Hey Dave,
greetings from the colonies ;')
Yesterday, as I was researching my own problem (50Hz hum in rg75g3) I noticed some comments about people wanting to check if their valves were 'rattling', and the advice (fwiw) is to put on some oven/pot mittens, and while the tubes are 'hot', and the rattle is coming from the amp, hold the valve(s) in the mittened hand. Obviously not tightly, nor for long (they would still get hot) but long enough to determine if the noise is coming from that part of the system.
disclaimer:
Of course, there is no warranty expressed or implied in the above advice - and you should seek separate advice from a qualified technician before proceeding.
I know *I'd* think twice before some barmy stranger from the antipodes told me to stick my hand in the back of a hundred-bazillion volts!
kind regards,
.h
