This Mullard EN91 is also marked with the American designation of 2D21. The valve is a xenon thyratron. 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 box anode seen from the side.
The ceramic insulation shields the anode connection on pin 6.
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 EN91 was first introduced in 1944.