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:
FLUORESCENCE (Photo 1)- 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.
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)
GAS (Photo 2)- 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.”
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.
The getter or getter flash is the silver/gray coating seen on the glass of a tube. Most tubes have the getter flash at the top of the bottle, but some have it on the side or even bottom of the bottle. There are even tubes that have multiple getters on the top and sides. Regardless, the getter flash is what indicates there is a good vacuum in the tube and that no air has leaked inside. The shade of gray can vary from chrome-like to black. Any shade of gray is good. Inexperienced or misinformed folks may think a black color is a burn mark. It is not a burn at all but rather a product of how fast the flashing was done. Here in Photo 3 we show various colors of healthy getters.
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 filament in a tube has the job of heating the tube. It is required to bring the tube up to a temperature where electrons will get excited and start flowing. The filament is also commonly called a heater for this reason. All common audio tubes will have a heater filament although in some tubes they are well hidden behind the plates and other elements making them difficult to see, even in a dark room. Regardless, if the tube is warm and sound is coming from the amplifier, you know the heater is doing its job no matter how bright or dim it appears. An excellent example of this is the reissue Tungsol 12AX7 tube shown in Photo 5. Its heater is extremely well hidden and casts little light. 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.