When they mug the output transformer of course. But seriously, other then total failure how do you judge? I just got my Pro Junior back from warranty work. And he stated all 4 power tubes I had were bad. I gave them the original GTs and some JJs I just got a few months ago. I really dont think the guy was just trying to sell me tubes, he didnt really push the issue after telling me they were bad. I am guessing a lot of us dont have a tube testing device around, so what do you look or listen for? Ive heard some great things about JJs and so I picked some up, but now they are bad already? I remember when I first got them they sounded pretty good, not a whole lot different then the originals. I know they get knock around a bit, but thinking of getting new GTs just to be on the safe side. I have to state the amp sounded like crap before I brought it in with the JJs in it and I have not had a chance to try it out since I got it back today. Ill have to swap some tubes around when I get time with some tested old RCAs I have.