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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 identification appears in the usual elliptical M-OV outline but is applied with paint not etched into the glass. Slight rubbing will remove this paint.

The Osram label is the later rectangular style.

The top mica showing the tops of the electrodes. The cathode tube is round, the oscillator control grid has copper supports, the second grid (oscillator anode) is two plain rods, the screening grids have their supports bridged as can be seen on the right and finally the signal grid connects to the top cap.

The classic envelope is 44 mm in diameter and, excluding the B7 base pins, is 108 mm tall.

References: Data-sheet, ¹⁰⁴⁰ & ¹⁰⁴³. Type MX40 was first introduced in 1934. See also 1934 adverts.

Pin Connections

Absolute Maximum Operating Conditions^¶

Thanks to Frank Philipse for supplying the above PDF datasheet.