Plexi 6V6 Pentode/Triode Switch

[morton]

Hello, new to the forum. Getting started on building the Plexi 6v6, wondering if adding a pentode/triode switch would be possible?

Thanks,
Gary

[sluckey]

Sure.

[morton]

Thanks, the Plexi is the Hoffman kit. I planned to install a dpdt switch to connect/disconnect the screen grid to the plate, looked like the existing resistor values would work. Just wondering if it's a worthwhile mod, or if it would create an issue biasing between the two modes.

Thanks,
Gary

[sluckey]

I've only done that to one amp. I played with the switch for about a week then removed it. I didn't think it was very useful, but I never gigged with the amp either. It's easy enough to do, so if you're curious, give it a go. Easy to undo also.

[morton]

That's what I was thinking as well, just wondering about the plate voltage around 390 in triode mode.

[HotBluePlates]

I planned to install a dpdt switch to connect/disconnect the screen grid to the plate, looked like the existing resistor values would work. Just wondering if ... it would create an issue biasing between the two modes.

Envision one pole of your switch. The center lug connects to the screen resistor, which then connects to the screen. One outer lug connects to the plate, the other outer lug connects to the screen power supply node. There are various clever versions of physically accomplishing those connections when you build the amp, which depend heavily on your choice of layout.

If your amp has a screen power supply voltage which is very similar to the plate power supply voltage (probably because you have a choke between those points in the power supply), then idle current will change little, if at all, when you switch.

If the screen voltage is very much lower than plate voltage in pentode mode (because you have a large resistor between those points in the power supply, or because you have a separate, lower-voltage screen supply), then idle current will rise when the screen is connected to the plate. The change is directly proportional to the difference in screen and plate voltages in pentode mode; if the screen voltage will rise significantly (like 50-100v or more) when switched into triode mode, expect to need additional negative bias to keep the plate current rise in check.

[morton]

Thanks, that helps. The Plexi is the Hoffman 6v6-the plate voltage is 394, screen is 399 with a 470 3w resistor. Choke is there, so theoretically it looks as if it will work? I planned to build it stock first, didn't want to throw too many variables in at first.

[HotBluePlates]

It will absolutely work.

[Backwoods Joe]

You might love it. I didn't really care for the tone and it was still plenty loud. $.02

[HotBluePlates]

Triode mode will be a bit cleaner than pentode mode. Often, that gets interpreted as "darker" because there's missing 3rd harmonic distortion you're used to hearing before the onset of heavy distortion, which your ear is likely to interpret as "edge" or "brightness" in pentode mode.