[Amps] Checking Filament Voltage
ve3zi at yahoo.com
Fri Mar 3 11:09:16 EST 2017
I posed a question last month about whether it was permissible to run a metal ceramic tube (filament) for a short period without cooling. I received a lot of replies ranging from 'don't even think about it' to 'it will be fine until the anode is too hot to touch'.
I decided to try it anyway. The transformer primary has a rheostat and fixed resistance in series and multiple taps. Because it is a directly heated tube, the filament reaches working current and presumably temperature quite quickly - less than 5 seconds. So I powered it on for 10 seconds, noted the voltage and changed the tap which took about 30 seconds. I then repeated this twice more at which point the filament voltage was correct. After this, the base of the tube appeared cold, and the plate just might have been marginally warm, but it was difficult to tell just by feeling it.
I cannot conceive that this could have stressed the tube - blowing the base could not have made any difference as it was still at ambient at the completion of the tests.
Whilst there is still work to be done to complete the amplifier, the tube and blower are now installed and the filament voltage has required no more adjustment after several hours operation.
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