The equipment operating manual should give more information on this. Radiotronics was published by the Amalgamated Wireless Valve Co. of Australia using technologies derived from RCA USA, Marconi, Telefunken and others. The colour of this glow and its intensity depend on the nature of the glass, the voltages employed, and the design of the valve. It has no deleterious effect, however, and actually is a sign of an extremely high vacuum. It is particularly prominent with type 42, but may also be observed on other types, such as 6A7, 6C6, 6D6, but since in these types a black coating is used inside the bulb the fluorescence is rarley noticed.
Blue glow is a glow between electrodes caused by ionised gas.
When any appreciable glow is observed inside the plate, the valve is definitely defective, due to gas, and a negative grid current will be observed if a micro-ammeter is placed in the grid return circuit. A valve reading less than 1 micro-ampere for each 10 milliamperes of anode current is quite satisfactory, and even two to three times this amount of negative grid current is usually quite permissible. When a valve becomes very gassy, the blue glow frequently extends in the shape of streamers radiating from the ends of the anode, and the valve in this condition is completely unusable. In high vacuum rectifiers used at voltages not exceeding 400 volts, a small amount of blue glow is not detrimental and may be rather beneficial. High voltage rectifiers, on the other hand, must have an absolute minimum of gas.