The UCH21 is an early example of all glass construction. This triode heptode frequency changer was designed to operate on a heater chain across the mains supply, and the anode voltage is correspondingly low to take account of the low HT voltage present in universal sets. Mullard describe the valve as designed to accept ACG to the heptode.

The rusty metal skirt and the red paper band round it suggest that this is an early (1940s) sample. It looks as though it is of Continental manufacture rather than British, although Mullard did make UCH21's in the UK.

The reason why there is no cathode/g5/s pin is that the metal spigot is used for this purpose. This meant that the metal skirt operated at cathode potential, not chassis potential, creating insulation problems unless a suitably-insulated base socket was used. It also creates problems with some commercial valve testers.

The electrode construction is obscured by the shielding screen. The triode is separate from the heptode and is required to generate a peak to peak voltage of 13 Volts to drive the mixer correctly.

The wide glass tube envelope is 26 mm in diameter and, excluding the B8B base pins, is 60 mm tall.

 
Pin Connections

B8B	1	2	3	4	5	6	7	8
	h	a	a(t)	g(t)	g2,g4	g1	g3	h

 
Absolute Maximum Operating Conditions

Heptode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	20.0	0.1	200	100	-2	3.5	6.5	1M	0.75

Triode	Va	mAa
	120	4.1

Thanks to Frank Philipse for supplying the above datasheet

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This exhibit was last updated on 24 November 2007 {find valves by name}