Randall RG50TC buzzing but not mains frequency!

[Stratfordade]

Mysteriously this Randall RG50TC (3x12AX7 pre-amp, 12AT7 PI, 2xEL34 power amp) will sometimes buzz when switching from clean to overdrive when input jack in socket. Buzz stops when jack removed (so input is then shorted to ground) and is gone when jack re-inserted, till next unpredictable time it happens.

I thought it must be a ground/power supply thing, but when I put scope on speaker the frequency is 45Hz, not the expected 60Hz of our local mains power. Does this mean the buzz is some oscillation?

The history is I was asked to look originally at low volume from amp, and found half of the OT primary shorted. Replacing OT restored full volume, but could the short have damaged one or both of the EL34s and created some instability?

[Helmholtz]

If you're in the UK, mains frequency should be 50Hz.
Your scope might be a bit out of calibration.
Is the time base knob in its arrested position?

[pdf64]

https://el34world.com/charts/Schemat...all_rg50tc.pdf
There's a muting circuit that seems to be relating the switching contacts at the inputs sockets to Q14&15 killing the signal at the fx send node. There may be something weird going on with that.
Are all the 15V supply ecaps good?

[Helmholtz]

I'd start with cleaning and if necessary re-tensioning input jack (switching) contacts.

Why another thread on the same amp?

[Stratfordade]

If you're in the UK, mains frequency should be 50Hz.
Your scope might be a bit out of calibration.
Is the time base knob in its arrested position?

Thanks. The time base knob is in calibrated position but I’ll check the calibration. I didn’t realise I’d typed 60Hz till read your comment! Fat fingers.

Started separate thread as it was same amp but different issue, but if that is not usual practice here I’ll stick to single thread in future.

[Stratfordade]

https://el34world.com/charts/Schemat...all_rg50tc.pdf
There's a muting circuit that seems to be relating the switching contacts at the inputs sockets to Q14&15 killing the signal at the fx send node. There may be something weird going on with that.
Are all the 15V supply ecaps good?

Thanks for both suggestions. I’ll check them out when I get the amp back from test sat a rehearsal. Your comment about the signal at FX send may also fit with Helmholtz’s sadvice to clean and retension switching contacts.