That's the worst part of using tubes in my opinion. Usually they just gradually lose their "vitality" over time rather than failing in a noticable way. Best thing to do in my experience is to get a second pair of your main tubes and try switching them with your primary pair every once in a while (6 months, a year). If they sound the same (or worse), you know the originals are still good. I'm over a year into my first set on the AM and still prefer that pair to the new ones.
Even though tubes have recommended life spans, some keep working well long after that time is up. Others don't make it that far...
Or get a tube tester (just kidding).
Even though tubes have recommended life spans, some keep working well long after that time is up. Others don't make it that far...
Or get a tube tester (just kidding).