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The VGT121 is a thyratron. Comparison with the T41 from Mazda that it is equivalent to suggests that this valve is also of Mazda manufacture. The fact that it has a MO base completes the picture.

The thyratron is a special gas filled valve that at a given grid voltage rapidly moves into full conduction, when the gas ionises, thus acting as a switch. Under the correct circuit conditions this action can be made to generate ramps or pulses.

These valves were used in early radar and television as ramp generators, and in the first computers as pulse generators.

A typical thyratron TV timebase circuit of the early 1950s.

The classic envelope is 30 mm in diameter, and excluding the MO base pins is 78 mm tall.

References: Data-sheet, ¹⁰⁴⁰ & ¹⁰⁴³. Type VGT121 was first introduced in 1940. See also 1940 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.