Trash the tube. End of discussion. Also, be very careful to watch for a while the replacement tube. Your tube is probably just worn out, but you never know for sure. Oh, did I say trash the tube?
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I basically agree with Jeff, but as a precaution, it might be advisable to have the unit checked out to make sure that some passive component (like a bad resistor) is not causing tube failure.
If your tubes were all placed in service at the same time and have been working reliably for some while, it is unlikely (but possible I suppose) that one would just fail catastrophically all of a sudden by itself.
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.
Newbee, I guess I should have qualified that comment.
I didn't mean that if a tube has been behaving it should never explode! And if it does it could just be random failure, but I just meant I'd look elsewhere too, especially if it's been giving good service and isn't at the end of its (sonic) life yet.