This is what happens to an FAA amp when the RF feed opens

Besides taking out the tube.  This is what happens to the Kapton insulators in the plate circuit that make up the capacitors in the cavity.  There are 2 of these; one on the side for the output coupling and the other is the tuning cavity where the plate D.C. is applied to the tube.

These are very difficult to find as replacement parts, but since I have a sheet of Kapton of the same thickness I made a new one, installed it and tested the amp.  Seems to work fine.  Sure beats buying a new cavity if they are even available.

I also convert these amplifiers to run on a 4CX250B instead of the 8930.  The 8930 is the same as a 4CX250R which is a 4CX250B with more rugged structures and a much larger anode assembly.  This combination allows the tube to withstand more vibration than the 4CX250B and it will dissipate heat better.