1979 135w Fender Twin distorted output

[mresistor]

Hello all. I received a '79 Fender 135 watt ultra-linear Twin that had a blown apart power switch. It was fried and shorting to the chassis. I have temporarily subbed in a new on-off switch to trouble shoot the amp.
It obviously was blowing the 4 amp fuse but is not now. However the output is extremely low and distorted on both channels.  All the power tubes check good no shorts. All the small bottles check good. This amps OT has the taps for ultra-linear operation. The filament voltage is good all along the string. At the standby switch in standby there is 503 vdc. I've got 509 vdc on one side of the plates and 507 vdc on the other side of the tube pairs. It does still contain all the original ps filter caps and cathode bypass caps, which are the metal cased Spragues with no signs of leaking. I suppose I should re-cap this amp before going any further. However it appears that the OT may be shorting. I don't have an ultra-linear OT to sub in. My thought is to use a Super OT I have on hand and jumper the screen voltage pin on the power tubes to the plate for testing. I would only use a set of two power tubes, i.e in half power mode.
Would this work?

[mresistor]

-or I think I could tie the screens of the 6L6s  to the OT center tap and to run Pentode mode on the substitute OT for t-shooting purposes.  right?

[mresistor]

Well I unsoldered the OT center tap and pulled all the 6L6s and measured across the OT from primary to secondary and it was infinite. So, OT looks to be OK.
Put one pair of power tubes back in and checked the bias voltage and it is right at -58v on each side.
Turned the amp on and pulled the Master Vol switch and the amp is now working normally again. Pushed to MV switch back in and still works fine.
I guess I must have moved something probing around in the amp.


I guess a near 40 year old Carling on'off switch can go bad with use and short internally.