> I understand that high filament voltages is a contentious issue among
> hams and amp manufacturers. One camp claims it drastically shortens
> the tube life, whereas the other claims that in ICAS operation that
> other factors will kill a tube long before the excess filament voltage
> will do so.
Data is needed to assess the effect of relatively short duration power
cycles on filament life. We can read all the conjecture about monitoring
rated filament voltage in an effort to extent filament life, but much of
that data pertains to VT's with directly-heated cathodes in continuous
service emission modes like that found in the broadcast industry.
Anything we glean from this mail list is nothing but an opinion unless its
backed by valid, measured data. Comments by amplifier designers are of no
use either unless those comments are based on valid, measured data that can
be cited.
Last year, I undertook a project to lift the high-current filament return
off the chassis of my Alpha 77Dx. The side "benefit" was that what had been
high filament voltage of approximately 5.5 V was brought down to 4.9 V with
just enough series resistance from the filament choke I added. Will the
lowering of filament voltage from 5.5V to 4.9 extend tube life? Does the
fact that the amp is power-cycled several times a week undermine any benefit
offered by the reduction in F voltage? I don't know and I doubt anyone else
does, including the amp designers unless they have solid evidence to back
their claims.
I would like to know more about the subject myself. If anyone knows, I'd
put my money on John Lyles. But unless the data has already been assembled
by one of the major power tube suppliers in past years, then I doubt it will
ever be presented as high-power RF technology gradually shifts to
solid-state devices. Notwithstanding that data, it is then left to amateur
experimenters like ourselves to measure and present it. There's nothing
wrong with that if the results are valid. Anything less is just a guess or
it's based on limited empirical data that may not have universal relevance.
Paul, W9AC
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