RCA 6bc8 weird things

I recently purchased a supposedly matched pair of RCA 6bc8 from evilBay. I have proceeded to put them into my drip 436c comp and when I put out the lights to inspect how the 0a2 regulator tubes light up I saw something moderately scary on the 6bc8s..

here's a pic, and a closeup:




A friend is suggesting that the tubes are not holding vacuum anymore? They sound pretty well and really not microphonic as opposed to some other 6bc8 that I have, so I'd be inclined to keep them if they'll last and aren't dangerous. On a side note, both tubes from the bought pair do this.

Cheers,
B.

This is normally a sign of some gaseous contamination in the tube, but there should be a 'getter'
in the tube which absorbs the gas over time. If these tubes had sat for some time unused, this would be
typical. Run them in for a while. Incidentally, if you had no vacuum in the tube, it would not work very long, as the filament would oxidise and fail. Think broken light bulb!

What you have is an example of Fluorescence, electrons bombarding the glass envelope and exciting some of the elements in the gas.  Usually indicates a high quality vacuum.  More common on power tubes, some  receiving tubes handling a lot of current will also do this.  I wouldn't be concerned unless you've noticed noises, distortion issues or the new operating point is different (significantly) from the original.

A blue glow or haze in space usually indicates gassy conditions, sometimes the colour varies according to the gas.

Do a search on Google for "vacuum tube blue glow", you'll get 10 of thousands of hits, many with pictures useful for comparison.

David