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The three companies covered here were all creations of a prolific engineer and entrepreneur, Harold W. Ulmer. His life is summarized in the biography link to the left. All his companies were located in Oceanside, California, a sleepy coastal town at the north end of San Diego county, just south of the Camp Pendelton marine base.

Vacuum Tube Products was run out of Ulmer's parents garage and basement, starting in 1936. Ulmer went back east during World War II, working at Federal Telegraph Co. and then Raytheon, but upon returning in 1946, restarted Vacuum Tube Products. He started making picture tubes in 1948, and in 1949 made the first rectangular CRT in the United States. He went on to make many special tubes, vacuum equipment, welding machines, and CRTs, including the first storage tubes for Hughes Aircraft. Hughes was so impressed that they bought Vacuum Tube Products in 1959.

Harold Ulmer wasn't a big company guy, and left Hughes in less than a year. He then started H.W. Ulmer Co. in 1959, concentrating on welding and high-vacuum equipment. Several employees from Vacuum Tube Products rejoined him at H.W Ulmer Co.

In 1962, Harold Ulmer combined his H.W. Ulmer Co. with McEwen Enterprises to form MU Inc. (McKewen-Ulmer). Lillian McEwen was a production supervisor at Vacuum Tube Products. MU (which is pronounced "Em You", not mu, the Greek letter) specialized in making small production runs of tubes for the military, typically tubes no longer manufactured by other tube companies. Ulmer's past background in making many types of tubes allowed him to tool-up and make pretty much any tube needed. One of his daughters, Ronalee Elsberry (known as Roni), worked at MU starting in the 1970s. When Harold Ulmer died in 1997, Roni took over the company and continued serving the military tube market. In 2018, Roni decided to retire, and MU was shut down.

Various documents from Harold Ulmer's three companies are provided here, separated by company and listed chronologically.