Re-bias when changing from Triode to Pentode

[ronenash]

Hi,

I was wondering if I need to re-bias my amp every time I change from Triod operation to Pentode operation. Anyone has an idea?

Thanks

 

[TG]

Hi,

It depends roughly on the voltage at the g2 in pentode mode. If it's the same as in triode mode, than you don't need to re-bias the output tube.
If you can share the schematics, it would be easier to figure out.

 

[ronenash]

Attached is the schematic. There is a switch I put in that either connects the screen to the Anode through a 150 ohm resisitor for triod operation or connects it to the output tranny as currently shown in the schematic.
I appreciate your help TG.

 

[TG]

Ah, so you want it to be triode/ultralinear rather than triode/pentode.
OK, here's what we have (at idle):
1. In the triode mode screens are ~1..2V below the plates (I presume that output tubes are something KT88/EL34-ish, so screen current is unlikely higher than 5-10 mA).
2. In the UL mode screen voltage depends on R38/R39 values. The 'tranny' ends of these resistors will be a couple of volts above the plate, but some voltage will be dropped across them due to screen current.
In any case, screen voltage will not change much with mode switching, so I'd say you may not bother to re-bias the tubes. Or, if you want idle current to be exactly the same, you can adjust R38/R39 values (200-220 Ohm is a good start) and make it so.

 

[ronenash]

Thank you TG. This is most helpfull.