The 6L1 is a low impedance double triode. The 20L1 has identical operating characteristics but is designed for use with 12.6 Volts at 200 mA on the heater thus making it suitable for vehicle use.

The thin glass tube envelope is 19 mm in diameter and, excluding the B8A base pins, is 53 mm tall.

References: Datasheet & ¹⁰⁴⁰. Type 6L1 was first introduced^¶ in 1951.

 
Pin Connections

B8A	1	2	3	4	5	6	7	8
	h	a(2)	g1(2)	k(2)	a(1)	g1(1)	k(1)	h

 
Absolute Maximum Operating Conditions^¶

Triode	Vh	Ah	Va	Vg	mAa	ra	gm
	6.3	0.4	250	-11.5	10.0	6,200	2.8

Thanks to Frank Philipse for supplying the above PDF datasheet