05/10/2016
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[1], 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.
[1] http://www.arrl.org/arrl-vintage-amateur-radio-equipment-exhibit
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