I`ve had a power tube gone wrong recently. These were the signs or omens: after turning the standby on I heard a pop from speakers, when I looked at tube it was glowing bright blue and the metal plate in the center of tube was getting bright red.
Burnmark doesn`t have to mean tube is bad (I have 4 x 440 6l6 tubes in my amp and all of them have silver burn marks on top, which is normal, and 2 x 420 tubes which have no burn marks), but as far as I know only silver burn marks are normal, if the burn mark is white the tube is bad (somebody correct me if I`m wrong), but if you`d had bad power tube you would definetely notice it even without looking at it. For example volume changes or strange noises coming out of speakers, something that your amp wasn`t doing before.
But since you mentioned that only 1 of your power tubes have burn mark it could also mean you possibly have a pair of unmatched tubes in your amp. I was told that I should put matched pairs of tubes in my amp, otherwise I could damage the amp. Changing just one tube makes the amp work and won`t damage it immediately, but could after using it this way for too long, as far as I understood well...I wouldn`t want to risk it since rectos are kind of pricey.
