[SteveFord]

Is this sucker about to go BrrrrZaaap! on me?
The top of the tube wouldn't photograph too well but there's a long black mark bisecting the length of the silvering with  black circle in the middle. 
This is from a VTL MB250.



According to this

http://thetubestore.com/blueglow.html

I guess it'll be okay.  It bothers me when the appearance of tubes change on me but perhaps I am worrying about nothing.

[weitrhino]

This is purely anecdotal, Steve, but you sure seem to have more than your share of tube gear issues.

[JohnR]

Did it test OK?

[pehare]

in my experience when the top gettering turns from silver to dark over time the tube is aging & might call for testing before it goes south & hopefully dies peacefully or takes out an output fuse instead of anything else depending on the design.

[SteveFord]

I haven't tested it but it doesn't make any weird noises, it isn't glowing weird and biases okay.
I have spares so I should just chuck in a new tube and keep this as a backup.
I've also been told that it's nothing to worry about, it happens due to impurities in the metal.
Problems with tube gear seems to run in batches with me.
My guess is there's around 100 tubes in actual use here between the upstairs and downstairs and a lot of the gear (and tubes) are over the half century mark.  An occasional hiccup is to be expected.
I also have problems with solid state gear, too!  Pops, buzzes, ach, lemme tell ya, you don't want to know.