These tubes are okay...but that is the best they've ever been for me, just okay. I've owned a Mesa Express 5:50 for roughly 6 months and it blew one of these on the 2nd gig. I know it happens...sometimes, ya just get the bad one out of the batch. So I decided to give them another try, although I used them in my twin reverb. I blew 3 of them over an 8 month period. That's when I learned that these tubes are completely unpredictable. You never know what you're going to get when you buy a pack of these. My Mesa has not blown a tube since the first one, but my twin blew 3. These tubes don't even come close to JJ's as far as reliability. JJ's are cheaper too. I think Mesa sacrificed the quality and reliability for the convenience of not having to bias the amp after changing output tubes. I probably shouldn't have used them in a fender amp anyway, seeing as Mesa output tubes are generally not designed to work as hard. Tone comes more from the preamp. In the end though, a tube is a tube and it should be able to hold up in any situation. Mesa: GREAT amps, not so great tubes.