Some tubes like the ones you mentioned, have a deep blue glow on the inside of the envelope due to x-ray bombardment. This is different than a gas glow, which is inside the tube, usually between the plate and heater. This is usually a violet glow. This is from gas that has either evolved from the elements or has gotten in due to a leak. If a tube glows brightly its not going to last long.
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On 10/27/2021 5:24 PM, Mark Bielman wrote:
Most power amp output tubes (6AQ5, 6L6, EL34, etc) will glow a beautiful blue.
Suspect the EL86 would as well.
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Richard Knoppow
dickburk@...
WB6KBL
