>I believe it was mentioned on the reflector that leaving the filaments on
>indirectly heated tubes continuously would have very little effect on
>life. Perhaps someone could elaborate....John Brosnahan or Dick Ehrhorn
>maybe?
Is life expectancy dependent on whether the filament is directly or
indirectly heated?
I know that in commercial FM applications, leaving the filament running on
metal-ceramic tubes for extended periods of time results in increased
interelectrode capacitance, ultimately making the tube useless at VHF/UHF
frequencies. As I recall, a contaminating film slowly builds up on the
inside of the ceramic insulator, rendering the tube useless unless allowed
to "burn-off" during normal operation. At RKO Radio in Chicago, we finally
gave up on the idea of keeping our standby FM transmitter running with only
filament power when after having to continually retune the transmitter's IPA
stage over a three week period, the 4CX250B drivers would predictably fail.
Has anyone encountered this effect on their amplifiers at either H.F. or VHF
frequencies?
-Paul, W9AC
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