1976 Vibrolux OT issue?

[KilgoreTrout]

Hi folks:

I have a 1976 Fender Vibrolux Reverb that suddenly went silent. When I got it on the bench I discovered that the plate voltage of the first 6L6 (V7) had dropped below the screen voltage. I swapped tubes but the problem persisted. Here are the readings, with V8 for comparison:

V7
Heater 6.2 VAC
Plate 335 VDC
Screen 392 VDC
Control Grid -40 VDC

V8
Heater 6.2 VAC
Plate 397 VDC
Screen 392
Control -40 VDC

I changed the screen resistors for good measure. At some point, it started outputting again, at least through a dummy load and scope.

Resistance from the output transformer center tap to V7 is 78 ohms. Center tap to V8 is 69 ohms. Time for a new OT? Is there something else I should check?

Thanks,

Garry

[Helmholtz]

The DCR difference between the 2 OT primary halves is normal and cannot explain the difference in plate voltages.
I just measured a Fender Vibroverb OT and got 73/83 Ohm.

Please pull the PI tube and repeat plate voltage measurements.

Probing power tube plates often excites HF oscillation, causing the DC plate voltage to drop. Typically only one of the tube plates is concerned. Pulling the PI prevents the HF oscillation.

[KilgoreTrout]

Interesting. With PI removed, V7 has 409 VDC at the plate and 408 at the screen. V8 is 410 at the plate. So I guess we're okay. Hopefully the original problem was only a loose tube or something. Thanks so much for your help. I've learned something new.

[KilgoreTrout]

Mystery solved: bad speaker cable. I should've checked that first.