Below, pictures of a Jennings plug-in Treble Booster with its box (lid lacking) and instruction sheet. These units were produced in the UK from late 1964 to 1968.

Above, a detail from the Vox solid state catalogue of mid 1967. The four boosters are: Treble, Bass, Distortion and Mic.

In 1966 Thomas Organ, the agent for Vox in the United States, began producing boosters of its own. At first, they were simply copies of the English units. The design was later changed. The Thomas schematic for the JMI boosters can be found at the foot of this page.

Pages, dated June 1966, on the Vox plug-in boosters from the Thomas Organ Accessories Manual - the first US-made units simply replicated the English ones. The Mic. booster, however, was not adopted.