Rich Measures wrote:
>
>>This also acts as an inrush current limiter as a bonus.
>
>? ... only when the rheostat happened to be set at max. R at startup. .
> A step-start always limits fil. inrush current.
Normally the rheostat will be set wherever it was last time (unless
somebody has been messing with it). If the resistance is large enough,
it wouldn't need any more inrush protection.
I'm using a 100-ohm fixed resistor in the mains side of a 15V 6A
transformer, to drop the secondary voltage to 12.6V. The voltage at the
tube jumps quickly to around 11V at startup, but then rises more slowly
as the heater gets hotter and its resistance increases.
With a reasonably sensitive AC DVM, you can use the heater voltage to
monitor the heater's own temperature. It takes most of the 3-minute
warmup period before the voltage finally settles at 12.6V, which
presumably means that the whole heater/cathode assembly has warmed
through.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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