The MH40 is a high-mu, low microphany, audio triode designed to work into a 50 kΩ load.
The construction features ceramic supports and specialised glass work and sets this Type out as a professional valve rather than a mass produced consumer type.
The Type designation is etched into the glass within the standard M-OV lozenge shape and wording.
This valve was once part of the BBC stock. Here is one of the attached paper labels.
The BBC research section tested this exhibit in 1937. The label shows the parameters.
The mesh anode and the control grid shaped to fit round the cathode. The substantial top and bottom supports are ceramic. The rigid construction prevents the relative electrode movements that cause microphany. The MHD4 double diode triode also has a mesh anode but the supports are single sheets of mica holding the rods.
Looking at the lower end of the control grid, it seems probable that the after the grids were wound the sides were pressed together whilst a mandrel was inserted to define the central shape. The grid wires do not appear to be held into the outer edges of the support rods, a technique known as notch and peening.
Extra support for the tops of the electrodes is provided by the side struts that locate into glass rods that themselves are fixed into the glass of the pinch stem.
The balloon envelope is 50 mm in diameter, and excluding the B5 base pins is 103 mm tall.
References: Data-sheet, 1040 & 1043. Type MH40 was first introduced in 1936. See also1936 adverts.
