> Here's the question: Would it be worthwhile to place a small resistor (say
> something between 0.05 and 0.1 ohm) in series with the filament while in
> standby? Or is this a no-win scenario, because the small amount of
thermal
> cycling would stress the filament enough to send it to an early grave?
>
Hi Kim,
In all my conversations with engineers at Eimac, they always assured me any
reduction in filament voltage below the specified range would "be lost in
the noise" of other failures.
My own experience over the past 20 years with production tubes where data
was tracked seems to bear that out. The vast majority of failures I've seen
have been broken welds on anodes and gas problems. Only recently have
filament to grid shorts appeared, and only then with certain batches of
Amperex tubes. There isn't anything that has shown those failures to be
avoidable by anything like reduced voltage.
I've seen a few low-emission tubes, but most of them have been imports that
failed very early.
It is always a good idea to run the rated filament voltage on the tube no
matter what service. I can't imagine you would see any change in tube life
by reducing standby voltage, unless you have some really high operating
hours! In high operating hours commercial service it is often a good idea to
reduce the voltage of thoriated tungsten tubes to extend emission life. In
amateur service it is probably not useful.
MOX cathodes are another thing. NEVER reduce the voltage in a MOX cathode
below minimum voltage ratings, unless you are sure anything near the peak
emission will never be reached.
73 Tom
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