The CV858 is the UK service designation for the ECC91. This miniature double triode was used for VHF signal amplification.

The construction has a common flattened tube cathode, with the control grids either side. The anodes are U channel sections either side of the central cathode. At first sight the valve appears to be a single device.

The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins is 46 mm tall.

References: Datasheet & ¹⁰⁴⁰. Type CV0858 was first introduced^¶ in 1956.

 
Pin Connections

B7G	1	2	3	4	5	6	7
	a(2)	a(1)	h	h	g(1)	g(2)	k

 
Absolute Maximum Operating Conditions^¶

Triode	Vh	Ah	Va	Vg	mAa	ra	gm
	6.3	0.45	100	-0.85	8.5	7100	5.3

Thanks to Frank Philipse for supplying the above PDF datasheet