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Filament bias

I am using a 10/vt25 as the input/driver tube for another 10/vt25. From the RCA 10 data sheet average plate characteristics indicate that -5 volt bias and 150 volts plate the current consumption should be 10 ma. So I use a large 4 ohm resistor in series with the filament. Filament current is 1.25 amps, so my bias voltage s/b (4 0hms)(1.25 amps)=5v.
I have a B+ of 380 volts and I need a voltage drop of 230 volts @10 ma so I use 23Kohm resistor. R=230volts/.01 amps = 23kohms.
When I hook everything up I get 176 volts on the plate and current is 8.9 ma. Obviously not 150 volts, 10 ma @-5v. Any thoughts on why such a discrepancy between actual results and those predicted from the plate curves? TIA for any responses I'm really at a loss over this one.


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Topic - Filament bias - vetmedrobert@gmail.com 13:01:05 08/05/17 (23)

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