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Rare de Forest Audion Donated to ARRL, Mated with Vintage Radio for Museum Display


An ARRL member from Virginia has donated a rare de Forest “round bulb” Audion to the League, which has paired the groundbreaking triode with a de Forest Receiver of similar vintage and rarity. Walt Bain, W4LTU, recently wrote ARRL Headquarters to see if the League would give the antique tube a home. Radio pioneer Lee de Forest filed his first patent for the Audion in 1907, describing it as a detector of sound, and he is generally credited with having invented the vacuum tube. Although it was first used as the detector in the de Forest Audion Receiver, the Audion subsequently was heralded as the world’s first electronic amplifying device. Bain, who is 86, said he inherited the Audion from his father, George Bain, a graduate of Wesleyan University in Connecticut in the 1920s who went on to work for Westinghouse.

“In the 1930s he was chief engineer at KenRad Tube and Lamp Company,” Bain told ARRL. “He would have met de Forest anytime during college, at Westinghouse, or KenRad.” This particular Audion likely dates back to the early 1910s and appears to be a somewhat later version of the device that de Forest had submitted on his patent application a few years earlier. An intact Audion such as this one is considered extremely rare.

ARRL Lab Test Engineer Bob Allison, WB1GCM, who curates the League’s museum collection, accepted the Audion and had it installed on the League’s own de Forest Audion Receiver, which lacked a tube.  “Each year, we have about 2000 visitors to the Lab; they will get to see that tube,” Allison said. The League’s Audion Receiver, once owned by Columbia University, bears the patents of de Forests Radio Telephone and Telegraph Company. The Audion’s three elements are clearly visible within the blown-glass envelope. Connections to the Audion’s rectangular plate and squiggly grid were made via wires that exited on one end of the bulb. The other end featured a candelabra-style lamp base, which screwed into a socket. The lamp base provided the filament connection. The Audion is mounted with the lamp base up, to prevent the filament from sagging and touching the other two elements of the tube.

In developing the Audion, de Forest had built on the work of John Ambrose Fleming, who invented a two-element vacuum tube in 1905. De Forest discovered that applying a radio signal to the grid instead of to the filament, or cathode, would yield a more sensitive RF detector than what was then available. De Forest also was an early radio broadcaster, transmitting speech and music in the New York City area in the early 1900s. De Forest came up with the idea of using a series of Audions to enhance their amplifying capabilities, an attribute American Telephone & Telegraph company capitalized upon, after securing de Forest’s patents.

In time, vacuum tubes supplanted solid-state mineral detectors in radio receivers, although in a “what goes around, comes around” turn of events, solid-state devices called “transistors” replaced the vacuum tube in the 1950s and 1960s. Today’s iPhones have the equivalent of 2 billion transistors packed inside.