First time I've seen anything like this.

[Winky Dink]

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?