How do you know when tubes are done?

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.

roberjerman you are showing your age! Heathkit? Us Dynaco guys use to sneer at the Heathkit guys but you had to be braver to do a Heathkit because you had to build the boards, Dynacos you just had to do the point to point wiring. Bravo! I wish manufacturers would offer kit versions of their stuff again. Anyway, I always keep a spare set of tubes on hand. If you think something is off you can always swap to make sure. Actually, I always keep a spare set of everything around which I think is a neurosis. I would have a spare wife but I'd get shot. Think of what would happen if I got Atma-sphere MA-3s. I would be buried in tubes then get shot.
Steamboy with less gain in both channels I would suspect a power supply problem. Check your voltages. Great that you built your own stuff!

Change all tubes asap!!