A tube that is glowing blue is often wrongly perceived as a defect, however, it’s really just a side effect of a power tube \u2014 a fluorescent glow in the blue spectrum. The tube is fine! It actually indicates that the vacuum inside the tube is very good, which is what allows this phenomenon to occur. It will not have any effect on the performance or tone of the amp.<\/p>\n\n\n\n
Below is an excerpt from Sylvania Engineering Data Service, vol. 1, pg 23 that explains “Blue Glow” further:<\/p>\n\n\n\n
\u201cBlue 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:<\/p><\/blockquote>\n\n\n\n
Fluorescence<\/h3>\n\n\n\n
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.<\/p>\n\n\n\n
Mercury Vapor Haze<\/h3>\n\n\n\n
Mercury Vapor Haze<\/strong> is a blue-violet glow associated with those tube types which rely upon mercury vapor for proper operation. In such cases, the blue glow should be evident indicating proper operation.<\/p>\n\n\n\n
Gas<\/h3>\n\n\n\n
Gas<\/strong> produces a blue haze, generally confined to the vicinity of the mount structure. The proper function of gas types such as thyratrons, voltage regulator and voltage reference tubes, requires the presence of this glow as an indication of proper tube operation. Some voltage regulators use neon instead of argon and as a result exhibit a pink-orange glow. It is, however, a distinct detriment in vacuum receiving types, where the presence of gas in large amounts can cause malfunction of the equipment.\u201d<\/p>\n\n\n\n
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