12at7 Breaking Bad

My right 12at7 started to flash on power up. I didn't think much of it because the amp still sounded great, and it's not uncommon for triodes to flash on cold start up. After a couple weeks of this I thought it started to sound a bit flat, so I thought I'd check the bias. The left side was fine but the right side was running at 850ma. I dimed the bias trying to lower the bias but it only dropped to about 800ma. I swapped the suspect 12at7 to the left side and it flashed there too and drove up the bias so I knew the tube was the culprit. I didn't have another 12at7 so I put a 12au7 in the middle and kept the two 12at7's as outboard tubes, re-biased and everything is now working just fine.

Anyway, I thought it was worth sharing in case it happened to someone else. By the way, the 12au7 sounds pretty good. Hmmm???

Heaters on some tubes will glow brightly for a split second before running normally.  It is common at startup on few brands especially vintage.   I have some Telefunken ECC83s that do this.  It is normal and should not affect life of the tube.  And has been explained by the heater wire is lower resistance until it heats up.

What surprised me is that with identical tubes, same production date and everything, some will flash ... some won't. I've got that with a pair of Amperex Bugle Boys. I've also got an RCA clear top that will sometimes flash, sometimes not. Go figure.

Almost all Mullards will do this and they are the creme de la creme.

tubes do go bad, seems for some reason that one did go bad but fortunately you got it working again with a sub,
although that sub is not optimized for that particular resistor setup, it will work.