There are a few types of blue glow in Vacuum Tubes.
1) FLUORESCENCE. This type of glow is normally dark blue. It can appear outside the anode, on the glass surface inside the bulb, and as spots on other parts inside the tube. The spots may have the appearance of stains. With very used tubes, you can sometimes see a faint stain on the glas, at the position where the fluorescence appears. In most cases, you will find it with power tubes. It results from electron bombardment on glass taking place within the tube. It generally has no negative effect upon the performance, and in fact, tubes displaying this phenomenon are particularily good with respect to having a high vacuum. Mesh plate tubes will display this blue glow under high Anode current, since these have holes in the Anode, letting electrons pass. You may also see it in tubes like EL34, or 6550, which have holes in the anode. These holes are for adjusting the grids during production. Typical for this kind of glow is, that if you hold a conductive surface against the fluorescent spots, (if on the glass) the spot will change it's shape. (The electrons take a differnent path)
2) GASSY AMPLIFIER TUBES. These produces a white-blue haze, of low brightness, inside the Anode. (The color is like this: ====== ) You can best see it, when looking from the top, inside the tube. The brightness is much less than the filament lighthing. You can only see it in a dark room, and it appears as a uniform cloud, no spot building. It is this kind of blue glow that indicate the vacuum is not as good as can be. The electrons on their way from the Cathode to Anode hit gas molecules, that get ionesed from the incidents. The recombination of Ions and Electrons produce the light effects. The Gas iones are much heavier than electrons, and hit the Cathode with a considerable amout of energy. This causes early wear-out of the cathode, and grid current if it gets worce. The tube will self destroy eventually.
3) REGULATOR AND THYRATRON TUBES. These tubes are high vacuum , but there is a residue of gas added to the tube, on purpose. They light up pink, violet, or orange, depending on the used gas, like Crypton, Neon, etc.
4) MERCURY RECTIFIERS. These light white-violet, and have a mercury gas residue. The glow indicates proper operation. Used tubes may look worn out and extreme dirty, but can be 100% ok still.
Taken from JacMusic website. Original page is
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