I've been retesting boxes of old tubes and attempting rejuvenation on some of them. A couple of nights ago, this happened:
For the safety of my equipment, I should have just cut the power and left it alone. But I couldn't resist repeating it to get a quick video:
https://flic.kr/p/2hcncrU
I won't give you any more information. The photo and video are unaltered. Who can guess what's happening?