The 12K7GT variable-mu screened pentode was designed in the 1940's for intermediate frequency amplifiers in superhet receivers where the stage gain was controlled by the application of negative bias on the grid. That bias or AGC voltage was derived from the received signal by rectification. It is high impedance valve allowing for a high Q in the tuned anode circuit.

See also 6K7.

The wide glass tube envelope is 28 mm in diameter and, excluding the IO base pins, is 67 mm tall.

References: Datasheet, ¹⁰⁴⁰ & ¹⁰⁴³.

 
Pin Connections

IO	1	2	3	4	5	6	7	8
	m	h	a	g2	g3	nc	h	k

 
Absolute Maximum Operating Conditions^¶

Pentode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	12.6	0.15	250	125	-3	10.5	2.6	0.6M	1.65

Thanks to Frank Philipse for supplying the above PDF datasheet