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Note: numerous references for further information appear at the end of this article.
The vacuum tube, a seemingly simple device born from the humble incandescent light bulb, was the cornerstone of the electronic age for the better part of the 20th century. Its history is a tale of scientific curiosity, incremental improvements, and a revolutionary impact on communication, computing, and everyday life. From its origins as a two-electrode rectifier to its reign as the dominant technology for amplification and switching, and its eventual supersession by the transistor, the vacuum tube’s story is a fundamental chapter in the history of technology.
The Dawn of Thermionic Emission and the Fleming Valve
The story of the vacuum tube begins with Thomas Edison. In 1883, while experimenting with his light bulbs to prevent the blackening of the glass, he observed a curious phenomenon. He placed a metal plate inside a bulb near the filament. He found that if the plate was connected to the positive terminal of a power source, a current would flow from the hot filament to the plate, but not if the plate was connected to the negative terminal. This one-way flow of electricity in a vacuum, known as the "Edison effect" or thermionic emission, was a scientific curiosity at the time, but Edison did not find a practical application for it.
Two decades later, English physicist Sir John Ambrose Fleming, who had previously worked for Edison's company, revisited the Edison effect. Working for the Marconi Company, he was searching for a better detector for radio waves, which were then being used for wireless telegraphy. He realized that the one-way current flow in a vacuum could be used to convert the alternating current (AC) of a radio signal into direct current (DC), effectively "rectifying" the signal.
In 1904, Fleming patented his "oscillation valve," later known as the Fleming valve or thermionic diode. The device was essentially an evacuated glass bulb containing a heated filament (the cathode) and a metal plate (the anode). The hot filament, when heated by a current, would emit electrons. These electrons, being negatively charged, would be attracted to the positively charged plate, creating a current. If the plate was negatively charged, however, the electrons would be repelled, and no current would flow. This simple, two-electrode tube was a significant breakthrough, marking the birth of modern electronics and providing the first practical electronic detector for radio signals.
The Audion and the Age of Amplification
Fleming's diode was a rectifier, but it could not amplify a signal. The next major leap in vacuum tube technology came from American inventor Lee de Forest. In 1906, he took Fleming's diode and added a third electrode—a zigzag wire mesh he called a "grid"—between the cathode and the anode. He named his invention the "Audion."
The grid was the key to the Audion's revolutionary capabilities. By applying a small voltage to the grid, de Forest discovered he could control the much larger flow of electrons between the cathode and the anode. A small change in the grid voltage would result in a large change in the anode current. This meant the Audion could take a weak electrical signal and make it much stronger—it was the first device capable of electronic amplification.
This ability to amplify signals transformed the world. It made long-distance telephony practical by allowing signals to be boosted along telephone lines. It paved the way for broadcast radio, enabling weak radio waves to be amplified into strong signals that could drive a loudspeaker. Sound recording and reproduction, television, and radar all owe their existence to the principle of amplification first realized in de Forest's Audion, which became known as the triode (for its three electrodes).
The Evolution and Proliferation of Vacuum Tubes
The invention of the triode kicked off a rapid period of innovation and development. Scientists and engineers quickly recognized the potential of adding more grids to the tube to improve its performance.
* The Tetrode (Four-electrode tube): In 1919, Walter Schottky invented the tetrode by adding a second grid, called a "screen grid," between the control grid and the anode. This reduced unwanted capacitance, or the "Miller effect," which had limited the triode's high-frequency performance and caused instability. The tetrode was a more efficient and stable amplifier, opening the door for higher-frequency applications.
* The Pentode (Five-electrode tube): In 1926, Bernard Tellegen and Gilles Holst of Philips added a third grid, called a "suppressor grid," to the tetrode. This further improved performance by preventing secondary electrons from the anode from being attracted to the screen grid. The pentode became the workhorse of the electronics industry, offering high amplification, efficiency, and stability, and was widely used in radios, televisions, and audio amplifiers.
By the "Golden Age" of vacuum tubes, which stretched from the 1920s to the 1950s, a vast variety of tubes were developed for specialized purposes. Magnetrons and klystrons, for example, were developed for high-frequency applications like radar and microwave ovens. The cathode ray tube (CRT), another type of vacuum tube, became the display technology for oscilloscopes and, most famously, for televisions and early computer monitors.
Vacuum Tubes and the Dawn of Computing
Before the invention of the transistor, vacuum tubes were the only practical devices for building electronic computers. While mechanical and electromechanical computers existed, they were slow. The ability of vacuum tubes to act as high-speed switches, turning an electrical current on and off thousands of times per second, was a game-changer.
The first large-scale electronic computer, the ENIAC (Electronic Numerical Integrator and Computer), completed in 1945, was a testament to the power and limitations of vacuum tubes. It contained over 17,000 vacuum tubes, weighed 30 tons, and consumed 150 kilowatts of power. Its tubes frequently burned out, requiring constant maintenance and limiting its reliability. Other early computers like the Colossus and the UNIVAC also relied heavily on vacuum tubes. The tubes made these machines incredibly fast for their time, but also large, hot, and prone to failure.
The Transistor Revolution and the Decline of the Tube
The limitations of vacuum tubes—their large size, fragility, high power consumption, and limited lifespan—pushed researchers to find a solid-state alternative. This quest led to one of the most important inventions of the 20th century: the transistor.
In 1947, at Bell Laboratories, John Bardeen, Walter Brattain, and William Shockley invented the point-contact transistor. The transistor, a tiny solid-state device made from a semiconductor like germanium, could perform the same amplifying and switching functions as a vacuum tube. But it was smaller, more reliable, consumed far less power, and generated almost no heat. The invention of the transistor, and the subsequent development of the bipolar junction transistor and the integrated circuit, spelled the end for the vacuum tube's dominance in most applications.
Beginning in the 1960s, transistors began to replace vacuum tubes in radios, televisions, and computers. The shift was dramatic and rapid. The development of integrated circuits, which packed millions of transistors onto a single chip, made modern microprocessors and personal computers possible, something that would have been unimaginable with vacuum tubes.
The Legacy and Enduring Niche of Vacuum Tubes
While the transistor relegated the vacuum tube to a technological relic in most fields, tubes have not disappeared entirely. They retain a loyal following and a niche presence in several areas, largely due to their unique sonic properties.
* Audio Amplifiers: Many audiophiles and musicians prefer the sound of vacuum tube amplifiers, particularly for electric guitars. They argue that tube amplifiers produce a "warmer" and more harmonically rich sound than their solid-state counterparts. The distortion characteristics of tubes, which are generally considered "musical," are highly sought after by guitarists and high-fidelity audio enthusiasts.
* High-Power Radio Frequency Applications: In certain specialized, high-power applications, such as radio and television broadcast transmitters, particle accelerators, and some types of radar, vacuum tubes are still used. They can handle higher power and voltage levels than solid-state devices and are more resistant to electromagnetic pulses, making them suitable for military and industrial use.
* Scientific Instruments: High-power tubes are still used in some scientific instruments, such as electron microscopes and certain types of medical imaging equipment.
The history of the vacuum tube is a powerful reminder of how a seemingly simple invention can set off a chain reaction of innovation that fundamentally changes the world. From its origins in Edison's lab to its role as the driving force behind the electronic age, the vacuum tube's journey from a scientific curiosity to a technological workhorse and, finally, to a specialized niche product, is a compelling narrative of human ingenuity and progress.
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Volumes have been written about the vacuum tube. The following references will direct the reader to numerous articles appearing in popular magazines. Most of these magazines are readily available online free of charge at worldradiohistoy.com
The 23 part “Saga of the Vacuum Tube," by Gerald Tyne, appeared in Radio News magazine. Part 1 was printed in March 1943. While the numbers only go to 22, chapter 9 was accidentally used twice, making for a total of 23 chapters.
In 1977, these articles were combined into a book. A PDF is attached below.
Articles related to the vacuum tube
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Radio News, May 1930, p. 991. ("Evolution of the Vacuum Tube")
Radio News, Mar 1934, p. 520. ("Lilliput" metal tubes)
Radio Craft, May 1935, p. 646. ("Metal Tubes in the USA?")
Radio Craft, Jun 1935, p. 726. ("Now–Metal Tubes")
Radio Craft, Oct 1935, p. 197. (editorial on metal tubes by Hugo Gernsback)
Radio Craft, Oct 1935, p. 202. ("An Inside Story About Metal Tubes")
Radio Craft, Oct 1935, p. 228. (a chart of the first ten metal tubes)
Radio Craft, Nov 1935, p. 284. ("How Metal Tubes are Made")
Radio News, Dec 1940, p. 8. ("The History of the Radio Tube 1900–1906")
Radio News, Mar 1943, p. 25. ("The Saga of the Vacuum Tube, part 1")
Radio News, Apr 1943, p. 54. ("The Saga of the Vacuum Tube, part 2")
Radio News, May 1943, p. 26. ("The Saga of the Vacuum Tube, part 3")
Radio News, Jul 1943, p. 26. ("The Saga of the Vacuum Tube, part 4")
Radio News, Aug 1943, p. 26. ("The Saga of the Vacuum Tube, part 5")
Radio News, Sep 1943, p. 26. ("The Saga of the Vacuum Tube, part 6")
Radio News, Oct 1943, p. 26. ("The Saga of the Vacuum Tube, part 7")
Radio News, Nov 1943, p. 26. ("The Saga of the Vacuum Tube, part 8")
Radio News, Dec 1943, p. 30. ("The Saga of the Vacuum Tube, part 9")
Radio News, Jan 1944, p. 38. ("The Saga of the Vacuum Tube, part 9") (incorrectly titled as part 9)
Radio News, Mar 1944, p. 50. ("The Saga of the Vacuum Tube, part 10")
Radio News, Apr 1944, p. 54. ("The Saga of the Vacuum Tube, part 11")
Radio News, Jun 1944, p. 52. ("The Saga of the Vacuum Tube, part 12")
Radio News, Sep 1944, p. 46. ("The Saga of the Vacuum Tube, part 13")
Radio News, Nov 1944, p. 56. ("The Saga of the Vacuum Tube, part 14")
Radio News, Jan 1945, p. 54. ("The Saga of the Vacuum Tube, part 15")
Radio News, Mar 1945, p. 52. ("The Saga of the Vacuum Tube, part 16")
Radio News, May 1945, p. 58. ("The Saga of the Vacuum Tube, part 17")
Radio News, Jul 1945, p. 56. ("The Saga of the Vacuum Tube, part 18")
Radio News, Sep 1945, p. 54. ("The Saga of the Vacuum Tube, part 19")
Radio News, Nov 1945, p. 51. ("The Saga of the Vacuum Tube, part 20")
Radio News, Feb 1946, p. 54. ("The Saga of the Vacuum Tube, part 21")
Radio News, Apr 1946, p. 52. ("The Saga of the Vacuum Tube, part 22")
Radio News, Nov 1943, p. 50. ("Electron Tubes")
Radio Craft, Jan 1947. (special issue on Lee de Forest, and 40th year of the vacuum tube)
Electronics World, Oct 1960, p. 48. ("Compactrons")
Electronics World, Dec 1964, p. 42. ("Early Vacuum Tubes")
