The EF6 pentode was designed for radio receivers, where it would be used for RF and IF amplification. The later EF8 was a variable μ type for automatic gain control in IF stages.

The classic envelope is 29 mm in diameter and, excluding the Ct8 base pins, is 81 mm tall.

References: Datasheet & ¹⁰⁴³. Type EF6 was first introduced^¶ in 1938.

 
Pin Connections

Ct8	1	2	3	4	5	6	7	8	tc
	h	m	a	g2	-	g3	k	h	g1

 
Absolute Maximum Operating Conditions^¶

Pentode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	6.3	0.2	250	100	-2	3	1	1.75M	2.0

Thanks to Frank Philipse for supplying the above PDF datasheet

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