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Being of old(er) age and questionable sound mind, in the "Old" days(and we're talking 60's here)when a tube glowed blue it had gas! The gas was excited by the plate voltage and it glowed blue like a neon light. We generally considered them bad then because they did have gas. I know when they glowed any at all in the horizontal or vertical circuit of an old tube tv, the picture would roll and flip and you couldn't stop it. I remember being pissed because I couldn't see the latest episode of Star Trek. I would trek up to the local drug store and stick those good old 12ax7 and 6dg8's in the tube tester. Back then, they cost about a buck and a half and everything was NS(new stock!) Only in recent years have we gottin into the NOS. We would buy good old RCA, Philips, Sylvania and what everyone loves now, we thought was junk. The 1b3 high voltage tubes just went caput---no glow, no nothing. You had a black screen. And how about those old tube radios in cars? They seemed to never quit.
I was SO happy when RCA introduced the 6H30 (supertube---ha) in their tuners. It solved a lot of problems. I still have a few of the originals laying around. Maybe BAT might want them!
I guess times change and now blue is good.