Positive supply rail Fuzz Face Oscillation

[RobA]

I've been breadboarding with the Fuzz Face circuit to do a positive rail version. This is using PNP Ge transistors. I've read that there are oscillation issues that some people experience when doing this. And, I did an etched PCB for this a bit a go that was a failure because of oscillation at high gain. But, I hadn't been seeing any oscillation with the stuff I had on the breadboard at any gain setting with any set of transistors.

Then I tried to add a sag control. Oscillation everywhere with every transistor at a gain above about 2/3 max. Interesting. I pulled the sag circuit back out and tested using the variable voltage on my bench supply -- no oscillation at any voltage setting. Weird. So, I put a 100Ω resistor on the power input before the filter cap. Oscillation again. I tried it with a 10Ω resistor and it still oscillates, not with all transistor pairs but with at least a few.

So far, it looks to me like I can do this with a positive rail with no risk of oscillation as long as I don't have any restive filtering on the power input. This feels like it makes sense because, to this circuit, it's like putting a resistor on the ground input and I can see that causing some issues.

Does this make sense? Is it consistent with what you've seen or heard? Or, really to the point, has anyone experienced oscillations on a positive rail fuzz face circuit that didn't have any restive filtering on the positive power rail?

Also, it seems workable to add a sag control as long as it's based on an active variable voltage circuit that can sink and source current. The one I'm using that works is built around an LM317. But, simply buffering a pot based voltage divider followed by an emitter follower did have oscillation problems.

[Matt]

I had a similar issue with a positive ground tone bender 2 knob. Strange thin was that it worked fine for a few months then one day it started oscillating at max gain. I thought I read that it has something to do with the current dropping too much and causing feedback in the circuit. (I could be dead wrong about that). I can't remember who to credit but the fix was to add a 220uf filter cap. It has worked fine ever since.

[RobA]

Interesting, current issues could be related to what I was seeing, but I was assuming that it had to do with adding an impedance to the ground channel. I'll have to do some experimenting with both negative and positive power rail configurations. I haven't seen any issues with oscillation in the negative rail setting yet. I've been using a 100µF cap followed by a 100nF ceramic cap to filter the power supply. I'll try the transistors I get to oscillate with the resistor on the power rail while using a bigger filter cap.

Thanks for the info, it should help me with some further experimentation.