Jo Jennings and Calvin Townsend

Jo Jennings

Above: Jo Jennings and his long time partner Calvin K. Townsend examine two experimental vacuum switches, c. 1957 (Perham Collection of Early Electronics)

At 12, Jo Emmett Jennings built his first radio receiver, with an oatmeal container to form the coil and a “catwhisker” detector. In 1931, the 20 year old ham operator was hired as the night and Sunday operator at San Jose radio station KQW.  After graduating from San Jose State College in 1936, Jennings went to work for Eitel-McCullough, and soon decided to start his own vacuum tube company. Jennings Radio Manufacturing Company was born in 1940 in a chicken house in his father’s San Jose orchard.

Jennings Radio chicken shed
Original San Jose site for Jennings Radio Manufacturing Corporation, c. 1956 (Perham Collection of Early Electronics)

The first products Jennings developed were vacuum dielectric fixed capacitors with glass envelopes. His first big order was for 1,000 capacitors, a device used in nearly all radio systems in which an electric charge can be stored. Lacking tantalum to make them — a metal in very short supply during the war — he used sheet metal salvaged from old motor oil cans for his first capacitors. Since he couldn’t afford a Litton lathe, he built his own and trained his own glass-blowers. The company quickly expanded.

Jo Jennings with vacuum switches
Jo Jennings inspects a factory installation of three of his vacuum switches, c. 1957 (Perham Collection of Early Electronics)

In 1946, he patented a variable version of his capacitor, which made it possible to quickly change the frequency of a transmitter and use a broad range of frequencies. His employees were encouraged to offer ideas for improving both products and manufacturing processes. The firm developed vacuum switches and relays, widely used in electrical circuits. Vacuum relays were used extensively in the early space program to modify the direction of orbiting satellites. The company was sold to International Telephone and Telegraph Corporation in the 1960s.

As well as multiple examples of Jennings Radio products, the Perham Collection of Early Electronics includes original technical drawings and photographs from the Jennings Radio company, and Jane Morgan’s research material and interviews for her book Electronics in the West.

In 2017, the original Jennings location on McLaughlin Avenue was closed permanently, with manufacturing processes distributed to other locations. History San Jose received a large donation of products, test equipment, photographs, and marketing material no longer needed by corporate offices. These are being cataloged and photographed and are available to view through History San Jose’s online catalog:, Catalog No. 2017-29.

Guide to the Collection on Jennings Technology (Online Archive of California)