Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BrianS on March 06, 2026, 06:42:38 pm
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This cap was across the output transformer primaries of 1960s Gibson GA-5. I could not for the life of me figure out why I had only a very small amount of heavily distorted output when everything seemed to be working and testing correctly.
One of my final tests was to disconnect the output transformer and inject 6VAC into the secondary. When it passed that test, I started looking closer at every connection, and found this cap hidden behind other components. The cap is not on any GA-5 schematic I could find, but it does look original to the amp per how it's connected to the sockets and the yellow spaghetti tubing that was on the leads.
I put the cap on my capacitance tester and it wouldn't read on any setting. That's when I decided to test the resistance...
Hooked the amp up without the cap, everything worked and measured as it should.
I found reference to caps being used in this position as a "snubber", but also being used as form of controlling the tone of the amp.
I think I will replace the cap with something of the same value, but with a higher voltage rating (it's a 400V cap).
https://photos.app.goo.gl/TUKNKRNFMnxo4gKh6 (https://photos.app.goo.gl/TUKNKRNFMnxo4gKh6)
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I suggest leaving the cap out of the circuit. It probably failed because someone was playing the amp cranked wide open without a speaker connected. When those high level power chords reach the end of the OT winding only to find there is no speaker connected, a high voltage flyback pulse will be sent back to the primary. This pulse likely arced across the cap plates and created a short circuit.
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I'm sure you are correct as to why it failed, and that was my assumption as well. When I took the back off the amp, I noted that the size 22 solid core speaker wires were not connected the speaker at all, and when they were, it looked like they were just hooked around the speaker connections with no solder.
I plan on replacing the cap with a .0022uf, 1200V rated ceramic cap. I am going to assume the folks at Gibson added the cap to help remedy some issue with the frequency response or tone shaping of the amp. The customer claims this is their favorite amp, and I want to minimize the changes to the circuit to just what they asked for.
To minimize the chance of the speaker coming disconnected, I have attached longer, more robust speaker wires & properly fitted Quick Connect terminals.