Fender Twin 135w - 120hz hum

[Garrett335]

Howdy guys,

I'm lookin for help once again ha.
Working on a Fender Twin 135w model that a friend brought to me.
He tried to recap it himself and put one of the filter caps backwards, turned it on and the amp went up in smoke, he asked if I would look at it.
Once opened the carling switch was fried.
I rewired the filter section under the dog house, replaced the screen grid resistors, and plate load resistors, the phase inverter bypass caps.
The amp sounds great however there is this large 120hz hum that is present when amp is on and no instrument plugged in. I can remove all preamp tubes and the sound persists. The sound only goes away when the power tubes are removed. (Tried a new matched quad pair, didn't fix the sound)

When checking on the scope the signal is strong and healthy right past the PI to the grids of the output tubes.
Testing at the jack the noise is there.

I'm wondering if he messed up the OT during the short.

-Tried disconnecting the NFB
-Measured the heater voltages (all normal)
-Retraced all my connections on the Filter board
-Chopstick test, nothing appears to be loose

Any other tests you can recommend?

Here are the voltages with his non matching quad (None of the tubes match)
V7
Plates 465
Screen 474
Control -46
Plate current 44mA

V8
Plate 461
Screen 474
Control -46
Plate current 50.2mA

V9
Plate 462
Screen 474
Control -46
Plate current 46.5mA

V10
Plate 462
Screen 474
Control -46
Plate current 58.8mA

Thank you guys! This one has been stumping me for days.

[shooter]

there is this large 120hz hum

can you see it on the PS rail (at the caps in the doghouse?)

[sluckey]

New matched output tubes may fix that hum.

[pdf64]

Which cap was reversed?
Are you sure the rectifier diodes are ok?
Does the ‘output matching’ pot on the back panel make any difference?
How about the hum balance?
How about with the output valves fitted but the LTP valve in V6 removed?
https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_135_schem.pdf

[Garrett335]

Which cap was reversed?
Are you sure the rectifier diodes are ok?
Does the ‘output matching’ pot on the back panel make any difference?
How about the hum balance?
How about with the output valves fitted but the LTP valve in V6 removed?
https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_sf_135_schem.pdf

I believe the 20uf cap feeding the PI plates. It was partially disassembled when I received it.
I replaced the bridge rectifier diodes two of them were shorted. I have not replaced the bias rectifier, it appears to be working as I'm getting negative voltage etc.

Hum balance and output balance doesn't affect it.

Sound is still there with V6 LTP removed

I'm going to check the rail voltage with the scope. The signal looks clean everywhere else in the circuit, going to and after phase inverter, on the grid of the output tubes, the plate is where it's super funky, and the output looks like a shaking sine wave.

[Garrett335]

there is this large 120hz hum

can you see it on the PS rail (at the caps in the doghouse?)

I can actually, the 22uf caps that feed the output transformer has a lot of noise shaking back and forth.
There is ripple after those caps also, and the negative bias caps are showing dual lines idk what that means.

[sluckey]

I can actually, the 22uf caps that feed the output transformer has a lot of noise shaking back and forth.
There is ripple after those caps also, and the negative bias caps are showing dual lines idk what that means.

Probably just a scope triggering issue. Select normal triggering mode, Line for trigger source and turn the trigger level knob for a stable display. Also slow the time base down enough to see several cycles on the display.