How do you know when tubes are done?

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!!

Thanks everybody for the help.

Now I have to find a tube store!

And check the power supply!

I didn't make these myself. My landlord, David Yee, built these from scratch. He gave me several amps and pre amps to try and I picked the ones I liked best.

I can find no trace of him now and I wonder if he's back in Hong Kong.

He found Vancouver "too slow"! He called himself "DavidYeeAudio"

all posters gave good advice. IMO, the best was to keep a replacement set on hand at all times. If your tubes sound "tired" the new set should easily allow you to hear a difference.

A tube amp owner is often a tube geek to some degree, and "replacement" sets become laughable as one likely has a drawer full of the things. I could go another 10 years on my current stash. I recently broke a preamp tube (and had the power amp driver tube start showing its age) which launched me into a 6SN7GTB maelstrom of tube option messing about (mostly NOS various things moved around until the mojo returned)...hard to pinpoint when I got it all dialed in other than to say it all got right somehow eventually...changing out a rectifier (!) tube was surprisingly helpful, as I hadn’t noticed it needed changing, but there ya go...you know when tubes need changing when while listening your brow becomes furrowed instead of not so much...