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 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:
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. (Note from thetubestore: Mercury Vapor tubes are rare and almost never found in common guitar or hifi amplifiers)
To summarize, almost any blue glow you see inside a tube is perfectly fine and will not cause any problems in your amplifier. However, if you see bluish/violet glow around a single element such as a wire for example, it could indicate an air leak into the tube. There could be a small crack in the glass or a leak around one of the tube’s pins. A tube showing this needs to be replaced.
An article by Electronics Magazine published an in-depth article about tube getters in October 1950. It’s very long but below is an excerpt about the getters you will see in most common audio tubes. We also have the entire article available for download.
“Flash getters are outgassed at temperatures between 600 and 700 C, usually by r-f heating from the outside of the tube, and flashed at temperatures between 900 and 1,300 C. The barium vapor condenses on the cold surface opposite the getter material, usually on the envelope of the tube. The appearance of the condensed getter deposit depends upon the vapor pressure in the tube at the time of flashing. If the getter is vaporized very slowly, the first barium atoms evaporated will absorb the gas present so that the remaining getter is deposited in a very high vacuum, exhibiting a shining mirror. If flashing is done very rapidly, however, the getter deposits in a rather high vapor pressure and the getter mirror will be discolored due to dispersion of the barium. If vaporization is carried out in the inert atmosphere of a rare gas the condensed deposit will be black, resulting in a dispersal getter. This condition does not mean that the getter is contaminated, but merely that the deposit is finely divided and therefore absorbs light. Such deposits exhibit higher efficiency than the bright deposits.”
The only time the getter color will indicate a problem is when it turns white. A tube with a white getter will not function and cannot be used. This happens when air leaks into the tube. See Photo 4 for an example of this. The tube on the left has lost it's vacuum and no longer functions. The tube on the right is a normal functioning tube.
The Chinese 12AX7A also shown casts much more light even though it is operating in exactly the same circuit. On occasion we have people wonder why one tube appears brighter than another in a matched set they have. In almost all cases it’s simply a matter of one heater being more exposed than the other and therefore casting off more light. This is not a sign of a problem. Remember they are all hand made and a variance like this is normal. Photo 6 shows two EL34 tubes where one is brighter than the other due to the filament height. Both tubes test and perform perfectly.
Another thing that needs mention is red-plating or "cherry" plate glow. This will happen when a tube is incorrectly biased, causing the plate to overheat. Generally speaking, tubes do not like this unnecessary stress and will not last long if rebiasing is not done. See Photo 7. In this case the plate itself is actually casting a red glow.