appearance of burn on 7308 Amperex tube


I am fairly new to tubes and have been doing some tube rolling of late in my preamp, a Blue Circle BC 21.1. It uses 2 tubes from the 7308/6922/6DJ8 family. Only recently when I was swapping out the 7308 Amperexes (white label USA)for some 6922 PQ's USA white label did I notice a 'burn' or the appearance of something like that on the shoulder of one of the 7308 tubes. I don't notice any sonic degradation and both channels seem OK at the moment. Any ideas? Thanks all for your help. Dave
You are probably setting the after effects of the tube's getter. This is an element that is burned in the last stages of a tube's production to get rid of any residual oxygen left in the glass bottle after it has been evacuated.

If it sounds good, don't worry about it.
Herman is correct. If you see that there is no "burn area" or if it looks like swiss cheese, with bubbles of clear glass, the you probably have a problem! Depending on the tube, this getter flash can appear on the top, side, or bottom of the tube glass.
Thank you both for your responses. I learned something and can sleep again. Cheers. Dave