[Milsurplus] [Boatanchors] "Blue Glow" mystery.

[David Stinson]

>    The dark blue glow that seems to be right at the glass
> is caused by x-rays from the tube.  Its commonly seen in
> tubes like audio output tubes with fairly high voltage on
> the plates. Some older tubes had a black coating on the
> inside of the glass to stop it.

That's my big question and point.
You are of course correct about the blue glow,
but why is this one happening in a cut-off tube
with no current flow, then disappearing when 
current is flowing?  That's opposite of what
I've seen before.