Burn marks on ecc83's

[belch]

Just picked up an old pa with 5 old brimar ecc83's & a telefunken ecc81, the tubes work & sound ok but 3 of the brimar's have burn marks around the top of the tube ( some worse than others ) ,i was wondering what causes this & is it detremental to the tube.

[leslie]

This burn marks on blackburn or some valvos and tungsrams and even american tubes is not wear but is part of process of making tube.
http://www.tubemonger.com/v/vspfiles/photos/2012M-2.jpg