[kevchy]

I noticed a small tiny crack in my 2A3 tube while it was running in my audio system. It still works, but is the tube now running on borrowed time?

 

[megabigeye]

I'm not a tube expert by any means, but my guess is that as the tube heats up, the glass will expand and the crack will spread.

 

[tomb]

You can probably use it until the getter turns white. When that happens, the tube is definitely bad.