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.

Don't make the mistake which comes around time and again* of switching a
charged capacitor directly onto a discharged one, the switch won't last
long. Use a resistor to limit the surge current.

* The AR88 communications receiver and a Philips transistor car radio
(40 years apart) spring immediately to mind.


--
~ Adrian Tuddenham ~
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