Blue glow just inside the glass isn't anything to worry about. The electrons are hitting and causing the glow. The blue will move around with volume or content. It's especially noticed with output tubes running on high voltage.
You don't want blue glow within the elements. This is a gassy tube which may cause distortion and damage to your radio.
Norm
:Hello again,
: I wanted to thank you all for your help in answering my questions, it is very much appreciated. My question this time is : In the RCA 811K, I have two 6L6G's for the audio. In one of them, there is a blue glow at the top of the tube which moves with the tone of the output. What is this and what causes it? Is this common? Thanks, Justin