Germanium: High pitched frequency

[JJPJ83]

I'm messing around with a few Germanium Transistors (IT308B or GT308V if you insist). These are low gain PNPs, my batch varies from 60 - 80 hfe. I've flipped the circuit in the same manner as a Fuzz Factory to accommodate the PNPs with Negative GND. The problem is, I'm hearing a harsh high frequency pitch during the decay. I've tried various capacitors from B to C and from various points to GND and this does seem to help in some areas, but this makes the tone dark and muddy.

Has anyone experienced anything like this? Could it be leakage? or the DC ripple? or maybe I'm too stoned any my ears are playing tricks on me.

[R.G.]

This comes up over and over

...and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over and over.

Trying to reverse the ground for convenience on PNP circuits and especially PNP germanium circuits works often enough so that many people think it's fine and there's no problem if you just do it right. While that may be true, the exact formula for "right" in this case remains unknown, and it's not for lack of some very detailed, long, theoretical, practical, and just downright exhausting trying.

People who think reversing the ground for convenience simply haven't hit the problem - yet. There are many dodges that might get you by, until the next funny one comes up.

Flip the grounds back to positive ground and it'll probably work. There are other ways to mess it up and get oscillation, but putting the grounds back will remove at least one of them.

[rrroo]

i once had high pitched squealing with russian PNP transistors and it was with positive ground wiring. the "problem" with some russian germanium transistors is that they are actually quite good transistors. transition frequency of GT308 is about 120MHz where AC128 has only 1MHz. So the capacitance of AC128 or other classic Fuzz Face transistors is naturally few hundred picofarads whereas GT308 has only ~10 pF. So frequency-wise some of these russian transistors are closer to silicon transistors. So a 100-200pF cap across B-C of the first transistor could help.

also change back to positive ground. that should clear things up.

[italianguy63]

I use these trannies with zero issue.  They are super low leakage, and super quiet.  You should not need filter caps.

Go back to positive ground and us a voltage inverter to make the pedal compatible with your pedal-board (charge pump).

MC

[pinkjimiphoton]

use a gigantic decoupling cap in the power supply... that can help.
you may find reversing the beta of the transistors may help too. i know it's bad, and all, but i've read about a hundred articles from the early days about transistors, and flipping them 180 degrees from what you expect can help.
a smallish cap to snub oscillations and crap can help.... sometimes between b and c, or c and ground.

if ya wanna run a pnp circuit on a negative supply, you HAVE GOT TO DECOUPLE THE LIVING @#$% OUTTA IT. and even then... no matter what tricks ya try... it may or may not work as RG said.

but the biggass decoupling cap can help... try something ridiculous, maybe 1000u. add another snubber to it too, for the real high frequency crap... somewhere between 10-1000p or even more. breadboard it and keep pluggin' 'em in til it goes away, or you give up.

remember, the leakage from the ge's is gonna make it an issue. directly grounding the emitter may help, but ya may need to decouple that with a smallish resistor and a bigass cap.

and electrolytics have leakage too... what would be ripple in an npn circuit can become a feedback loop in a pnp circuit very easily.

in my experience, what works on ONE may not work on a for all intents IDENTICAL one. keep plugging... you'll get it.

one last trick to try is temporarily wire in a huge-ish linear pot for the c resistor, sometimes transistors can have multiple "ranges" where they will work... if you're getting weird behaviour, sometimes going way past where you'd expect can lead you into an area where things that don't work right suddenly do... particularly if you have a really tiny "window" with a trimmer where it biases right.

i had a toneblaster fuzz come back to me a while back from someone that went south. i couldn't get the damn thing to bias at all, and when i COULD, the freakin fuzz knob was working backwards.  rebiasing by ear instead of using my meter made a huge difference.

the joys of diy....  i love the magic smoke 

[JJPJ83]

Thanks for all the helpful replies. I think I'm just gonna switch back to +GND to make it easy.