The 2D21W thyratron is a gas filled tetrode valve. The W denotes a special quality valve that has been strengthened so as to operate reliably in areas of shock and vibration.
The gas at low pressure is xenon. This example is of tetrode construction and was designed for relay or grid-controlled rectifier applications.
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.
The GE logo and the rectangular box anode.
Here the double micas are visible. It is this extra thickness of mica that reduces the wear on the punched holes and thus prolongs the alignment of the electrodes and consequently enhances the life of the valve.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 47 mm tall.
References: Data-sheet & 1040. Type 2D21W was first introduced in 1958. See also1958 adverts.