded, or the pot is directly grounded. It doesn't matter if the ground point is the back of one pot or another, or the jack or whatever. For most guitar situations ground is ground is ground no matter where it is. That does not hold for amps and things with significant ground currents, but the (passive) guitar does not suffer from this.
What about a bass cut?
Most controls are just treble cuts. To lower the bass output, the cap must be put in series with the signal. This is often done with a switch and rarely done in conjunction with a potentiometer. A good example of a switched bass cut is the Fender Jaguar's mysterious third slide switch. A good example of the potentiometer controls bass cut can be found in the Fender Rhodes 'Stage' model which has two passive controls, one of which is a bass cut (labelled bass boost to be more optimistic sounding I guess).
If a potentiometer controlled bass cut is desired, place a relatively low value cap (Jaguar uses a 3nF, aka 0.003µF, aka '302' cap) in series with the signal. This requires breaking a wire and placing the cap in the break. Verify that you have a thin sound now. Next, wire up the CW and wiper terminals of a very large value pot (try 1 Meg Ω) across the cap. Now you should get full bass response at fully CW and a thin tone at CCW. If the change is too abrupt and switch like, you may want to try a reverse audio taper pot. This type of control is never quite as pleasing as the normal, treble cut tone control, so it is rarely seen.
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