The 6BA6 is a variable μ amplifier pentode for HF use. This is a re-badged valve of unknown manufacture, as Bentley were resellers and not manufacturers.

Looking into the anode the control grid can be seen in the top window very close to the cathode. The screen grid and suppressor grid are also clearly seen with increasing diameters and reduced pitch.

The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 52 mm tall.

References: Datasheet, ³⁰⁰² & ¹⁰⁴⁰. Type 6BA6 was first introduced^¶ in 1950.

 
Pin Connections

B7G	1	2	3	4	5	6	7
	g1	g3	h	h	a	g2	k

 
Absolute Maximum Operating Conditions^¶

Pentode	Vh	Ah	Va	Vs	Vg	mAa	mAs	ra	gm
	6.3	0.3	250	100	-1.0	11.0	4.2	1.5M	4.4

Thanks to Frank Philipse for supplying the above PDF datasheet

More