I have read that SED tubes glow blue because of the metallurgy, I have some =C= el34s that glow blue. My TS re-issue 6550s started glowing blue after a while, which bothers me because glowing tubes can also be caused by a vacuum leak, which is bad and will kill the tube, and maybe damage your equipment. The TS 6550s show some getter wear, not much, but that's also a possible indication of a gassy tube. Maybe others have a bit more detailed explanation?...