The DK91 is a miniature B7G dry battery heptode, it was designed for frequency changer use in battery portable radio receivers. The valve was designed for self oscillating service.

The thin glass tube envelope is 17 mm in diameter and, excluding the B7G base pins, is 45 mm tall.

References: Datasheet & ¹⁰⁴⁰. Type DK91 was first introduced^¶ in 1947.

 
Pin Connections

B7G	1	2	3	4	5	6	7
	f-	a	g4,g4	g1	f-	g3	f+

 
Absolute Maximum Operating Conditions^¶

Heptode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	1.4	0.05	90	67.5	0	1.6	3.2	0.6M	0.3

Thanks to Frank Philipse for supplying the above PDF datasheet