Silicon FF (BC108c) on bredboard sounds great but oscilates like hell

[yeeshkul]

I put together my second silicon FF (i had to ditch the first one because of oscillation) - this time just on breadboard. And again - when i turn the fuzz pot all the way up, it explodes with oscillation. Both trannies are BC108c (high gain) and the sound is brilliant. I am just a bit disappointed, because i don't want to loose that last bit of fuzz-all-the-way-up by adding a resistor in front of it. I have seen pictures of the original units and they also included 108Cs.
Any idea? This project seems to be cursed

[R.G.]

Did you use a power supply decoupling cap?

[slideman82]

So, are u listening f*cking treble noise? Put a 220p cap between C and B of 2nd tranny, and try it too other in the C and of the 1st tranny too. Also, put a 100u prom 9V to ground, and a 1n in parallel with this one.

Try the Ge one, sounds better. I believe you are a Gilmour fan, aren't you? 

[yeeshkul]

I am hehe. I have Ge FF already and really want to try Si version, because Dave was using Si FF between 70-76. Actually when the fuzz pot is turned a bit down the sound is excellent. Very Gilmourish . I put 100u the cap on battery and it made it squeal even worse (more toppy). I just can't believe both my builds oscillate ... grrr. Dunlop used to produce their JHs with even more powerful trannies. Just my bloody thing doesn't wanna do good.

[slideman82]

Cufk JD factory! The sound like crap! I say my FF are the best in all Argentina... none of them oscillates, and I alwaays use AC128, some of the last AC151 too, great combo...

Nevermind the Si version, just keep the Ge one, if not, try those caps I told ya.

[ambulancevoice]

could be a short somewhere, check for shorts (on the power supply and everything)
mis bias, try some trimpots 100k 0r 50k
also, try and audio probe and poke around and find where the oscillation starts

[yeeshkul]

It is on breadboard, so no shorts, and bias of Q2 is tuned to 4.5V. But the audioprobe sound like something i need haha. Where can i find some info on it?
thanks mate
Jan

[slacker]

It's oscillating because there's too much gain, adding the caps that Slideman mentioned should sort it out.
If that doesn't work try adding some extra resistors as shown on the AMZ YAFF schematic http://www.muzique.com/lab/yaff.gif R7 reduces the gain of the first transistor slightly and R8 does the same for the second. If you play about with the values you'll probably find some that stop the oscillation without affecting the tone too much.

[yeeshkul]

thanks that sounds good, by the way i found about the audioprobe already ...

[tcobretti]

It is almost unavoidable for a high hfe Si FF to oscillate with the fuzz dimed.  The mods above should help but will change the tone slightly.

Here's my high gain Si FF that doesn't oscillate.

[ambulancevoice]

It is almost unavoidable for a high hfe Si FF to oscillate with the fuzz dimed.  The mods above should help but will change the tone slightly.

Here's my high gain Si FF that doesn't oscillate.

is it alright to use a 1kl pot for the gain??? or is there a way i can increase the resistance of the 1k pot?

[Gus]

schematics link at the top of the page? 

http://www.diystompboxes.com/pedals/schematics.html

vox 2 transistor fuzz circuit has a hint

[col]

Try some different transistors, even different 108s. I have had that oscillation and there was also a strange tail off to the notesusing two 108s. I finally settled on a 108 for T1 and a 109 for T2. The lower gain power transistors in low energy lightbulbs sound fantastic too but they are ECB pinout.
The cap across the BC junction works well but yopu have to slowly work up the values starting at around 100p until you find the smallest value that works. If you go too high it sucks all the treble out of the circuit and it's really dull.

[Paul Perry (Frostwave)]

The stray capacitances associated with a breadboard can be enough to make a high-gain circuit oscillate.

[analogmike]

also check wiring. it can come fom having wires too close together.

[ambulancevoice]

http://two.flash-gear.com/acid/acid.php?c=v&id=969827488

http://four.flash-gear.com/eye/eye.php?c=e&id=517615&k=27441549

CLICK THEM MIKE!

[tcobretti]

BTW, yes, you can use a 1k pot.

[ambulancevoice]

BTW, yes, you can use a 1k pot.

phew, good, cause ive started to build it
with switchable transistors and the input diode mod (like on Joe D's Antiquity)

[analogmike]

http://two.flash-gear.com/acid/acid.php?c=v&id=969827488

http://four.flash-gear.com/eye/eye.php?c=e&id=517615&k=27441549

CLICK THEM MIKE!

fun with flash!

Check out my peppermint fuzz page, (go to the bottom).

Thanks!

http://www.analogman.com/pepper.htm

[ambulancevoice]

YAY!!!!