The 2D21 thyratron is, as usual, a gas filled valve, in this case the gas at low pressure is xenon. This example is of tetrode construction and was designed for relay or grid-controlled rectifier applications.
Hewlett Packard is a name associated with the very best in electronic instruments but not valve manufacture and this valve was made for HP by General Electric.
When a certain critical anode current flows the ionisation of the gas begins an avalanche discharge that overrides the grid control and leads to saturation current flow. The anode current can then only be extinguished by reducing the anode volts below the ionisation threshold.
The gm value in a normal hard valve is replaced by the control ratio parameter. In this valve the ratio is 250. At saturation the valve voltage drop is 8 Volts.
Side view of the box anode.
The flat face of the anode has been formed with a small flange.
The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 44 mm tall.
References: Datasheet & 1040. Type 2D21 was first introduced in 1952.