EricM wrote:

Forgot to mention that the 'buzzing' only starts after the relay
closes -

Well of course it does, because before that the amp isn't working at
all. A tube amp without anode voltage is just a refined heater.

Do not forget that this is a tube device. Some of the tube cathodes
are heated with 6.3 Volts alternating current of 60 Hz. If there is a
cathode-heater-leak (due to tube age) you get AC into the signal path.

Note that the preamp stages are operated with 400 Volts off the
stabilizer circuit around V9 - V10. So have a look at C20 / C2 - they
should neutralize AC ripple there.

I'm scratching my head about connectors 3 and 4 on TS2 named
regulator filament. Somehow TS2 in the ps-schematic doesn't seem
to be TS2 in the amp schematic.

Regards,
H.