Pentode/Triode switch

[Paultergeist]

Greetings!

I am interested in the idea of incorporating a "Pentode/Triode" switch in a current amp build (Trainwreck 'Rocket'-inspired, quad of EL84s).

From my reading of this, "triode" mode is pretty much based on the operation of the screens. What continues to be confusing for me are some of the layouts and descriptions I see for such a "pentode/triode" switch. Most commonly, "triode" function is approximated with pentode tubes by conecting the screen supplies to the plates. This really seems odd, as doing so would make the screens more positive than they were in pentode mode. So.....I am trying to wrap my brain around the idea that making the screens more positive than normal (but still a bit less positive than the plates since the screen resistors are still in place) come somehow close to "triode" mode of operation?

Also, the screens are still operational, so is it really "triode" mode? Is the term "triode mode" in such a context a misnomer?

Thanks to all for the helpful dialog.

[martin manning]

Most commonly, "triode" function is approximated with pentode tubes by conecting the screen supplies to the plates.

Not the screen supplies, but the screens themselves, through the screen resistors. The screen supply node of the power supply is left unconnected.

This really seems odd, as doing so would make the screens more positive than they were in pentode mode. So.....I am trying to wrap my brain around the idea that making the screens more positive than normal (but still a bit less positive than the plates since the screen resistors are still in place) come somehow close to "triode" mode of operation?

Also, the screens are still operational, so is it really "triode" mode? Is the term "triode mode" in such a context a misnomer?

The screen grid electrically becomes part of the plate, so the tube effectively becomes a triode. The screen grid is a bit more positive than it was at zero signal, but when the plate current goes up and the plate voltage goes down, the screen voltage now goes down with it.