[Milsurplus] 2X2 question

[Jack Antonio]

Hi all

Recently I've been working on a WWII countermeasures
receiver, that uses a 5" scope for display, sort of
an early "spectrum analyzer". And it was starting
to work, the scope had a nice bright sharp trace,
and the signal circuits were working, but needed
a little troubleshooting to solve a calibration
problem.

High voltage for the scope tube is generated by about
1250 VAC delivered to the cathode of a 2X2 rectifier,
about -1500 taken from the anode.

After a disassembly/re-assembly trying to troubleshoot
the IF, when I fired it up, the 2X2 glowed bright blue and
the line fuse instantly blew. Of course I thought it was
something I did in the re-assembly, and spent a couple
of days getting nowhere. Then I pulled the 2X2 plate cap and
applied -1100 VDC from an external supply  to the circuit with no 
problems(thinking a capacitor was breaking down).

Then I put the -1100 on the plate cap of the tube, and
the tube drew lots of current, and glowed a nice blue.
(No filament voltage)

I pulled the tube and put a variable DC supply across the
tube, reverse biased, and the tube breaks down at about 290 volts,
at that voltage it has a nice blue glow inside, the higher the voltage,
the more intense the glow. The manual says it should withstand 12500.

My question is, anyone have an idea of what the failure mode is?
(I have some replacement 2X2s coming).

TIA

Jack

Jack Antonio WA7DIA
scr287 at sbcglobal.net