Are my tubes defective if they glow blue?

The blue glow (fluorescence)

A blue glow primarily occurs in the upper area of the tube between the glass bulb and the mica support plate (mica spacer), as well as at openings in the anode plate.

Physical cause:
This phenomenon is caused by high-energy electrons (barium-strontium electrons) emitted by the filament. When they hit the mica surface, they release nickel atoms. Through collisions, these atoms are ionized, creating the characteristic blue fluorescence on the glass.

Important features:

Vacuum dependence:
The glow is a function of the anode voltage and the residual gas molecules present inside the tube. Even in a very good vacuum, there are still billions of molecules that make this effect possible.

Variability:
The intensity varies depending on the individual tube and operating voltage – from barely noticeable to clearly visible in daylight.

Performance and service life:
This blue glow is not a quality defect. Extensive characteristic curve measurements show that neither the electrical performance data nor the life expectancy of the tube are negatively affected.

Critical warning signs: pink glow and air leakage

Two potentially dangerous conditions must be clearly distinguished from normal blue glow:

Ionization caused by high-voltage flashover:
In the case of massive gas ionization caused by sparking, the gas turns pink. This leads to unstable anode currents and clearly indicates a defect.

Loss of vacuum (air leakage):
If air enters the tube, it is ionized. The glow often appears bluish, but it leads to immediate oxidation of the filament and rapid loss of emission. In modern audio tubes, this case occurs only very rarely.

Summary – quick diagnosis