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Blue Glow....again!

MD,
You really should replace the 2 resisters that go from cathode to ground with a resister per power tube. Getting the power tubes properly biased will cut the heat down quite a bit. Order some nice 2 watt, low tolerance carbons from Electra-Print or Angela. It aught to improve the sound too.
I will be doing that to a 101-B after I get done screwing around with it.
 
The harmless blue glow on the surface of the glass itself is known as "fluorescence" and is a sign of an especially hard vacuum. Fluorescence usually happens only in tubes that are operating with more than +350V plate supplies, though. You'll see fluorescence most often on output tubes, occasionally on driver tubes, but rarely on preamp tubes which are typically operated from +300V or less.
 
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