Nsgarch, re your last sentence about the unlikelyhood of catastrophical failure in a well used/tested tube being unlikely. I guess I've been unlucky as this has not been my experience on several occsions.
But in all other respects I agree it could be something else especially in view of the poster's observations regarding another tube which also flared up and quieted down. I've a tube amp with an auto bias circuit and had a tube's plate start to glow red. I changed out the tube but there was no change in the red plate syndrome. Sent it back to mfg'r who adjusted bias circuit - all is well now with original tubes in place.
But in all other respects I agree it could be something else especially in view of the poster's observations regarding another tube which also flared up and quieted down. I've a tube amp with an auto bias circuit and had a tube's plate start to glow red. I changed out the tube but there was no change in the red plate syndrome. Sent it back to mfg'r who adjusted bias circuit - all is well now with original tubes in place.