[Amps] Testing 3CX800s?

[Floyd Sense]

I recently acquired an older AL-800H, which uses a pair of 3CX800s.  When I
first checked it out, I found that diode D117 was shorted - this is the
diode that protects the meters and overload circuit.  According to the
manual this diode will usually short "if there is a large
high-voltage-to-chassis current fault".  Upon close inspection, I could find
no obvious problems in the high-voltage area, and suspected a case of
"flash-over" of one of the tubes, as I've read about in this forum.  I
replaced the diode and ran the amp for several weeks, with full output on
all bands.  At one point, I heard a muffled bang (more like a "ting"), but
never hear it again and the amp continued to work OK.

I recently installed a new pair of 3CPX800s and again found the D117 diode
shorted prior to firing up the new tubes.  Replaced that again and all is
working just fine.  When inspecting the old tubes, I hear a faint noise
inside one of the tubes when I rotate it about its axis, as if something
were loose inside.  I seem to recall that this was also a symptom of
internal flash-over damage.  So, the question is - where can I get these
tubes tested to see if one or both are flashing over?

Floyd - K8AC in Angier, NC