The 708A was developed in 1941 as a triode mixer for the receivers used in 10 cm radar operating at 3 GHz. The triode was used as it could withstand the transmit pulse. The later introduction of TR cells enabled semiconductor rectifiers to be used in place of the UHF triode.
The 708A would be used in the grounded grid configuration. The steel body is connected to the control grid with the anode and filament connected by the base pins.
The filament pins are indicated by the arrows on the base.
The evacuation tube is at the top.
The wide metal tube envelope is 27 mm in diameter, and excluding the base pins is 53 mm tall.
