Vacuum tubes

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Vacuum tubes aka electron tubes or (thermionic) valves were the workhorse active components in electronics up to the mid-1960s.

Tube failure

Important failure modes of vacuum tubes include:

Loss of vacuum

aka tube "getting gassy" - can be caused by glass leak, electrode/glass interface leak, broken glass

Can often be detected by visual inspection in glass-envelope tubes, either when the envelope is visibly cracked, or if the getter (silver-colored metallic patch) has turned white or entirely disappeared.

Gassy tubes will draw more than the nominal heater power because convection transports heat away from the filament.

Slightly gassy tubes exhibit a blue glow from inside (ionization of gas molecules by the electron beam).

Filament failure

broken/burned out filament (open circuit)

Loss of cathode emission

Can be measured on a tube tester or using a test jig.

Electrode damage

  • electrode damage (e.g. melting) caused by overloads - grid wires are most sensitive
  • bent electrodes or internal structures (especially noticeable in CRTs) - may be caused by excessive G force application such as dropping

Either of these can also lead to inter-electrode shorts.

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