Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ncusack on July 09, 2018, 09:42:06 am

Title: What switch rating for switching coupling caps?
Post by: ncusack on July 09, 2018, 09:42:06 am

What should the switch rating be for a switch used to switch a parallel coupling cap on a standard 12ax7 gain stage?

Title: Re: What switch rating for switching coupling caps?
Post by: 92Volts on July 09, 2018, 10:13:00 am

The standard 125v, 1a switch used for mains current should be OK. These are cheap and easy to find. And in a coupling cap application, it would be used within its official limits (while switches are abused in other parts of tube amps all the time).

I'd put it on the ground side so if voltage jumped from the internals of the switch to the outside it wouldn't be a shock hazard. That's also less likely to happen when both the internals and outside of the switch are near ground potential. This rule of thumb is also used with pots.

I'd recommend bypassing the switch with a large resistor, maybe 2.2meg ohm. This will block most sound when the switch is open, but let the DC voltage across the cap stay in the same neighborhood as the difference in the circuit (and the other cap). If the DC voltage across the unused cap is allowed to "float" you'll get one hell of a pop when you reconnect it.