Strange preamp voltages in a Gibson GA-5 (like 5F1 tweed Champ)

[dadydg]

I recently picked up a tweed Gibson GA-5. It's nearly identical to a 5F1 Champ, save for maybe the preamp plate resistors being 220k instead of 100k and a couple other resistor/cap values. The layout and tubes are the same.

The problem I'm having it I'm getting strange voltage reading around the second pre-amp stage (second half of a 12ax7 just like a Champ) which vary inversely to how the volume pot is set. Volume at "10", plate is 41vdc, cathode 2.47vdc, volume at "0" plate "135vdc, cathode 1.09. The first half's plates and cathode stay around 135/1.15 regardless of volume setting.

I checked both plates resistors and they are with in spec. I checked both cathode resistors and they are also within spec. I tried a few different 12ax7s and no change, they all react the same way. I stubbed in a new cathode cap on the first stage - no change. I tried an new volume pot - no change.

The only alterations to the amp are that I just replaced the filter caps.

Even with the strange voltagse, the amp sounds OK, but I know it's not as loud/gutsy as it would be if the second stage was operating correctly.

I need some ideas, because I'm out of them.

Dan

[Enzo]

What is the DC voltage on the grid of that second stage when volume is at zero, and then again when it is maxed? Got a little positive DC? ALL it takes is a 10th of a volt. Should be NO +DC there.

Is the volume control a bit scratchy? That would be a result of DC on the pot.

Are those .02uf coupling caps original? You mention changing all the caps except those, and they are the ones that will leak DC.

What is the DC voltage at the grid of your power tube. SHOuld be NO +DC there either.

[dadydg]

Sure enough! There's about 30vdc on the second stage grid and 2vdc on the power tube grid. As the volume pot is turned the DC on the second stage grid drops to 8vdc and the DC on the power tube grid rises to aabout 20vdc.

I'll replace the coupling caps and report back.