[Amps] Checking Filament Voltage

Jim W7RY jimw7ry at gmail.com
Fri Mar 3 15:44:51 EST 2017

Well done!

That is exactly what I would have done !

73 Jim W7RY

On Mar 3, 2017 10:16 AM, "Roger Parsons via Amps" <amps at contesting.com>

> 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.
> 73 Roger
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