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Expanded Audion and Vacuum-tube Development (1917-1930)
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The wartime consolidation of the radio industry under government control led to important advances in radio equipment engineering and manufacturing, especially vacuum-tube technology, with receiver design seeing the introduction of superheterodyne, super-regenerative and neutrodyne circuits. Still, some would look toward the day when vacuum-tubes would be supplanted by something more efficient and compact, although this was another development which would take decades to be realized.
Wartime and Post-War Advances -- Superheterodyne Receiver -- Super-regenerative Receiver -- Neutrodyne Receiver -- Search for Solid-State Alternatives


During World War One the military took over control of the entire U.S. radio industry, and in conjunction with the major electrical firms made great strides in radio engineering using vacuum-tubes. In addition, wartime work exposed thousands in military service to the changes which were taking place, especially with respect to vacuum-tube equipment. The Vacuum Tubes entry by Major General George O. Squier, in the Signal Corps section of the 1919 edition of War Department Annual Reports, reviewed the advances made in vacuum-tube manufacturing and engineering from 1917 to 1919, with the prediction "That vacuum tubes in various forms and sizes will, within a few years, become widely used in every field of electrical development and application is not to be denied." And shortly after the war ended articles started to appear that showed a comprehensive scientific understanding and explanation of the design and operation of vacuum-tubes, for example L. M. Clement's The Vacuum Tube as a Detector and Amplifier (extract), from the April, 1920 issue of QST. H. Winfield Secor's The Versatile Audion, which appeared in the February, 1920 Electrical Experimenter, reviewed the advances taking place in thirteen areas of vacuum-tube engineering. In April, 1919 American Telephone & Telegraph, employing vacuum-tube versatility from six of Secor's categories, transmitted speeches and entertainment by phone lines and radio to a Victory Liberty Loan drive, as reported by Speeches Through Radiotelephone Inspire New York Crowds, from the May 31, 1919 Electrical Review. Magic Touch of California Wizard of Electricity Has Harnessed Sound Waves and Linked Continents, from the January 13, 1920 San Francisco Chronicle, provided a very generous look at an independent manufacturer, Otis B. Moorhead. Lee DeForest continued to develop transmitters, with the De Forest Radio Telephone & Telegraph Co.'s OT-10 model used for the introduction of a broadcasting service by the Detroit News radiophone in August, 1920. This same model was also promoted, in an advertisement in the December, 1920 issue of Motor Boating, as suitable for providing communication "for yachts and motor boats".

By 1922 vacuum-tubes had been firmly established as a major technological advance, and the Vacuum Tubes chapter of William C. Ballard, Jr.'s 1922 Elements of Radio Telephony reviewed the device and its construction. Research accelerated, resulting in increasingly more powerful vacuum tubes for use in radio transmitters. These advances were led by research done by Harold Arnold at AT&T and Irving Langmuir at General Electric. The October, 1922 issue of Popular Science heralded Irving Langmuir--Creator of the Super-Tube, as the G. E. engineer posed with a 20-kilowatt tube, which was compact enough to hold in his hands. However, in the next month's issue of the same magazine came the announcement that Langmuir's work had been eclipsed by The World's Most Powerful Vacuum Tube, produced at AT&T's Bell Laboratories, which at 100 kilowatts was five times as powerful, yet still small enough to be held by a single person. It was particularly ironic that one of the main corporations involved with producing high-powered vacuum-tubes was General Electric. This company had formed the Radio Corporation of America in 1919, in large part to provide a market for its high-power Alexanderson alternator-transmitters, which set the standard for reliable long-distance radiotelegraph service. But now, just three years later, the high-power vacuum-tubes made G.E.'s alternator-transmitters obsolete -- "ponderous and costly" in the words of the review of the AT&T tube -- and the company would soon cease the production of what it originally expected would be one of its showcase offerings, although many of the approximately twenty 200-kilowatt Alexanderson alternators that had already been produced would be used for two or three more decades. Meanwhile, more powerful tubes continued to be produced, and in Experiments with High Power, from the November, 1930 Radio Digest, Howard Edgar Rhodes announced Westinghouse's development of a "monster" 200-kilowatt tube.


The development of high quality vacuum-tubes made possible improvements in receiver design. Initially, receivers tuned directly to stations' transmitted frequencies, but, while during military work in France during World War One, Edwin Howard Armstrong developed an approach that first converted the signal to a fixed intermediate frequency, which was then processed and amplified. This allowed reception of a much broader frequency range, in particular providing superior reception of signals higher than 1500 kilohertz.

Armstrong described this new tuning method as employing the "production of a superaudible frequency by heterodyning", which was soon contracted to "superheterodyne" or, more informally, "superhet". The February, 1921 issue of the Proceedings of the Institute of Radio Engineers published his December 3, 1919 review of A New System of Short Wave Amplification, which introduced the concept to the American engineering community. (The U.S. Patent Office eventually assigned primary credit for the idea to a French engineer, Lucian LÚvy.) Although the basic concept was straight-forward, extensive engineering work was needed before superheterodyne receivers were affordable and easy enough to use by the general public, and commercial sets would not be widely available in the United States until the Radio Corporation of America began offering them in 1924.


On June 7, 1922, Armstrong returned to the Institute of Radio Engineers in New York City to present his latest breakthrough, an enhancement of the basic regenerative circuit that he called "super-regeneration". Paul F. Godley's review of the promise of Armstrong's Super-Regenerative Circuit appeared in the September, 1922 Radio Broadcast magazine. However, despite the initial optimism, super-regeneration turned out to be too complicated and limited to be broadly adopted.


Early tube radio designs had a characteristic that they readily squealed if not tuned perfectly, marring reception, so Professor Louis A. Hazeltine of the Stevens Institute of Technology developed the "neutrodyne circuit" to eliminate this problem. Jack Binn's Tuned Radio Frequency and Neutrodyne, from the October, 1923 Popular Science, gave a detailed looked at the innovation. RCA initially was unwilling to licence its superheterodyne circuit to competitors, so, in the interim, a group of vendors working together as the "Independent Radio Manufacturers, Inc." licenced Hazeltine's patent to produce receivers that, although not as sophisticated as a superheterodyne, were less expensive and adequate for most customer's needs. The neutrodyne sets had a distinctive "three dial" tuning arrangement, and dropped out of favor once RCA began to allow other firms access to the superheterodyne patents.


In spite of their impressive versatility, vacuum tubes still had significant drawbacks -- they were expensive, had to be replaced when they burned out, and required heavy batteries for the relatively strong electrical currents they needed in order to operate. So some looked for a simpler, more compact device that would perform the same functions, for example, Hugo Gernsback, who, in Developing the Radiophone, from the December, 1919 Radio Amateur News, suggested that experimenters "should look for substitutes of vacuum tubes" which would be worth "a king's ransom for the patent".

One promising line of investigation was the "oscillating crystal". Since 1906, crystals had been used as rectifiers for simple detectors, although even in this basic role they were often temperamental and prone to getting out of adjustment when jarred. And to fully replace vacuum tubes, oscillating crystals would have to work in circuits that produced steady electrical oscillations, for amplifying signals and producing continuous-wave transmissions. There were tantalizing hints that some day this might be possible. In March, 1920, QST magazine included a short note in the Strays column about a simple, low-power oscillating crystal circuit designed by Greenleaf W. Pickard. (Pickard later noted that the basic idea dated back to work done by Dr. W. H. Eccles in 1910). In the August 12, 1922 Radio Digest, Crystal Set Amplifies reviewed C. E. Butterfield's development of a circuit design that used three crystal detectors to (slightly) boost received signals.

Then, in late 1924, there was a brief flurry of excitement, as Hugo Gernsback announced A Sensational Radio Invention in the September, 1924 Radio News, proclaiming that Russian O. V. Lossev's work on oscillating crystal circuits meant that "The crystal now actually replaces the vacuum tube". The same issue of the magazine included an article with simple construction projects, to provide hobbyists a chance to experiment with The Crystodyne Principle. Gernsback predicted that, although development was admittedly still in the experimental stages, within three-to-five years receivers using oscillating crystals in place of tubes would go on sale to the general public. However, it turned out that for the next few decades radio enthusiasts would have to make do with incremental improvements in vacuum tube design -- tubes that required less current, lasted longer, and could run on household electrical current instead of storage batteries. It was only when a deeper knowledge of solid state physics made it possible to refine oscillating crystals into much more practical and reliable "transfer resistors" (transistors) that, beginning in the mid-1950s, the lightweight radios running on flashlight batteries envisioned by Gernsback finally became available to the general public. (In its October, 1948 issue, QST magazine reviewed the development of the transistor by Bell Laboratories, which included the construction of a simple superheterodyne receiver, in The "Transistor"--an Amplifying Crystal, noting that "These clever little devices are well worth keeping an eye on.")

"There has arisen within the last few year a new and important type of sustained radio frequency generator, namely, the hot cathode vacuum rectifier, usually with three internal electrodes. As will appear, the ease and certainty of control of circuits formed by pure electron streams in a vacuum has rendered these devices suitable not only for use as generators, but also amenable to telephonic modulation and control of radio frequency output. Since the mode of action of the devices described here is still, in many cases, under judicial consideration in the courts of this country, we shall confine ourselves to giving without comment the explanations advanced by various investigators."--Alfred N. Goldsmith, Radio Telephony, 1918.