The MHD4 is a double diode triode for detection and first AF amplifier in superhet receivers. It was a cost saving to make all three devices in a single envelope with a common cathode. This style became the normal configuration.

Eventually the MHD4 with its B7 base was replaced with the DL63 that employed the by then standard octal base.

The metallic gauze anode is easily seen in the inset picture.

The specimen shown is an example of later construction, probably late 1940s. The earliest MHD4s were much more bulbous.

Although the later Type DL63 had characteristics rather similar to those of the MHD4, the octal successor to the MHD4 in cheap superhets was in fact the DH63, a copy of the American 6Q7G, whose characteristics were more suited to the job.

The classic envelope is 40 mm in diameter and, excluding the B7 base pins, is 100 mm tall.

References: Datasheet & ¹⁰⁴³. Type MHD4 was first introduced^¶ in 1933.

 
Pin Connections

B7	1	2	3	4	5	6	7	tc
	a(d2)	m	a(d1)	h	h	k	a	g1

 
Absolute Maximum Operating Conditions^¶

Triode	Vh	Ah	Va	Vg	mAa	ra	gm
	4.0	1.0	250	-4	4.0	18,200	2.2

Thanks to Frank Philipse for supplying the above PDF datasheet

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