Help! Wah Mod Problem

[CA_Dan]

Based on the ultimate wah thread I made some mods to my crybaby (GCB-95 rev G). I put in a 3-way selector for the vocal mod (33K 51K and 68K) and a 3-way selector for the sweep range mod (0.01uF 0.015uF and 0.022uF). I also made it true bypass and increased the input gain by changing the resistor from 390 ohms to 270 ohms. I removed the input buffer and changed the remaining two transistors on the board to BC109B. While I was at it I changed out the remaining poly caps to metal film.

Here's the problem:
When I switch the sweep range to 0.022uF, I get a feedback sound in the partial to full "heel" position. It's almost like the pedal is self oscillating. I can't hear any guitar when it makes this noise and it does it even if an instrument isn't plugged in. What went wrong?

[CA_Dan]

UPDATE: Problem Solved

The culpret was the input gain resistor that I changed from the stock 390 ohms to 270 ohms. As the resistance is decreased toward zero the gain goes up but I didn't notice much difference in output volume. The sound gets "fatter" as the first transistor's gain going up and a little distortion is introduced. When the resistance gets near zero ohms the wah will self oscillate at the low end of the range. I swapped out the resistor for a 1K linear trim pot mounted to the circuit board and tweaked this to the lowest value I could acheive without the pedal self ossicallating - around 810 (!) ohms.

I think (could be wrong here, just a guess) that changing out the transistors to BC109Bs accounts for why I had to change the input resistor value so much, actually reducing gain significantly over the stock pedal to properly bias the new transistor. Anyone know for sure?

[CA_Dan]

This is turning into more of a blog than a thread, but this should be my last post on the subject. Turns out I had a few capacitors switched when I replace them with metal film. Once I made that correction, I was able to sweep the linear pot I installed for the transistor gain from unity (zero ohns) all the way up to 1000 ohms without any self oscillating or cutout to try out a wide range of fat, balanced, and mid range bump tones. Totally fun!!! The lower gain BC109B transistor allowed me to lower the input gain so low without self oscillation, which makes sense. The higher 810 ohm gain I referenced in my last post seemed counter-intuative - I guess it was!

Bottom line, I recommend doing these mods. Made a world of difference and allowed me to custom taylor what I want it to sound like. I know this is useless without a sound clip, so I will post shortly. In the meantime, here are my final mods:

I started with a stock GCB-95 rev G crybaby, date code 12-8-94. I put in a 3-way "on-on-on" selector for the vocal mod with resistor values 33K, 51K, and 68K. I also added another 3-way "on-on-on" selector switch for the sweep range mod with values 0.01uF, 0.015uF, and 0.022uF. I performed the “mid mod” by changing the stock 1.5K resistor between the base of Q2 and the inductor to a 2.2K resistor which made a beautiful difference in the vocal quality of the wah. I made it true bypass and eliminated the input buffer. I increased the input gain by swapping out the fixed 68K input resistor with a 100K trim pot and tweaked the resistance until I got a slight volume boost with the wah on (final value was 33K) I changed the Swapped out the transistors from the stock high-gain MPSA18 to lower-gain BC109B, and added a 1000 ohm trim pot to adjust the gain on the input transistor. Sweeping it to the lowest value resulted in a nice fat tone with a little grit to it - perfect for blues. Sweeping it to the highest value gave it a thinner sound with less bass and more mids and treble. It also cleaned up the sound and lowered the overall volume a bit, which could be corrected by lowering the vlaue of the input resistor to maybe 20K. This setting would be great for funk or cutting through the mix in rock, metal, country, etc. The final value I settled on for this pot was 0.7 ohms, which is just shy of really bass heavy and still has a nice fat tone to it on the bass and mids while still keeping the treble there. This is good for my style of playing. Last, I changed out the remaining poly caps to metal film which made no audible difference to me so I recommend skipping this step unless you want bragging rights. I also don't plan on changing the stock fasel to the red or yellow aftermarket units as I hear this has a very minimal effect on the sound. I'm completely happy with it now. The only other mod I might do is mounting the transistor gain pot externally.

[Edit: corrected a few typos]