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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. 

I had a bright blue glow in one of my KT 120 tubes on a Cronus Magnum Integrated.  Then it turned bright orange and poof. Tube was gone.  I Had to replace the fuse for that tube and the tube of course.  I would definitely keep my eye on it.