My first tube amp was a Cary SLA 70 which used a pair of CV729s/5R4GYs/CV378s and I remember when one of them started doing something weird (this was almost 30 years ago, but maybe it arced or glowed or flashed, I don’t remember precisely) and when I called Cray (this was when Cray offered much better customer service than they do now) I was told that it wasn’t anything to be afraid of, but that the tube was probably on its way out & I should probably keep one on hand.
Since that time, I did have a couple of rectifier tubes fail with a flash or an arc on startup, and that would blow the AC power fuse, but no other damage.
I am not an electrician or a tech so this is not advice, only my experience with a somewhat similar situation.