From time to time I’m asked about the different colours that appear inside the warmed up valve, especially valves with glowing blue colours – are they an indication that all is well, or is there in fact a problem with the tube?
For purposes of this post I’ll break the types of emitted blue colour glow into 3 general categories, which were all best described by the manufacturers Sylvania back in the hey day of classic valve manufacturing. The following was written about valves with glowing blue by their manufacturing engineers at the time and still applies today –
FLUORESCENCE – this type of glow is usually violet in colour 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.
GAS – produces a blue haze, generally confined to the vicinity of the mount structure. The proper function of gas types such as thyratrons, voltage regul
ator 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.”