The 6A8G is a heptode or pentagrid mixer and is typical of the 1930/40s frequency changer. The triode oscillator is formed between the cathode and g2 acting as an anode, and has to generate a peak to peak voltage of 25 Volts. The mixer is screened by G3 and G5 with the signal injected into the oscillating electron stream at the top cap g4.

The central cathode tube surrounded by the five grids they appear to be equally spaced. It is not possible to see if g2 is wire wound or just a pair of rods.

The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 99 mm tall.

References: Datasheet, ¹⁰⁴⁰ & ¹⁰⁴³. Type 6A8G was first introduced^¶ in 1937.

 
Pin Connections

IO	1	2	3	4	5	6	7	8	tc
	m	h	a	g3,g5	g1	g2	h	k	g4

 
Absolute Maximum Operating Conditions^¶

Heptode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	6.3	0.3	250	100	-3	5	2.7	0.3M	0.49

Thanks to Frank Philipse for supplying the above PDF datasheet