1955 Gibson GA-30

[Helmholtz]

Interesting.. I think the schematic shows 75V plate for the mic channel first stage. I'm at 85V or so.
The amp sound really really good till it gets to about 4. I'm getting what I thought to be speaker fart initially, but I have put 4 different 12" speakers on it today, and they all are exhibiting the same sound....nasty farty breakup only on the E and A strings, when hit hard when it starts to get loud. Note that it's starting to breakup in a pleasing way around 4 with no issues, but when it really starts to crunch, weird sounds happen..
I even changed the 1st coupler to .02 with no change.

If this only happens with the mic input it's likely grid conduction of the grid leak biased input tube.
Another reason to try a buffer.

[Chuck H]

Ordinarily I'd suggest changing the input stage to something more typical and modern. Higher plate volts and cathode biased. This would make it consistent with what is expected from an amp. But I digress. If there's anything about the vintage design that is to be captured then the inherent flaws must be accepted. In other words, it's possible that the "problems" cannot be solved for without making the amp into something other than what it is. Or was. And how much modification is acceptible is up to anyone who would own and play through it.