[AMPS] cycling filaments for gas?

[John Lyles]

Dave asked if running spare tubes for a few hours of filament time is good
practice, to activate getters and such, to scavenge any gas accumulation.

IMHO, yes, but depends on the tube. As a former broadcast engineer, we
always rotated our stock yearly. Not just for a few hours, but to ensure
that the spares were really operable. I think others may do similar
rotation.

Somewhere in the middle, (5000-100000 watts?) tubes get complicated enough
that the seals are many, and cover a lot of square area. And the big tubes
have tremendous internal volumes. The Burle 7835 super power triode has a
warranty condition that it be rotated on an external ion pump every 2
months for a day or so. Or left on ion pump. This tube and others will
slowly accumulate gas molecules, from SOMEWHERE. Is is seals? Or outgassing
inside (hard to imagine when the tube is cold)? If one waits for years,
then connects an ion pump, it can be totally swamped by the volume of gas,
and not able to recover. Then it's all over, an expensive mistake. I had a
4CW250,000B go up to air on the shelf. It was determined to be a defective
seal, so we thought. Eimac gave partial credit in the warranty, as it had
been run for so many hours as a good tube, then shelved as a spare. It just
had a slow leak, and normal operation didn't show this as the getter was
adequate to keep up with it.

As for smaller tubes, I am not sure it is worthwhile. Like the 250 Watters.
Nah. The volume is so small, the seals are so tiny. Also, don't know if
they use filament to reactivate or improve gettering. I know that some of
the big ones do, as mentioned earlier. In asking people at Eimac, Econco,
Burle, and others, I have heard quite a diversity of opinion. None of them
want to ever admit that a tube could possibly have a bad seal or a gas
problem. In general they say that the smaller tubes (where is the
division?) don't have problems with increasing gas on the shelf.  But in
general all agree that rotating spares is smart. Hmmmm.
I heard an interesting comment from the Litton engineer. We use Litton
Injectron beam tubes to turn on the mod anodes on big Klystron RF
amplfiers. They suggested that we store our Litton spares in a heated room,
away from moisture and temperature extremes. We had been keeping them in a
trailer outside, so we moved them indoors. In their opinion, the tubes are
more prone to leak when subject to temperature extremes in storage.

I'd be interested in hearing from Ham amplifier manufacturers (ETO?) here.

John
K5PRO




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