Some tubes are more prone to do this than others. It's from trace gas left over when it was manufactured and is being ionized causing it to glow. Usually harmless but can sometimes indicate a tube that is gassy. Have access to a tube tester? They will usually have a test for that. 6L6GC tubes can sometimes have a beautiful glow.

BillWojo

@israrami

https://www.thetubestore.com/blue-glow

Blue Glow in Tubes, Tube Getters, and other Tube Info

Blue Glow – We often get asked if blue glow inside of a vacuum tube is a sign of a defect. Thankfully, it is rarely a sign of a defect at all. In fact, many tubes have a blue glow, especially modern tubes. Photo 1 shows a tube with a healthy blue glow. This was also seen in tubes from the golden age of vacuum tube production.

In the 1960’s Sylvania printed the following article:

 

“BLUE GLOWS:

 

Blue Glows are not tube detriments per se. They are, however, suspects in the eyes of many receiving tube users for lack of a full understanding of their origins. There are several types of Blue Glow which can be described as follows:

FLUORESCENCE (Photo 1)- this type of glow is usually violet in color and most noticeable around the inside surface of the glass bulb. It is most pronounced on power tubes and is the product of electron bombardment of the glass taking place within the tube. It generally has no adverse affect upon receiver performance, and in fact, tubes displaying this phenomenon are particularly good with respect to gas content.

And FWIW the Tung Sol 7581 tube ( 6L6 / KT66) has the prettiest blue glow I’ve ever seen. Sounded good too.

I’ve always liked the blue glow. I think my Quicksilver Monos did it. I’m almost certain my Fender Vibrolux tubes did it. What were those tubes anyway? 8417’s?

One of my RCA Globe 45 tube (from the 1930s) dead soon after the purple glow show.