1956 Gibson GA-6 driving me NUTS!

[Fred G.]

Pin #4 is connected to the "microphone" volume pot wiper. I expected a pop when probing, but it actually begins to hum louder and louder as I get the probe nearer and the pop is unusually LOUD and persistent - more like thunderous continuous static!

OK, an update: checked grids and pots for any DC. None. Disconnected dummy load, connected speakers back up. Moved the wires from the preamp filter caps away from the terminal strip ( I had inadvertantly moved them too close when I installed the 3-prong power cord). The "ticking-motorboating is gone. Now - the "instruments" volume still pot gves a "sssssst" when turning it up in the very early rotation, and hum of course increases as you turn it up. There is also a similar "ssssstt" when the Tone pot is turned all the way down, it goes away as you turn it up. Finally - the input cap from the "instruments" input seems VERY microphonic - when tapping it with a chopstick, you can hear it in the signal - similar to tapping a microphonic tube. the voltages on v1 tube seem normal.

On testing the cathode resistors of V1, I'm finding one of the 4.7M has drifted up to 5,5M, and this is indeed on the "instruments" leg of the circuit. This is one of the resistors I left unchanged.

Thoughts?

[g1]

One of the biggest problems with Gibson amps is finding the correct schematic. That is what I'm concerned about here.
Your description does not really match the posted schematic.

This schematic shows the pot wipers going to the previous stage. And the 2 pot 'hots' tied together at one of the 6SL7 grids. My questions only relate if the schematic is the right one for this amp. For this drawing, I don't see why it should matter much which wiper you touch with your probe.
You might want to check for DC voltage on any of the pot terminals though.

[Fred G.]

I think I may have made an error in the power supply - I substituted a 30uF e-cap in place of the 20 uF for the first filter cap shown in the schematic. Looking at the tube specs for the JJ 5Y3, I see that the "input capacitor" is specified at 20 uF. I "thought" I was actually doing a "good" thing here (based on a blog someone posted about restoring these amps) - helping to firm up the bottom end a bit - but did I instead exceed to recommended value, and actually cause more problems? What would be the net effect of this?

Thanks!

[Chuck H]

I think I may have made an error in the power supply - I substituted a 30uF e-cap in place of the 20 uF for the first filter cap shown in the schematic. Looking at the tube specs for the JJ 5Y3, I see that the "input capacitor" is specified at 20 uF. I "thought" I was actually doing a "good" thing here (based on a blog someone posted about restoring these amps) - helping to firm up the bottom end a bit - but did I instead exceed to recommended value, and actually cause more problems? What would be the net effect of this?

Thanks!

The 30uf is fine there. When that amp was made power supply caps commonly had a spec tolerance of -20%/+80%. Tube manufacturers knew this too. Spec tolerances are typically much tighter now. I've used 47uf in Champ type builds three times with no trouble. Two of those builds were over fifteen years ago and the latest was maybe five years ago. The last one is a guy that's still local to me and I know that he uses the amp often.