Tom et al,
Just to give you an example of how tube envelopes can and do leak,
our RCA/Burle 7835 triodes (3 MW, 200 MHz, pulsed) have a 'legal'
warranty of ~50 hours. This 'warranty' is only valid if we keep the
appendage ion pump on top of the tube connected to a power supply at
least every 60 days. So in storage we have ion pumps running on all
tubes. We have large klystrons from CPI - that we keep the ion pumps
connected at all times, even for spares. These large tubes (bigger
than a person can lift) will slowly degrade into worthless
counterweights if they are kept in storage and not pumped
continuously or at least at regular intervals. Trying to recondition
a tube that is very gassy, without a lot of damaging arcs, is risky
and time consuming. Functioning power supply crowbars are, of course,
mandatory to meet the manufacturers warranties as well.
73
John
K5PRO
at a particle accelerator
>
>Myself, I'd be very reluctant to pay anything for an as-is
>power grid glass tube without knowing for sure it runs. The
>reason is the seals always leak some air, especially if the
>tubes have been used or if they are in a humid environment.
>
>Years ago I was at Dayton with a friend, and he bought a
>pair of unused 3-400Z tubes from a guy. I bought two the
>same fellow said were just pulled from operation. The tubes
>had the same date code, they were 5 or so years old.
>
>His unused tubes were both so gassy they were unrecoverable.
>The used tubes with slight envelope discoloration were
>perfectly good. The difference was for several years the new
>tubes slowly gathered air, while the used tubes were
>operated and the zirchronium getter on the anodes did its
>job keeping them gas free.
>
>So again the fair thing to do is make it clear they are
>untested, and just take whatever you can get. How they look,
>unless something you can see is broken or melted, has little
>to do with if they actually will work.
>
>73 Tom
>
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