Arc from 1Be plate cap lead to high voltage cage

[Electronic M]

I agree with oldcoot and Bill.
If the tube was good before the arcing,then it should still be fine. The arcing from that point should not hurt it.
I would probably replace that wire with a 15-40KV rated wire....In lieu of that corona dope is one option, but I'd first try to put as many layers of heat-shrink tubing over the wire as I could. The tubing I get at Ace Hardware is 600V rated and some brands may be rated higher. Two layers of 600V insulation is 1.2KV extra insulation 5 layers is 3KV and so on.

Three other observations: Dress the lead as far from the cage as you can. It looks like the grab fingers/terminals on the 1B3 top cap/spring have been pointed at the side of the cage....You probably want them pointed away as they could form an arc point. Have you measured the HV, and compared it to spec?...If the HV is substantially higher than designed insulation may break down, and arcing may occur.