[Amps] 4 x GU-81 Grounded Grid Penthodes

[Fuqua, Bill L]

If you have AC supply the emmissive current would have to be added to get the total filament current.
That is because it is  not the average current but the RMS current that heats the filament.
We don't do it because the emissive current is much less than the applied filament current in
either case. 
73
Bill wa4lav

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   You can always use DC on an indirectly heated cathode, but with a directly heated filament you
must consider the fact that one end of the "cathode" will have 12.6 Volts more or less cathode to
grid bias than the other, this will cause one half of the filament wire to emit more electrons than
the other.  Generally this is to be avoided.  It can reduce the life of the filament, as the
emissive current must be added to the filament current in that part of the filament causing that
part to operate at a higher (than normal) temperature.