Looking at the specs for the N43 tube https://frank.pocnet.net/sheets/060/n/N43.pdf and I notice that it says you should only use the tube with "full automatic grid bias." Presumably that is because of the sensitivity of the screen makes it easy for the tube to run away. Does running it triode solve that problem? Is it still in danger of runaway with fixed bias?
In other words, use a cathode resistor, NOT fixed bias, as the valve may overheat and damage itself.
Wiring it in triode mode will not reduce any risk, just reduce the overall voltage gain.
Wiring it in triode mode will not reduce any risk, just reduce the overall voltage gain.
The N43 has high gm (10mA/V) and very low bias voltage for a power pentode.
These two parameters combine to make grid bias (fixed bias) somewhat hazardous, because the usual sample-to-sample variations in Vgk vs. Ia will mean that swapping the valve could mean a very large change in anode current. And all the usual contributions to drifting of the grid voltage can lead to excessive anode current too.
These two parameters combine to make grid bias (fixed bias) somewhat hazardous, because the usual sample-to-sample variations in Vgk vs. Ia will mean that swapping the valve could mean a very large change in anode current. And all the usual contributions to drifting of the grid voltage can lead to excessive anode current too.
Thanks for the info. Using this tube was always an outside possibility anyway. Got them by accident, meant to order n34!
I don't think the screen (G2) has much to do with it.
This is a POWER valve, 10W. If you want to run it as a preamp or driver down around 1 Watt or 2 Watts (200V 5mA-10mA) resistance-coupled it may not be as critical.