Safest way to make a high-voltage switch


First of all, I've dabbled in electronics on and off over the years. I have no formal training.

I'm designing a vacuum tube guitar amplifier (seems like everyone is these days). I want to add a switch to change the voltage of the screen grid of a tetrode (basically to switch it into triode mode. This is in the power amp section.)

The plate voltage is 384V and the screen grid will be somewhere in the vicinity. The current will be a few mA. I don't want to use some big fat gigantic 600V switch to switch it and I don't feel comfortable having almost 400V that close to being touchable. I'd like it to be a little switch in the back of the amp. Either a toggle or a slider.

The leading idea I have at the moment (and this may be a terrible idea, which is why I'm here) is to have two N-Channel MOSFETs, each with the source connected to the screen grid and the drains connected to the two different voltage sources and then an SPDT switch that activates the gate of one of the MOSFETs. (The gates would have pulldown resistors to ground.)

Would this work or is the better way? Would I have to worry about it introducing any sort of noise?

(Incidentally, the switch would be on the back of the amp, probably covered, and meant only to be switched when the amp is powered off).

Best Answer

You could use a slide switch such as "Arcoelectric" (great name- dealt with a Taiwan company called "Sure Fire Electric" for some stuff, but I digress..) screwdriver-slot-switched K22208EB DPDT, UL CSA rated at 300VAC (424V peak) 4A. It does not have a DC rating but since it will never be switched under load.. it might be okay depending on your regulatory requirements.

These are of the same ilk as the voltage selector switches used to switch the capacitor doublers in PC power supplies.