The VP1321 is a variable μ pentode with external metallising. The 200 mA heater requirement was to allow the valve to be operated in a chain across the mains and thus be used in mains receivers without mains transformers. This both saved cost and allowed operation on DC mains as well as AC.

The classic envelope is 38 mm in diameter and, excluding the B7 base pins, is 104 mm tall.

References: Datasheet & ¹⁰⁴³. Type VP1321 was first introduced^¶ in 1934.

 
Pin Connections

B7	1	2	3	4	5	6	7	tc
	m	g1	g3	h	h	k	g2	a

 
Absolute Maximum Operating Conditions^¶

Pentode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	13	0.2	250	200	-2.8	7.4	1.85	1M	2.0

Thanks to Frank Philipse for supplying the above PDF datasheet