Tone stack mod

[J Luth]

Got a request to do a mod to the tone stack on a Fender amp, it has the usual T M B controls. He wants to add a second mid pot along with another cap of different value to the circuit. It would have an .047, an .022 and 2, 25K pots. I told him I could add another cap, a switch and a couple of resistors so he could switch between the caps. I think it's a waist of time but he wants the pots if possible.

Questions before I try it.
Would this be a waist of time and cause problems?
Would they not interact well?
Should the second pot be wired to the same bass pot terminal along with the current mid pot?
Is there a better way to do what he wants?

[Roberto]

I'd do it this way: use a standard tonestack, then two options:
- a supplementary cap in parallel with the treble cap, and a pot in series to dampen its effect;
- a bright cap on the volume pot, with a pot in series to dampen its effect.

Both make the sound brighter. The former is not volume dependant, the latter it is volume dependant.

[Bob M.]

Well, controlling the midrange is a big part of getting a good guitar sounds and the Fender/Marshall/Vox tone stack only goes so far in doing that. A couple of ideas for more midrange options - 1) adding a cap in parallel to the treble cap (usually 250pf in a Fender). It's an old idea but pretty useable. 2) adding a slope control for lower midrange/high bass adds alot of fatness to amps. I guess it works a little better w/ Fender type guitars than Gibson types. Take out that 100K slope resistor and use a 47K ohm resistor in series with a 50K pot in place of it; linear works best. Then you can control the midrange in various frequencies and not just one place in audio spectrum.

Bob M.

[J Luth]

Thanks

He is not sure what he wants to do now, the ball is in his court.

[eschertron]

Changing the 25k (?) midrange pot for a larger one allows the user to dial in less "loss", and emphasize the midrange. Combining that with a way to change the notch freq, as suggested above, can make for more variety. If you haven't tried Duncan's tone stack calculator, it'll provide you with some idea of how these mods can work.