The VT50 is a service designation for the HL2K.

See HL2K for details.

The classic envelope is 27 mm in diameter and, excluding the B4 base pins, is 70 mm tall.

References: Private communication, ³⁰⁰² & ⁴⁰⁰¹. Type VT50 was first introduced^¶ in 1934.

 
Pin Connections

B4	1	2	3	4
	a	g1	f	f

 
Absolute Maximum Operating Conditions^¶

Triode	Vh	Ah	Va	Vg	mAa	ra	gm	μ
	2.0	0.1	150	-3	1.75	18k	1.5

Thanks to Frank Philipse for supplying the above PDF datasheet

More