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Triotron designate the HD2 triode for detector and HF service. The HD2 features in the Wireless World data booklet in 1932.

The low consumption filament indicates that it is oxide coated. Probably by the Azide process.

The remains of the lettering. Triotron can found as can HD2 even though the H looks like an M. MH2 did not exist.

The box anode at an angle in the envelope. The box is open at either end and this was important to let the Azide breakdown products escape.

The angled supports emerging from the pinch. The grid winding is just visible near the bottom of the image.

The balloon envelope is 48 mm in diameter, and excluding the B4 base pins is 89 mm tall.

Reference: Data-sheet. Type HD2 was first introduced in 1929. See also 1929 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.