The MH4 is an indirectly heated amplifier triode. The MH4 was also produced in Catkin form in 1933/4. The glass envelope version has a totally different electrode construction but gave the same results. The Catkins were excellent valves but were more expensive to make and were phased out after a few years.

The identification is printed onto the metallic paint screening.

The balloon envelope is 50 mm in diameter and, excluding the B5 base pins, is 113 mm tall.

References: Datasheet, ¹⁰⁰⁵, ¹⁰⁴³, ³⁰⁰² & ¹⁰⁴⁰. Type MH4 was first introduced^¶ in 1929.

 
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	-4	5.0	11,100	3.6

Thanks to Frank Philipse for supplying the above PDF datasheet