Listening for tube degradation

What I have noticed is that the sound does get a little muddy (or fuzzy) and you get less gain. This latter one is how I ended up confirming it - I had to turn the volume up higher for the same loudness. Since my preamp has a numerical volume display, this was easy to notice.

I have since bought a restored tube tester. It turns out all the tubes in my amp were worn out indeed. I am very happy being able to monitor tube performance other than with my ears because I want to keep the sound at peak performance and don't want to risk having a tube die on me and breaking something... Tubes can fail catastrophically if you use them too long. I have witnessed this first hand before.

Arthur

I also never had the equipment (or the knowledge) but learned a way after I made Heath and Dynakits, take cover off the amp and let it warm up,do not touch any part of the amp (be sure the selector switch is set to Lp or extra/cd whatever you are using, volume up a bit) and tap the tubes with a pencil. This may not tell you how much life is left in a tube as a Tester would, but if any tube makes any noise comming from the speaker when you tap it, static, spitting, burping, harmonics etc, it is done for and should be replaced.
Len W

Len W- not to be contrary, but what you are hearing is microphonics. Some tubes are microphonic when new, some are not. I don't know if this can also occur due to aging. It may be sign that the tube needs to be replaced, but it is NOT a reliable indication of aging. BTW, some microphonic tubes can benefit greatly from a tube damper.

Performance that deteriorates slowly, in tubes or anything else, is difficult for a user to recognize. Think about your eyes. They seem to be OK until the optometrist says you need new glasses. One nice thing about ss amps is that, unless they emit a puff of blue smoke, they are probably OK. With tubes I found that I was constantly worrying about them, listening for problems, which detracted from the musical experience.