odd tone control

[woodyc]

Here's one I bet you haven't seen before...

This came about when a guy dropped off a box full of old Thomas Organ parts and asked me to make a guitar amp out of it, which I'm just finishing up now.

After I got into it, I learned that the organ produced tones by filtering square waves, so all but one of the "stops" work for guitar. Only the flute tone was different.

I had to change a couple values and replace the original inductor as a mouse had eaten some of the turns. Otherwise, its just a stock Thomas tone control.

[markr14850]

Should there be a coupling cap on the exit from V1?

[woodyc]

Should there be a coupling cap on the exit from V1?

Thanks, that was a typo -- fixed it.

[Tilman]

And how does it sound like ? Is it usable for electric guitar too ?
Tilman

[woodyc]

And how does it sound like ? Is it usable for electric guitar too ?
Tilman

Sounds good. It reminds me of a Neve preamp where you have hipass, lopass, bandpass, and notch controls. In this case you don't have independent control over the frequencies, but somebody obviously spent some time thinking about ones that would work.

[jrfrond]

Not weird at all. From the bottom up, it: Low-pass, Band-pass, Notch, and High-pass. Feed it the harmonic-rich square wave signal from the divider circuit, and you can selectively filter harmonics. The only thing that surprises me is that it doesn't have summing resistors at the top of each pot, but hey, I didn't design it.

[Steve Conner]

Not weird at all. From the bottom up, it: Low-pass, Band-pass, Notch, and High-pass. Feed it the harmonic-rich square wave signal from the divider circuit, and you can selectively filter harmonics. The only thing that surprises me is that it doesn't have summing resistors at the top of each pot, but hey, I didn't design it.

It is obvious enough how it works, so not weird in that respect. But compared to the cliched tone circuits usually found in guitar amps and pedals, it is way out there. I think building it into a distortion pedal would be an interesting experiment, and easier than finding an amp with four spare knob holes. Distortion circuitry more or less gives a square wave, so it should feel at home.

Any idea what impedance levels it was intended for? I guess it's supposed to be driven by a low-impedance buffer, but what about the input to the next stage?

Jrfrond, I think they put the summing resistors in series with the wipers, then popped a bright cap across the "Reed" one.

[woodyc]

...Any idea what impedance levels it was intended for?

A buffer might be a good idea, but the organ tones came off one plate of a 12AU7 operating at relatively low current. (I'm driving it from both plates of a paralleled 12AX7.)

[Chuck H]

What could be really cool about it is that since it's designed to work with a square wave it's tuned to create very specific effects. I've done experiments with EQing square waves for unusual tones and it can be really different. It may actually make a distorted guitar input signal sound like a "reed" setting on an organ (provided the impedance is matched up as Steve noted).

[jrfrond]

It is obvious enough how it works, so not weird in that respect. But compared to the cliched tone circuits usually found in guitar amps and pedals, it is way out there. I think building it into a distortion pedal would be an interesting experiment, and easier than finding an amp with four spare knob holes. Distortion circuitry more or less gives a square wave, so it should feel at home.

Any idea what impedance levels it was intended for? I guess it's supposed to be driven by a low-impedance buffer, but what about the input to the next stage?

Jrfrond, I think they put the summing resistors in series with the wipers, then popped a bright cap across the "Reed" one.

I'm "iffy" on this one. Summing resistors would usually be something like 10K across the board right at the summing point. The resistors you are pointing out seem to be for more for signal attenuation, but I could be wrong.

Since this is a "lossy" circuit, a buffer and recovery amp would be a great idea. There's also a lot of latitude here for mods. With something like this, it would help if to have access to spectral analysis and a pink noise generator to see what's really going on, but tuning by ear always works the best in the end.

If you page through "The Tube Amp Book", or poke around on Schematic Heaven or Free Information Society, you will find a lot of guitar amps with seemingly wacky passive tone networks, both fixed and variable, especially some of the left-of-center brands you don't often see. Most everyone is used to the tone stack, single-knob tone control and even Baxandall circuits, but each segment of the schematic shown can be found somewhere. I'd actually like to try this one myself, just for the heck of it. As Steve pointed out, it should fly best with distortion. Could be fun! Or, I could just need to get out more.