The PL81 was designed for universal mains sets without a mains transformer and with the heaters on a series chain. The parameters given are for a pair in push pull audio service.

As a line output valve in television service, the peak anode surge voltage is 7,000 Volts. The anode dissipation is 8 Watts.

Mullard also specify the PL81 for use as a series pass element in voltage regulators.

The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 71 mm tall.

References: Datasheet & ¹⁰⁴⁰. Type PL81 was first introduced^¶ in 1951.

 
Pin Connections

B9A	1	2	3	4	5	6	7	8	9	tc
	ic	g1	k	h	h	ic	ic	g2	g3	a

 
Absolute Maximum Operating Conditions^¶

Pentode	Vh	Ah	Va	Vs	Vg	mAa	mAs	Pout	D
	21.5	0.3	200	200	-31.5	87	12.5	20W	5.5%

Thanks to Frank Philipse for supplying the above PDF datasheet