The blue tube is probably gassy and no good if the blue glow fills the space inside the tube. That means air has leaked inside and contaminated the vacuum. The fact that the blue tube is getting hotter is not a good sign.
If the blue glow is on part of the inside of the glass surface only and not filling the space then it is just harmless florescence caused by stray electrons hitting the glass.
If there is any doubt, replace the blue tube because a gassy one can go into a runaway condition where the current keeps rising until something eventually burns up.
thanks,, this is a new old stock tube i got from Gary up in ohio,,
i will order another.
I dont want to use my original Philco tubes ,, all of them were good except 1 and would like to save them back as spares.
Sometimes the flourescence will "dance" to the music in an audio amp. As stated, if it's probably gassy, even NOS tubes can get this way. I've heard about running 'em for a while can sometimes clear up some gas to the degree possiible with the internal getter. I can recall an ancient Eico that did this. The 6L6's lasted for years!
