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.

[LarBal]

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.

I seem to remember it still happening with the instrument inputs also, but not as bad. I chalked that up to considerably less output and overdrive in those channels. I will check with a boss tuner.

[LarBal]

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.

Exactly, this is in the forefront in my mind. The last thing I need is another 15W fender amp around here. This amp has a ton of character that I want to preserve, so the goal is to keep that, but clean up the overdrive. It's got an incredibly good crunch in there if I can get rid of the fart.
Speaking of Fender amps ha I was looking at the 5C4 Super, and the resistor values in the paraphase stage and preceding stage are consistent with other gibson 9pin values(GA-20) values, 220k instead of 470K, and a higher shared cathode. I'm going to experiment in this area also. I'll attach them all here for comparison

[Tom Phillips]

Reviving this thread to add a schematic , as I lucked into one of these rare 12AY7 GA-30 amps. Got it for $400 in non-working condition. 100% original ...

Any chance that you took before & after photos? If so, it would be interesting to see them.

[LarBal]

Here are the pics on my phone. Looks like I had just started to do a cap job.

[Helmholtz]

Did you measure output power?
Can be quite revealing.
I'd expect 12W to 15W clean.

[Tom Phillips]

Here are the pics on my phone...]

Thanks. I know from experience that taking photos is sometimes a pain and can slow work progress.
Please let us know if any additional photos are found.

[LarBal]

Thanks. I know from experience that taking photos is sometimes a pain and can slow work progress.
Please let us know if any additional photos are found.

There's some pics at the start of this thread from another poster, but there are some mods there. Mine appeared to be bone stock except for maybe the green tone cap

[LarBal]

Did you measure output power?
Can be quite revealing.
I'd expect 12W to 15W clean.

No I have not, but that's what I figure

[Helmholtz]

No I have not, but that's what I figure

Sure, but the question is if the amp does produce something like 12W clean.
If not, the waveshape might give some clue regarding your farting issue.
I would use a 100Hz signal.

[LarBal]

Sure, but the question is if the amp does produce something like 12W clean.
If not, the waveshape might give some clue regarding your farting issue.
I would use a 100Hz signal.

Armed with my new scope, it took 2 minutes to see that the waveform went crazy at the tone pot, when the volume got up past 3, which was when the farting happened.
I replaced the pot and the amp came alive, and is much louder now. I couldn't even get it up to the breakup volume at this hour, so I think my problem is solved. The waveform looks completely different at the pot now too, and looks good even with the volume dimed.