Difference between revisions of "Vacuum tubes"
(stub article on tubes - will be expanded and illustrated)
Revision as of 09:15, 8 November 2019
Vacuum tubes aka electron tubes or (thermionic) valves were the workhorse active components in electronics up to the mid-1960s.
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).
broken/burned out filament (open circuit)
Loss of cathode emission
Can be measured on a tube tester or using a test jig.
- 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.