Fender 57 Deluxe 5E3 reissue problem?

[drewl]

Got one of these lovely tweed Deluxe reissue 5E3,
Customer says volume drops when turned past 6 or 7.

Looking at it on a scope, it's just the amp distorting/compressing, normal, right?

The thing that's odd is I'm getting negative voltage on the input grids of the 6V6's when the amp is cranked, around -8 to -13v

is that coming from the bias supply? I swapped out the coupling caps C8 and C9 just to check, but it's still there.

I know I've seen this on some old Ampeg amps

Also the output of V2-B from pin 8 looks funny when cranked, it's not a clean sine wave, but a wave which thins at the top to a point, almost. That's from the Cathodyne phase inverter, right?


I know the output trans is bad, the output is low. I subbed in another and got full power, but the same conditions as with the original OT.

[Helmholtz]

The thing that's odd is I'm getting negative voltage on the input grids of the 6V6's when the amp is cranked, around -8 to -13v

I think that's bias shift, caused by grid conduction charging the the coupling caps negative at the grid side.
Consequently average grid voltage gets (more) negative.
Happens more or less with all tube amps to some degree

[pdf64]

Grid rectification. Grid to cathode is a diode.
Take out the output valves and clip diodes between control grid and cathode terminals, banded end to cathode, and you’ll see the a similar effect.