The MHD4 is a double diode triode for detection and first AF amplifier in superhet receivers. It was an 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 international octal base.
This MHD4 is an early example of the type and has a clear bulb rather than a carbonised envelope of external metalisation. The top cap is the early 1930s screw type and the other examples that we have use the metal cap.
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 Gecovalve is a brand name of M-OV and this example was exported to France. Thanks to Michel Marignan for sending these images.
The cylinder at the lower end of the cathode is one of the diode anodes. The second diode anode is a wire inside the cylinder.
A close-up of the etched lettering and the mesh anode.
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.