If the blue glow is between the elements of the tube, that's a bad
sign (gas). If the glass alone is glowing blue, it's
unimportant (phosphorus in glass).
If the tube has sat idle for a very long time, and the gas isn't too
bad, you might getter the tube by running very low anode voltage and
positive grid and screen voltage. Cook the anode red hot for a day or
two (with enough airflow) and try again.
Watch the grid dissipation. If you use dc the waveform will have
no harmonics, so you can multiply voltage (between the grid and
cathode) times current (in the same path) to get dissipation.
Of course MOST of us know that won't work with non-sinusoidal
or non-steady dc current.
I've recovered a few 4-1000A tubes this way, but only when they are
tubes that sat idle for many years (very slow leakers).
73, Tom W8JI
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