Blue Glow In Tubes FAQ

Originally written and maintained by Larry no longer messes with tube amps, but kindly gave me permission to reproduce and maintain the FAQ.

This document is intended to address the frequently asked questions reguarding the blue glow frequently seen in power electron tubes or valves. I welcome contributions, criticisms and corrections.

Please send your questions and/or comments to: (remove the XYZZY- or the mail will bounce). Please include "[tubes!]" in the subject to get past the spam filters.

Q: "My tubes are glowing blue. Should I worry?"

A: No. Blue glow is perfectly normal. It's an artifact of operation of the tube. If the amp is otherwise working fine, relax and don't worry about it all.

Q: "Does the blue glow mean my tubes are 'gassy'?"

A: No. Gassy tubes are characterized by excessive grid currents and other poor performance characteristics. Extremely gassy tubes may glow with a pinkish cast throughout the entire tube. A faint blue glow on the glass, or inside the plate structure is completely normal. In fact tubes exhibiting this glow are frequently lower in residual gas.

Q: "My amp cuts out sometimes. When it does, I see the tubes stop glowing momentarily. Does this mean I should change my tubes?"

Q: "My tubes glow blue when the standby switch is on, and stop glowing when it is off. Is something wrong?"

A: Tubes will usually glow blue when the tube is in operation with voltage applied. When you cut the standby switch off, this interrupts the voltage applied to the output tubes, and they stop glowing. If your amp has a problem and the sound cuts out, taking the glow with it, the problem is not caused by the glow. The glow interruption is merely a symptom of whatever is causing your amp to cut out in the first place

From "Sylvania Engineering Data Service", vol. 1, pg 23 (1960?):


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-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.

MERCURY VAPOR HAZE-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

All About The Glow

Tubes operate on the principle of thermionic emmission. A heater filament heats the cathode until it glows a dull red. The cathode is made of materials that can emit electrons at high temperature. The tubes plate is held at a high positive voltage, which attracts the electrons emitted from the cathode. In amplifier tubes, the flow of electrons is controlled by one or more fine wire screens or grids placed in between the cathode and plate. The flow of electrons is controlled by varying the voltage applied to the grid. A small change in grid voltage results in a larger change in electron flow through the tube, amplifying the signal applied to the grid.

Many materials flouresce or "glow" when struck by electrons. Not all the electrons in a vacuum tube strike the plate. A few stray off and strike other things in the tube, such as the glass envelope. Many types of glass will give off a faint blue glow when bombarded by electrons of sufficient energy. This is the most common type of blue glow seen in power output tubes. Often you will see small patches of glow that correspond to the holes in the plate or other structures in the tube.

Another type of glow you may see in power tubes appears on the inside surface of the plate structure. The pattern of the glow will aften appear in strips that vary along with the signal applied to the tube. (i.e., the music.) This type of glow is generally normal as well. It is often caused by a thin layer of atoms that have boiled off the cathode and deposited on the surface of the plate. What you see is the slight flourescence of this material when it is hit by electrons from the cathode. The stripes correspond with the alignment of the grid wires. The varying widths of the stripes demonstrate the operation of the tube. The grid allows restricts the size of the electron beam according to the grid's voltage.

Of course, you might have a serious situation if the blue glow looks like this.

1996-1999 Larry Barras, all rights reserved. May not be reproduced without permission.

Last Modified: 12/31/1999

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