[Amps] 8560A tube failures and tube reconditioning

[N1BUG]

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.

Ep: 1500V

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 
the tube.

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 
a result.

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 
gassy tube?

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!

73,
Paul N1BUG