Looking at the constant current curves, DC filament supply with a
centre tap probably won't be a problem, maybe 20% variation in
emission from end to end.
I still have some radios with 1.5V filament battery valves/tubes
and more earlier ones with 2V DC filaments; they were specifically
designed to run with dc powered filaments which might be a bit of
difference from 3CX3000.
I'm curious, why does the low HT voltage make the emission
variation worse? I'd always assumed it's the filament to grid
potential which determined it.
Steve G8GSQ
Roger wrote:
Jim w8zr wrote:
"Seems to me this problem may not be so troublesome......"
I have to agree with Jim on this, for various reasons:
There is no reason why a DC filament supply should not be centre tapped, most
simply with a resistive bridge;
The bias on the tube is related to the average of the emission over the whole
of the filament, and this is static with a DC filament supply;
Those of us who are old enough will remember battery tubes (such as the 1T4 and
many others). These (obviously) had a DC filament, and significantly an HT
voltage which was sometimes as low as 22.5V. Any problems due to a voltage
gradient along the filament would likely be much worse than with a power tube
with several kV on the plate. Yet they worked;
And most significantly. I cannot find any recommendation by Eimac or other tube
manufacturers against using DC filament supplies. This is despite their
specifications going into great detail about other matters such as permissible
filament voltage variation.
73 Roger
VE3ZI
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