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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.

The identification is etched into the body of the enveloped.

The first MHD4's had screw top caps, this 'top hat' style originated in the mid 1930's.

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

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 Osram paper brand label seen from behind. The triode is at the top with the screened diode anodes below.

The gauze anode of the triode, the cathode tube is a flat sided oval. Below, the diode screen is connected to the cathode and the connection can be seen through the identification lozenge.

The classic envelope is 45 mm in diameter and, excluding the B7 base pins, is 112 mm tall.

References: Data-sheet & ¹⁰⁴³. Type MHD4 was first introduced in 1933. See also 1933 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.