Tubescreamer-type tone control

[Matt T.]

I have an OD called a Blue Balls from Blues Pearl (now defunct AFAIK). After tracing the circuit and comparing it to some schematics of common pedals, it looks like the input buffer, output buffer, and tone control (using a single op-amp) are almost exactly like a Tubescreamer. Other than that it's a dual op-amp with both sides cascaded, then clipping diodes to ground (actually to Vref), then the tone control (third op-amp), then output buffer.

The only difference I can see from the TS tone control is that the 'fixed' LPF in the BB is 1K (series) and .1uF to ground instead of 1K/.22uF as in a TS. The tone control still uses the typical 25K pot/.22uF cap/220 resistor.

The problem is that the tone is too dark until about 3:00 and then too bright shortly after that, i.e. the useable range for the tone control is very small which makes it difficult to dial in. So what are some mods to try to increase the useable range of the tone control? If possible, I'd rather just solder something to the pot since it's so much easier to get to...but if I have to I'll lift the board up again. Any help appreciated.

I think the fact that this tone control is sort of 'half shunt/half feedback' contributes to the fact that I don't fully get it.

PS - I tried adding another .22uF cap, I soldered it to the tone pot wiper and moved the connection (that goes to the board) to the other side of the cap. Didn't really cure the problem and (unless my ears played tricks on me...entirely possible) I thought the pedal didn't sound quite as good.

[Matt T.]

If it walks like a bump...and talks like a bump...

[Matt T.]

All we are is
just another bump in the wall

FWIW, this pedal originally had the typical switching diodes (silicon I think) and I didn't like it at all. Distortion came on way too quickly, congested, nasally, and had a terrible static-y decay. Swapped the regular diodes for LEDs and the tone improved immensely and it cured the static-y decay but the pedal still had a too nasally 'voice'. Then I added 3 switching diodes to the LEDs, arranged like the lead channel clipping circuit in a Marshall Jubilee/Slash head and this pedal sounds really good now. Kind of like a poor man's Chandler Tube Driver...very liquidy/sustainy/smooth and fat. But the tone control is so sensitive within that tiny range that it's difficult to dial in.

[Matt T.]

FWIW, I saw on the Small Bear site what he called a; 20K, work-alike of the "G" taper pot in the TS-808, and installed one in place of the existing 25K linear pot. That pretty much solved the problem with the tone control.

[Steve Conner]

Is the tone pot log or linear taper? Sounds like it might be log, in which case you could try changing it for a linear one. D'oh I just noticed your last post and realised that you did something like that.

[Matt T.]

Steve, it was a 25K-L but the 20K 'G' taper pot looks like it might be reverse-audio taper.

[Steve B]

Try a 2k linear from Small Bear. It will spread the bright-dark range thoughout a larger pot rotation. I read of people getting the "correct" pot from Small Bear, & preferring the way the 2k pot reacted. It worked better for me.

Steve