Under certain circumstances a valve may emit a bluish glow, but this does not necessarily indicate that the valve concerned is faulty. This blue glow is, of course, quite distinct from the light emitted from a red hot cathode or heater.
Valves which contain a small amount of gas will usually glow in operation, this glow being emitted from the gas between the electrodes. Voltage stabiliser tubes and thyratrons, which contain some gas, always glow in normal operation. Similarly, mercury rectifiers always glow, as they contain mercury vapour. If, however, any ordinary valve which should have a high vacuum within its envelope emits a bluish glow from between the electrodes, this shows that the valve is soft (i.e. it contains some gas). Such a valve should be replaced as soon as possible, as the reverse grid current will be very high and this will affect grid bias and alter operating conditions.
In extreme cases there may be too much air in the valve for a glow to be emitted and the 'getter' (a deposit on the glass which should be of a bright metallic appearance) becomes white. Such a valve cannot be used at all.
Power valves may sometimes emit a bluish glow from their glass envelopes or from the mica supports within the valve. The intensity of the glow may rise and fall with changes of anode current or signal amplitude. Providing that this glow comes from a solid surface and not from the space between two electrodes, this is not an indication that the valve is faulty. The glow is caused by electron bombardment of the glass or the internal surface.
On one occasion known to the writer it was found that, if the input to a large audio amplifier was short-circuited, an extremely loud sharp noise was emitted from the speaker. The trouble was traced to the first valve in the amplifier, which had become soft. The resulting grid current flowing through the grid resistor had produced a steady voltage across it. When this voltage was short-circuited, the voltage change was amplified and resulted in the very loud 'crack' just referred to. The valve concerned emitted a slight glow from the space between its electrodes.