Hi Tom,
Thanks very much for your response.
It's pretty clear that the first thing I need to do is make sure that the
filament voltage is right. One of the ways I've heard to do that is via a
rheostat on the primary side of the transformer. That sounds great for an
isolated filament transformer, but the transformer in the AL80A contains
windings for the filament, 12 VDC supply, and the B+. So, my thought is to
check the filament voltage when the line voltage is highest (which I think
is in the evening or morning) and insert appropriate resistance,
appropriately rated, in the filament supply.
You're right, though: I don't operate enough to worry about the stand-by
hours. I should be so lucky!
Thanks again, Tom!
Kim Elmore, N5OP
At 08:00 PM 3/17/2003 -0500, you wrote:
> > 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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Kim Elmore, Ph.D.
University of Oklahoma
Cooperative Institute for Mesoscale Meteorological Studies
"All of weather is divided into three parts: Yes, No, and Maybe. The
greatest of these is Maybe" The original Latin appears to be garbled.
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