The 1N5G is a variable μ pentode for dry battery operation. Almost certainly used for domestic broadcast receivers at RF or as an IF amplifier. The relatively large size of the valve suggests that it was not designed for battery portable sets but for fixed operation. The filaments had to have the lowest possible consumption due to the cost of the dry batteries.

The wide glass tube envelope is 28 mm diameter, and excluding the IO base pins, is 80 mm tall.

References: Datasheet, ¹⁰⁴⁰ & ¹⁰⁴³. Type 1N5G was first introduced^¶ in 1937.

 
Pin Connections

IO	1	2	3	4	5	6	7	8	tc
	m	f+	a	g2	-	-	f-	-	g1

 
Absolute Maximum Operating Conditions^¶

Pentode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	1.4	0.05	90	90	0	1.2	0.3	1.5M	0.75

Thanks to Frank Philipse for supplying the above PDF datasheet