The MX40 is a 1934 Osram heptode frequency changer. The British radio industry was slow in adopting the superhet for a variety of reasons one of which was that British valve manufacturers were slow in developing the specialised types of valve required in an efficient, low-cost set. The American radio industry showed the way and the MX40 is simply a home-brewed copy of the American 'pentagrid converter.
The oscillator voltage was designed to be 10.0 V peak to peak.
The B7 base and pinch assembly put the electrodes far away from the under chassis wiring this extra inductance limits the maximum frequency of operation but most heptode frequency changers were used for low frequency long and medium wave broadcast reception.
The Osram paper label. The Marconi version of the MX40 differs only in the paper label.
The circular wire grids: The inner grid is the oscillator control grid, grid two can be seen to be a pair of plain rods and this is the oscillator anode. Grids three and five are screen grids with the signal coupled from the top cap to grid four.
The classic envelope is 44 mm in diameter and, excluding the B7 base pins, is 114 mm tall.
References: Datasheet, 1040 & 1043. Type MX40 was first introduced in 1934.