The T-21 or T.21 or T21 is a gas filled triode relay or Thyratron from the 1930s. The gas filling is argon.
It was designed for timebase circuits where the maximum repetition rate is 15,000 times a second. Typically use would be in television receivers and similar magnetically deflected CRT systems.
A full explanation of the workings of this valve are given in the data-sheet.
The internal structure looks like an ordinary triode.
The anode side view. Note the cooling fins on the control grid supports.
The classic envelope is 39 mm in diameter and, excluding the B5 base pins, is 109 mm tall.
Reference: Data-sheet & 1043. Type T-21 was first introduced in 1936. See also1936 adverts.
