Vox Tone Bender Debugging

[diemilchmann]

I built a Tonebender a while back, and once you bring the volume up to full and the tone up to anywhere good, it immediately starts a really bad screech, just constant noise. I'm pretty sure its something to do with the gain, but i have no clue where to put a cap or anything to remedy it.

Here's the schem, I did use the 2n3906 for both transistors.

http://www.freeinfosociety.com/electronics/schemview.php?id=612

[Chugs]

You could try connecting a 100 Ohm resistor from the emitter of Q1 to ground. That will reduce the gain slightly and may cure your screeching problem. Germanium transistors were originally used in this circuit and the silicon 2n3906's may have a little too much gain which could be causing the noise. If you still have a screeching problem after trying that you could also try placing small capacitors across the base and collector of Q1 and Q2. Try 47pf. 

[zombiwoof]

I would definitely try some lower gain germaniums in that pedal, if you want it to sound like an original Vox Tonebender.  Small Bear has matched trannies that don't cost much.

Al

[diemilchmann]

I tried putting in a resistor, because i didnt have a good value of a cap around. But i ended up putting 2 trimpots in parallel and using that. I have a really nice tuner now for the distortion! Thx for the help.

[daganyb]

I'm working on this same project (with the same transistors) and having a similar problem. Can you say more about what you did with those pots? How's the sound?

[diemilchmann]

All i did was put about a 750 ohm resistor between Q.....1? and the ground. i actually put 2 10k trimpots in series and i have a cool little control thing ot tweak.