Epiphone futura static – sudden volume increase

[booj]

Just spent two days working on this beast. Re-capped it and got the tremolo and reverb working real nice. For some reason I can’t get a decent sine wave output from it past the wiper of the volume controls. The signal goes through a resistive capacitive filter that really messes with the signal. But that isn’t the main problem. After I put the chassis back in the cabinet and tried it out with guitar, it sounded ok but seemed to lack gain and headroom. I turned it up all the way and hit a chord and BAM! Mucho signal accompanied by static, then back to low output again. A strong signal almost acts like a switch. My guess is vibration related. But where to start? I’m just frustrated and wonder if anybody has had a similar problem and fixed it.

[booj]

By the way, the GA-45RVT is the proper scat as far as I can tell. The tube layout is slightly different in that the OA2 is on the other side of the 6L6's. None of the regular Epi Futura scats that I've found look close.

[Enzo]

Gibson changed things and changed them some more. Sometimes they completely the innards with a different model, as long as the knob functions were all the same, that was enough. I have 6 different schematics in my file for one model name. And yes, a Gibson model might be in an Epi amp and an Epi circuit might be in a Gibson amp. Is there no small EA-xx on the panel?


If you suspect vibration, find out. Play something gently through it, not enough to trigger it on, and while doing that, whack the top of it with your fist. If that jolt beings the music up momentarily, then we do have a loose connection. If it remains quiet, probably not.


You have been swapping a lot of parts out? I start to suspect you left out a connection or even a resistor. Does this occur to both channels?

Look at each tube. Your schematic shows the channels mix at V4, have you swapped out the tubes through the amp to look for duds?

The drawing is chock full of voltages. Any grids floating away? ANy cathodes not working? even better, are there any points that restore the sound when you touch a meter to them?

[g1]

For some reason I can’t get a decent sine wave output from it past the wiper of the volume controls.

Not sure if you mean level or waveform. Crank it wide open and use a small input signal. Should be the same coming out of the pot as going in. If it's not, maybe it's just dirty pots giving you your problems, is it the same for both channels?

[booj]

Found it It was the Trem LDR unit. Bad solder connection from the factory no less. The absence of the load of the resistive unit would explain the increase in volume. thanks much for your input guys!