The 28D7 is a double beam tetrode designed to work as a class A audio amplifier. When both sections are used in push-pull the output is 175 mW. The anode and heater circuits are rated at 28 Volts. This is the voltage one would expect to find in a lorry or truck when the engine was running.
The twin electrode towers.
The side view of an anode plate. This is ribbed for strength and blackened to act as a more efficient heat radiator.
The wide glass tube envelope is 28 mm in diameter, and excluding the B8B base pins is 62 mm tall.
References: Data-sheet & 3002. Type 28D7 was first introduced in 1943.