Output Stage Operation Question

[rp]

I thought strapping the screen grid to the plate made a tetrode or pentode into a triode, the classic "1/2 power switch" seen in 2x PP amps, particularly in hifi. So why aren't the 5E7 (and kin) and the Vox AC10 considered as low powered triode configured amps?

[sluckey]

Because the screen is not connected to the plate. It's connected directly to B+, just happens to be the same B+ node that the CT of the OT is connected.

[pdf64]

That arrangement is very hard on the screen grids, as there's nothing much to limit the current flow into them during high signal levels.

[rp]

My question was sort of a mix of dumb and rhetorical as I figured Fender and Vox didn't accidentally market a triode wired amp. But one more dumb question, is it not triode because of the resistance in the half winding, or is it only triode if the SG is directly tied to the plate and so disappears completely?

[martin manning]

The difference between triode and pentode operation is the screen voltage and what happens to it as the plate voltage is pulled down on the positive signal swing. The key is whether the screen is connected on the supply side of the the OT primary where the voltage will remain high, or the plate side where the screen will experience the voltage drop across the load (hundreds of volts) like the plate. The primary's DC resistance is only a small part of that. In Ultralinear (or distributed load if you like), it's a mixture of the two, where the screen sees a fixed fraction of the voltage drop that the plate sees.

[Dave5881]

Great explanation Martin. Thanks!

[rp]

^ +1
Three top faculty members. It's fun going to Amp U.

[Firestorm]

A pentode or tetrode can pull its plate almost to zero volts. Like 70 to 50 depending on tube type. This makes the plate dramatically less attractive. But with the screen at a "fixed" potential, the electrons see that as the anode and keep flowing. It makes plate current mostly independent of plate voltage. One of the great inventions in the history of tubes. Also SCREENS grid to plate capacitance for significantly less high frequency loss. Since this was the era of radio and TV, I often wonder if that was the actual intent. Who invented the screen grid? Don't you dare say let me Google that for you.

[Firestorm]

I Googled it myself. Schottky. He was after both effects. Smart SOB.

[Firestorm]

Actually three effects since the screen also stabilizes performance. Damn it must have been fun to work on this stuff in that era.

[martin manning]

https://en.m.wikipedia.org/wiki/Tetrode gives a different version:
"The first true screen-grid valve, with a screen grid designed for this purpose [reducing Cg-a], was patented by Hiroshi Ando in 1919, and the first practical versions were built by N. H. Williams and Albert Hull at General Electric and Bernard Tellegen at Phillips in 1926."

[Firestorm]

https://en.m.wikipedia.org/wiki/Tetrode gives a different version:
"The first true screen-grid valve, with a screen grid designed for this purpose [reducing Cg-a], was patented by Hiroshi Ando in 1919, and the first practical versions were built by N. H. Williams and Albert Hull at General Electric and Bernard Tellegen at Phillips in 1926."

Interesting. Like I said, it would be way cool to work back then. The whole virtual cathode thing was never fully explored.