On Wednesday, August 10, 2016 at 12:07:10 PM UTC-7, Gareth Magennis wrote:
I have a fairly vintage valve amplifier with a dead HT (standby) switch.
Now, this switches the rectified HT winding from the transformer, which is
around the usual 500v with no valves in place.

I'm particularly concerned that the switch originally fitted, and similar
generic types available from Farnell only seem to be rated to 24v DC.
No wonder it is faulty.

I'd be tempted to rewire the unit to activate a high-side PMOS power FET
switch. The plastic switch could bias it from any positive or negative
low voltage with a NPN level translator (emitter driven from low voltage
DC on the replacement switch). Depending on the current you're switching,
even an optoisolator might suffice.