The MX40 is a 1934 Osram frequency changer of heptode construction and some of the grids are clearly visible in the picture.

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' frequency changers popular there in the early 1930s.

The oscillator voltage was designed to be 10.0 V peak to peak.

The balloon envelope is 43 mm in diameter and, excluding the B5 base pins, is 115 mm tall.

References: Datasheet, ¹⁰⁴⁰ & ¹⁰⁴³. Type MX40 was first introduced^¶ in 1934.

 
Pin Connections

B7	1	2	3	4	5	6	7	tc
	g2	g1	g3,g5	h	h	m,k	a	g4

 
Absolute Maximum Operating Conditions^¶

Heptode	Vh	Ah	Va	Vs	Vg	gm
	4.0	1.0	250	80	-3	0.5

Thanks to Frank Philipse for supplying the above PDF datasheet