wait a minute....I know this sounds like a stupid question with an obvious answer: but as far as I know my amps are biased properly ( by an amp tech ) and the tubes don't glow at all when the amp is idling however, when I crank the amp up and start playing, the top of the ( plate? --> the metal structure inside the tube ) starts to glow orange in the centre, and then spreads to the whole top surface of the (plate?). if I stop playing, the glow subsides...but will come back as I play aggressively. the sides of the metal structures don't glow at all, just the tops. both amps are 2x6V6 powered, that I normally love to play at full volume....and the tubes are old RCA, so I don't want to destroy them prematurely if I can avoid it. thx!