soft "howling" oscillation on my tonebender MK II

[mordechai]

When I max out the fuzz pot on my TB build, I hear a moderately high pitched howling...it's not loud and by playing with the bias pot and fuzz level, I can cut it out, but how can I shunt it out completely?

[seedlings]

I've seen that on breadboarded FF circuits when the leads of the Q2 bypass cap don't make good contact in the sockets.  Try touching each component with your finger to see if it goes away.  Also if the 100K feedback resistor is connected to the Q2 collector instead of emitter you get positive feedback.

CHAD

[Electric Warrior]

If you used the 25µF power filter cap, try a 50µF. Sola Sound did the same thing pretty early on. Seems like they had that problem too, though I couldn't reproduce it with OC75s. With lower leakage transistors like 2sb175s I can get it to oscillate with certain setups.

[mordechai]

Well, I'm not worried about the feedback resistor not connecting to the right lead.  This is built on a verified PCB, Q2 is socketed and it's definitely oriented correctly.  The cap you mentioned is also properly soldered and connected.  I wonder if it's transistor choice/gain/leakage.  I'm using OC84's, Q3 has a fairly high gain (around 130 Hfe) and all of them have quite low leakage (surprisingly low, actually).

[mordechai]

BTW...my power filtering cap is the "standard" 47uF.  Should I go up to, say 100uF?

[Electric Warrior]

nah, 47µF should be enough.
Maybe try swapping Q2 and Q3 or try a lower gain transistor for Q3.
What resistors values are you using at Q1's base and Q2's collector?

[Mugshot]

is the box "earthed"?