Taming the high end bite of the big box 2-channel Carl Martin PlexiTone?

[Burstbucker]

Hi guys,

I bought a big box 2-channel + Boost, Carl Martin PlexiTone pedal last week, it's the one with the three footswitches.  The pedal certainly has potential but hit the mark a little bit since I think that it could have been tweaked to be even better than it already is.

For starters there's the Tone knob that should maybe be called Presence since it quickly gets into the ice-pick region if set anywhere beyond ten o'clock.  I might try doing a small mod to roll back on that super bright character.  What would be the best way to accomplish that?   

There were a couple different versions of the PlexiTone, the earlier models had four LEDs and the later models only had three LEDs.  Mine is the newer one with the three LEDs and by then they had apparently also fixed the problem where the effect was bleeding into the clean signal while the pedal was in bypass mode.

In the Tone section of the schematic that I've linked to, it shows that the Tone knob is a 5K pot that's connected to C13 which is a 0.068uF capacitor.  And both of those are in parallel with R7 which is a 22K resistor.  Is there something in there that I could change that would dial back the super ice-pick highs?


Here's the schematic that I am looking at:

[thermionix]

I would try a small cap in parallel with R7, something in the pF range.  Don't know how to calculate which value, I would have to experiment, but maybe 330pF is a good starting point.  The point would be to have the highest frequencies fully fed back at all times.

Just my noob idea, not necessarily the best approach.  Wait for somebody smarter than me to chime in.  That should probably be my signature line.

[anotherjim]

I'd put a cap across R7 also. Alternatively, it could go across the tone pot if that's easier to play with.
I dunno how distorting it gets - a lot of distortion harmonics might mean the required treble cut can be both stronger or starting at a lower frequency than theory suggests - it is only adding 1 filter pole. 330pF would be a good try, but be prepared to go over 1000pF.