A Vox PA mixer (six channel) - c. 1966
Below, pictures of a Vox Mark IV (one could just about term it also a Mark III, version 2) public address mixer: solid state, six channels - five for microphones, one for music. Transistors are germanium RCA 2N2613. Power was supplied by a PP9 9 volt battery. For an overview of the development of Vox mixers, which were part of the "catalogue" from 1964, see this page.
In terms of competition, "CTH Electronics" and PYE, both based in Cambridgeshire, were strong runners, but otherwise the field was fairly open. In the USA there were many more manufacturers (and re-branders).
At present, it is not known who assembled the mixers for JMI. Burndept seems unlikely; Triumph certainly did not. The contract will probably have been given to a smaller company, perhaps the one that produced the guitar booster units for Vox - see the examples on this page.
The body of the unit below measures: 4 3/4 inches deep; 5 inches high; and 14 inches wide. It stands a little higher than 5 inches on its feet, however. The metal case is finished in a sort of blue hammerite. Pot codes are "JM" = September 1965, so assembly probably in early 1966.
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