The MHL4 amplifier triode (mu = 20) was not much used in domestic radios because the MH4 (mu = 40) was better suited to the detector/first AF task. However, the MHL4 did see service in professional and military applications over many years until it was superseded by the L63, which had comparable characteristics.

The MHL4 was marketed from about 1930. It was certainly a year or so behind the MH4. The specimen shown looks 1940-ish. Early MHL4s were taller and bulbous, and were available either with B4 base caps + side terminal or with B5 bases.

The classic envelope is 42 mm in diameter and, excluding the B5 base pins, is 94 mm tall.

 
Pin Connections

B5	1	2	3	4	5
	a	g1	h	h	k

 
Absolute Maximum Operating Conditions

Triode	Vh	Ah	Va	Vg	mAa	ra	gm
	4.0	1.0	250	-8	8.0	8,000	2.5

Thanks to Frank Philipse for supplying the above datasheet

This exhibit was last updated on 02 December 2007 {find valves by name}