I'm just gonna go ahead and share my work. My first big mod project. Fender PA 100. Be gentle.

[Whatizitman]

Sometimes high gain amps have negative feedback loop between pre amp stages. If there is and you have cancelled stages perhaps it has turned to positive feedback which can cause oscillation?

Well, it started out as a very low gain amp. There are technically 3 gain stages in the stock PA 100. Similar to a BF/SF amp in that respect. But even with the stock master volume and the first small mods I did to the inputs and cathode values, it was still a very clean pre-amp. That's when I decided to cascade the first channels (V1A and V1B) and convert to a cathode follower driven tone stack. I guess I just figured that it wouldn't make this much of a difference, even with V5 still in the circuit. Since I set V2 up similar to a Marshall JCM 800, I really failed to take into account the added gain stage of V2A, in addition to the less loading of the tone stack. There's just too much gain. More than a very high gain amp. And that's not really what I want. If I can lessen some oscillation just by reducing gain (taking out a gain stage), then I should do that next. My plan was never to make a super high gain amp. I just wanted it to be like a JCM800.

 

[tubedude]

Low frequency oscillations, especially under heavier drive, may point to insufficient capacitance or insufficient isolation of power supply nodes.
On all high gain builds I have a supply node for each tube, 10-20uF, depending on function.
Also, not to be critical, but cleaning out wire trimmings, solder pieces, etc. before powering up is a good idea. An upside down shake, or compressor blowout of the chassis can keep them from becoming buss bars and fuses. Don't ask me how I know. Keep Calm and Solder On!

 

[mfguitar]

My very first PA system, so yes I was the band leader

 

[2L man]

You possibly could be able to isolate where the oscillation come "shorting and cutting" signal path. Typical cathode biased triode stages quit amplifying when grid leak resistor is shorted. Also when other lead of coupling capacitor is detached.

Eventually an oscilloscope is needed to troubleshoot electronics

 

[Whatizitman]

You possibly could be able to isolate where the oscillation come "shorting and cutting" signal path. Typical cathode biased triode stages quit amplifying when grid leak resistor is shorted. Also when other lead of coupling capacitor is detached.

Eventually an oscilloscope is needed to troubleshoot electronics

Got a lead on one close by for a good price. May have to take the plunge.

 

[Whatizitman]

Easy temp fix! I bypassed the extra gain stage and cathode follower, effectively making the tone stack plate driven off V1B, lowering the overall gain dramatically. Significantly less floor noise and no audible tremolo type oscillation. Gain and distortion levels where like it. I’m gonna clean things up a bit, maybe mess with the tone slope a bit to brighten it up. But I’m gonna call it done enough, for now.

Lol it’s pretty much where it was before I rewired it for cathode follower. With the addition of some more Marshally tone shaping.

What did I learn? Lots, actually. For the amp’s sake it would have better to not mod it so drastically. But my goals were to learn as much as it was to keep it playable. It would more of a risk IMO to spend the dough on a proper 2204/2204 clone build, given my novice level of understanding. So I’m good.