The 6SS7 is a variable μ RF pentode. It has a medium gain and was released in the 1940's and listed as obsolete by 1964.

It was designed for HF receiver use where it was applied to RF and IF amplification, but above 15 MHz or so the low gain would tend to impact on receiver noise performance.

The thin metal tube envelope is 25 mm in diameter and, excluding the IO base pins, is 50 mm tall.

References: Datasheet, ¹⁰⁴⁰ & ¹⁰⁴³. Type 6SS7 was first introduced^¶ in 1940.

 
Pin Connections

IO	1	2	3	4	5	6	7	8
	m	h	g3	g1	k	g2	h	a

 
Absolute Maximum Operating Conditions^¶

Pentode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	6.3	0.15	250	150	-3	9	2	1M	1.85

Thanks to Frank Philipse for supplying the above PDF datasheet