How do you know when tubes are done?

I agree that the best approach is to keep a new, or tested NOS, set of tubes to put into the amp for comparison purposes.  At some point, the tubes have to be changed, so it makes sense to buy the set now.  You could get "lucky" if the set you buy now goes way up in price by the time a new set is needed.

By the way, tubes are never "done."  They can always be sold on ebay "as is;" if they you have a set of totally dead tubes, you can sell them as a "matched pair."

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!

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!

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!

Hi,
tubes are towards or end of life when dynamics are gone. Preamp tubes may produce funny noises or extra hiss sound, power tubes will lack power to drive efficiently. If in doubt get or borrow a good tube tester and check before rather than sorry later.