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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).

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    \$\begingroup\$ what about a relay? \$\endgroup\$ Sep 12, 2014 at 20:09
  • \$\begingroup\$ @VladimirCravero Are there reasonably priced relays that handle that voltage? I haven't had much luck finding any. I'm not even sure what style of relay would handle that voltage. \$\endgroup\$
    – Pete
    Sep 12, 2014 at 20:17
  • \$\begingroup\$ well I did not search, I don't think mos are a good idea though. Is that something you are planning to switch often? you can use a connector inside the amp, you insert it here or there and have what you want. but you need to open the amp, or at least put some sort of window on the back \$\endgroup\$ Sep 12, 2014 at 20:22
  • \$\begingroup\$ @Pete Coto Technology makes high voltage relays. You can look for them on Mouser and DigiKey. Here's an example. \$\endgroup\$ Sep 12, 2014 at 20:23
  • \$\begingroup\$ @NickAlexeev I don't see any SPDT ones, though. I can't think of a way to make an SPST one work for this. Maybe using 2 relays and an SPDT switch? \$\endgroup\$
    – Pete
    Sep 12, 2014 at 20:40

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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.

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  • \$\begingroup\$ I actually like those kinds of switches for this application, but because they don't have a DC rating, I was concerned. When you say, "It might be okay depending on your regulatory requirements." My only regulatory requirements are that, 1> It's safe, 2> It works, and 3> It's legal for personal use (that is, I'm not making anything I'd sell.) Does it meet those requirements? \$\endgroup\$
    – Pete
    Sep 12, 2014 at 21:20
  • \$\begingroup\$ If it looks like the pictures, and if I could mount it in such a way as it was protected I would have no hesitation in using it myself. I can't give you a blanket statement on safety, it's going to be 'it depends' for both legal and ethical reasons. \$\endgroup\$ Sep 12, 2014 at 22:41
  • \$\begingroup\$ Problem is that it can be dangerous if you switch it when power is on. \$\endgroup\$ Sep 12, 2014 at 22:43
  • \$\begingroup\$ @VladimirCravero If it's marked covered so that it can't be casually changed or there's a sliding plate that interlocks the incoming power, I think there's less of a problem. \$\endgroup\$ Sep 12, 2014 at 23:11
  • \$\begingroup\$ Thanks guys. I'll go with that. I'll figure out a way to cover the switch. Though I'd never be messing with the back of an amp that's turned on (unless I were working on it). \$\endgroup\$
    – Pete
    Sep 13, 2014 at 3:09

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