How do you know when tubes are done?

Tubes are finished when the silver getter flash turns brown. I ran a set of four 7591's in a Heathkit AA100 integrated amp for years until this happened. Background noise increased under the music. I ended up discarding that set of output tubes and bought a brand new set of EH 7591's. Pristine clarity restored!

Speakers were the Quad 57's. The AA100 was purchased from the original owner/builder (who built it in 1963!). So the 7591 output tubes already had plenty of use when I got it ($25!). He even gave me the construction manual! After I put some more use on it, the silver getter flashes turned increasingly browner - and noise increased, too!

Having a good tube tester on hand is recommended! That way cathode emissions can be checked before the getter flash goes increasingly brown. When emissions fall below 50% that is time to replace tubes!