BLUE GLOW in Electron Tubes

Explanation 1:

Transcribed for your convenience from the "Sylvania Engineering Data Service", Volume 1


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:


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 particularily good with respect to gas content.


This 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.


Gas 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.

Submitted by: Doug Haugen

Explanation 2:

Transcribed for your convenience from "RADIOTRONICS"Technical Bulletin No. 39 - 24 February 1936
Radiotronics was published by the Amalgamated Wireless Valve Co. of Australia using technologies derived from RCA USA, Marconi, Telefunken and others.


With modern valves a blue colour is frequently observed (either steady or flickering) on portions of the bulb wall; this is a phenomenum known as "fluorescence" and is somewhat akin to X-ray fluorescence and is due to electron bombardment of the glass.

The colour of this glow and its intensity depend on the nature of the glass, the voltages employed, and the design of the valve.

It has no deleterious effect, however, and actually is a sign of an extremely high vacuum.

It is particularly prominent with type 42, but may also be observed on other types, such as 6A7, 6C6, 6D6, but since in these types a black coating is used inside the bulb the fluorescence is rarley noticed.

Blue glow is a glow between electrodes caused by ionised gas. It is never observed in a really hard valve, but there are certain types in which a very slight glow may be observed which, although indicating a very small amount of gas, is not deleterious to the valve; for example this sometimes applies to type 50.

When any appreciable glow is observed inside the plate, the valve is definitely defective, due to gas, and a negative grid current will be observed if a micro-ammeter is placed in the grid return circuit. A valve reading less than 1 micro-ampere for each 10 milliamperes of anode current is quite satisfactory, and even two to three times this amount of negative grid current is usually quite permissible.

When a valve becomes very gassy, the blue glow frequently extends in the shape of streamers radiating from the ends of the anode, and the valve in this condition is completely unusable.

In high vacuum rectifiers used at voltages not exceeding 400 volts, a small amount of blue glow is not detrimental and may be rather beneficial. High voltage rectifiers, on the other hand, must have an absolute minimum of gas.

Occasionally fluorescence may be seen on the inside surface of the anode or grid; this fluorescence may be easily distinguished from blue glow by the fact that it is a thin film and does not spread through the space between the electrodes.

All types of fluorescence are completely harmless."

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