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.

 
Pin Connections

B8B	1	2	3	4	5	6	7	8
	h	g1(2)	g2	a(2)	a(1)	k	g1(1)	h

 
Absolute Maximum Operating Conditions

Beam Tetrode	Vh	Ah	Va	Vs	Vg	mAa	mAs	rk	rl
	28	0.4	28	28	-	18.8	1.2	180	68,000

Thanks to Frank Philipse for supplying the above datasheet

This exhibit was last updated on 02 July 2007 {find valves by name}