[Tyson]

Noticed this just appeared on one of my tubes, a dark discoloration on the shiny silver flashing at the top.  Is this cause to be concerned?

[sfox7076]

It would be odd if burning like that was a sign of anything other than use.  Usually if it is more, the getter turns white.  Regardless, that looks like a pattern seen on many of the Mullard EL34s I have owned.  They all look a bit off, but my XF2s have lasted years looking like that.

[Tyson]

Thanks - I nosed around a bit on the internet and that seems to be the consensus, it's just a sign of use. 

[FullRangeMan]

I have various Ulianov new that have this rainbow and they are normal, GSTubes inform me this is normal and originated from the getter apply, if it is what I think it is, as I cant see anything in this photo.