The Jennings Univox (and organ) Conversion Unit
Late 1955 to c. 1960
January 1956
Developed in late 1955, the first dated instance of the unit appears in January 1956, its stated purpose being to "convert any type of electronic keyboard into an all-purpose amplifier with perfect tonal reproduction". In effect it was a "add-on" preamp.
By Spring 1958, the Conversion Unit seems to have been aimed more at owners - or prospective owners - of the Univox J6 and J10. The three inputs are described as being for "gram" (ie. Gramophone or Record Player), "guitar", "accordion", or "any type of microphone", presumably all indicated in the legends that one can see on the unit's fascia.
Detail from the JMI catalogue of Spring 1958.
In order to get going, a Univox user would first plug his keyboard into the Conversion Unit and then the unit into the main amplifier. When everything had been powered up, he or she could play the keyboard as normal and control the volume of any ancillary device that had been plugged into the box - guitar, microphone, gramophone, etc. - using the controls on its front panel.
Assuming that the unit contained at least one preamp driver valve, power, as Glen Lambert has kindly noted, will have been supplied by the Univox amplifier through the (multi-wire) connecting cable - two lines for the heater supplies (2 x 6.7v), one for the main DC voltage. Alongside those, lines for earth and (final) signal back out to the amp.
Connections in the umbilical cable provided by JMI to link the keyboard to the main body of a J10 from mid 1956. The six lines are: one DC voltage; one negative voltage for the thyratron valve; two 6.7v heater supplies; one earth; and one signal. The negative voltage will have been a pass-through in the Conversion Unit, necessary for the keyboard, but not required for the preamp.
Below, a Conversion Unit belonging to a Univox J6 from the later 1950s. It measures around 7" wide, 2 1/2" tall; and 6" deep. Jennings literature notes its weight as being 16 ounces. The inputs are captioned (top to bottom): "Moving Coil or Ribbon Mic"; "Crystal Mic"; "Gram[ophone]".
It is to be hoped that a little more can be given on the electronics of these units in due course.
Later mentions / instances
"Exchange and Mart", May 1962 - termed a "mixer unit" in conjunction with a "Univox J2" (most likely a J7).
"Exchange and Mart", December 1963 - the maker's "extra amplifier". Thanks to Glen for connecting this entry with the Conversion Unit.
"Exchange and Mart", April 1964 - "Vox converter".