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 selection

The mechanical tolerances in vacuum tube manufacturing lead to a slight variation in many tube parameters, e.g. the I/V curves, and the dynamic behavior. Further, tubes exhibit a slight change in performance in the first hours of operation. While those variations are acceptable in many applications, Tektronix chose to age most of their used tubes to avoid the alteration of performance in the first hours of operation of new equipment, or when replacing tubes. In many applications, the same tube was further selected after aging for parameters as low grid current or matched in pairs for a particular bias condition. (In contrast to popular belief in the audiophile world, a "matched pair" of tubes is meaningless, unless the exact bias condition in operation is known and tubes are matched in that condition.) This is the reason why there are often five or six different part numbers for the same tube, since they were differently selected.Tektronix used 157- as the part number prefix for aged/checked/matched tubes (e.g,157-0006-00 ), whereas raw tubes including CRTs were identified with 154- prefix (e.g.,154-0031-00).

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 may exhibit a blue glow from inside (ionization of gas molecules by the electron beam) when operated with plate/grid supplies present.

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.