Date: Fri, 3 Mar 2017 16:09:16 +0000 (UTC)
From: Roger Parsons <ve3zi@yahoo.com>
To: Amps <amps@contesting.com>
Subject: Re: [Amps] Checking Filament Voltage
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
VE3ZI
## Ok, so you only had the xfmr primary on for 10 secs, correct? Then shut
it down, then tweaked taps, then repeat a couple more times. 10 secs wont
damage anything. 30+ secs might. But you have it under control which is the
main thing.
Jim VE7RF
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