pentode PI

[andrew]

Check out this 6sj7 PI. I'll probably try it one day. http://www.freewebs.com/jonesamps/Jones-Angie.pdf

[LeftyStrat]

Check out this 6sj7 PI. I'll probably try it one day. http://www.freewebs.com/jonesamps/Jones-Angie.pdf

I'm going to have to stare at that a while.

[Colossal]

Very cool.

[printer2]

Can't see it working. How is the screen out of phase from the plate?

[LeftyStrat]

Can't see it working. How is the screen out of phase from the plate?

I'm probably very wrong on this, but since the suppressor grid is tied to ground, and at a lower potential than the cathode, the screen and plate seesaw around the suppressor grid. I can see how the grid would affect the screen, but having trouble understanding how it would affect the plate.

[tubeswell]

Can't see it working. How is the screen out of phase from the plate?

Pentodes are constant current devices, and when the plate current goes down, the screen current goes up, and vice versa.

In this case the screen is (effectively) just an un-bypassed screen (with the 'screen-supply' resistor taken from the HT) with a coupling cap attached.

That's my take on it anyhoo. Somebodyelse may care to chime in.

[tubeswell]

Can't see it working. How is the screen out of phase from the plate?

I'm probably very wrong on this, but since the suppressor grid is tied to ground, and at a lower potential than the cathode, the screen and plate seesaw around the suppressor grid...

The suppressor is attached to ground as any suppressor would be, so I don't think that makes any difference to the operation of the pentode.

I note that the tube current for the entire pentode must be quite low (given the highish resistance value of the total Rk), and this would help regulate the tube current (including the screen current) keeping it low enough to avoid meltdowns.

[Firestorm]

I guess that could work. Not really a pentode anymore though; kind of split load using a pentode. Drive would be pretty low, wouldn't it? I can't see the point. Just because you can do something doesn't mean you should.

[printer2]

I guess that could work. Not really a pentode anymore though; kind of split load using a pentode. Drive would be pretty low, wouldn't it? I can't see the point. Just because you can do something doesn't mean you should.

That is my take. The screen and plate current may vary a little with the signal but not enough to drive the output tubes to any great extent.

[tubeswell]

Well I'm just guessing on this. It is in RDHB4 Fig 12.29C, but there's very little to go by.

[printer2]

Tried it with a 6AU6. Both outputs were in phase.

[tubeswell]

But would that 'in-phase' aspect be the result of the screen compression from an unbypassed screen? Surely a pentode is a constant current device, whereby the screen is still at some +ve voltage (w.r.t. the cathode) that is attracting electrons even when the plate current goes down? Is it not the case that a screen needs to be triode-connected to the plate in order for the two to move in phase with each other? What is the effect of internal screen resistance and the screen resistor on the screen current phasing?

[tubeswell]

Some more information on this design (see article by Peter Sulzer on p10-11) http://www.americanradiohistory.com/Arc ... August.pdf