Sorry for the "QRP" amps question! (driver stage... :-)
I think I know what is going on here, but would appreciate comments.
I have a small VHF amp employing an 8560A tube. I have a box of used
and NOS tubes. So far this little amp has destroyed five tubes in
a short period of time. I *think* the tubes are gassy and flashing
internally but I'm not sure.
Configuration: Conventional... grounded cathode, grid driven.
Failure: audible "snap" followed immediately by drastically reduced
Ig, Iscr, Ip, and Po, and severe thermal instability in all those
parameters. Some tubes do this almost immediately after being placed
in the amp (during initial tune up). Others work OK for a few hours
or even weeks, then fail in the same way. After failure some tubes
drop to about 50% on the listed parameters, others drop to almost
negligible. Regardless they all show the severe thermal instability
after failure. I suspect major damage to the internal structures of
Is it likely (possible?) that a flash over external to the tube
could inflict such damage on the tube? Or is this most likely an
internal flash over as I suspect? I have had many a external HV
flash in similar (and larger) tetrode amps, but never lost a tube as
I note that every tube tried has exhibited a bright pink or purple
glow through the ceramic *only when RF is present*. HV alone does
not seem to produce the glow. Is this normal, or an indication of a
Last question: When reconditioning an old tube of this type, would
it be OK to use DC for the heater, or must one use AC? I'm looking
at possible ways to do more than one tube at a time on the bench as
a time saver...
TIA for any suggestions!
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