So I have been using my Heartbreaker with my band. Sounds awesome. I just put in a new set of Mesa 6l6’s about a month ago. A week or so ago, my little guy came in to the studio and flipped the standby switch on and off. Blew a fuse. I replaced it with a proper fuse and I have had no issues. I play a bit loud at home and nary a hiccup. Tonight at rehearsal which lasted a couple hours, the amp performed perfectly. After rehearsal, I stuck around and wound the amp up. Sounded glorious. I was at loud gig volumes. Amp was not even half way up. I keep it in tube recto mode all the time and variac switch up to full power and also keep it in the “Bold” mode. After about 30 seconds of glorious loud volume, the amp died. Fuse blew. I got it home and followed Mesa’s direction for trouble shooting tubes. Replaced fuse with proper size unit. Pulled all tubes, power and recto. Put the recto switch in Silicone Diode mode. Amp turned on and stayed on. Per Mesa, this eliminates any issues with the circuit. On to the tubes.. I installed the recto tube back and set it to recto tube mode. Powered up and flicked the standby. No arcing or flashes from the recto tube and the amp stayed stable fully powered up. Next, I reinserted each power tube one at a time. No arcing or flashing. All power tubes appeared to be operating at the same level. Per Mesa, this is how you determine a bad tube. So no tubes seem to be bad, now what.. I put the amp in to Silicone Diode mode and will play it hard tomorrow to see if the fuse blows again. If it does, I will run the head up to Mesa for a service I guess. My question now is, can a recto tube arc or temporarily fail if im running the amp hard? Either from vibration, or electrically from running the amp hard?