> One of the "facts" stated on this web page is that broadcast
> stations keep all their final amplifier tubes filaments powered
> at all times. I used to work at a broadcast station with
> three transmitters and we only had the filaments powered to the
> transmitter in use. Were we the only one ?
When I was in the broadcast business, I do not recall that being standard
practice -- even in major markets where funding is not an issue. Sure,
there may be a few stations that have tried it, but the benefit does not
offset the long-term result. In fact, the practice of leaving the filament
continuously heated in an FM transmitter in which ceramic tubes are employed
can result in severe de-tuning over time at VHF frequencies.
I know of one well-known broadcast engineer in the Chicago market who
documented the effect and presented a paper in the Proceedings during a
1970s NAB convention. In the case presented, a long-term filament-on
condition without HV and excitation resulted in semi-conductive deposits
forming on the inner ceramic of the tube. In one demonstrated case, a pair
of 4CX250B tubes used as the IPA in a Harris FM-20H transmitter had failed
after several weeks in the this condition which first manifest as a gradual
detuning of the IPA to PA stage. Finally, IPA tuning was no longer possible
owing to the contamination of the tube. The test was repeated several times
with identical results.
Since most commercial broadcast transmitters use directly-heated cathodes,
warm-up time is not much of an issue. Many transmitters use time-delays
that far exceed what is required in order to apply excitation to the PA and
IPA. That said, glass tubes used at HF may fare better than the example
described above, but the cost v. benefit cannot be justified even in the
most intensely competitive major markets.
Paul, W9AC
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