The Audion - the first triode
fig.1 the proto-Audion
fig.2 the Audion
The Audion is an electronic amplifying tube invented by the American engineer Lee De Forest in 1906. It is the first form
of triode consisting of a glass tube under partial vacuum containing three electrodes: a heated filament, a grid, and a plate.
The first component capable of amplifying a small electrical signal applied to the grid, it can control a larger current
flowing from the filament to the plate. Unlike the vacuum tubes that would succeed it, the primitive Audion tube therefore contained a small
amount of gas, deemed necessary by De Forest.
This "bad idea" had the effect of limiting the frequency operating range and giving it a highly non-linear character, as well as erratic characteristics. Comparable to a Fleming valve equipped with a grid, it is proposed by Lee Forest as a radio receiver detector with mixed success. Its amplification capability was discovered by several researchers around 1912. It was then used to build the first amplifying receivers and electronic oscillators of radios. The Audion was replaced after a few years by an improved version with high vacuum, the triode, developed by Irving Langmuir of General Electric, and then others.
This "bad idea" had the effect of limiting the frequency operating range and giving it a highly non-linear character, as well as erratic characteristics. Comparable to a Fleming valve equipped with a grid, it is proposed by Lee Forest as a radio receiver detector with mixed success. Its amplification capability was discovered by several researchers around 1912. It was then used to build the first amplifying receivers and electronic oscillators of radios. The Audion was replaced after a few years by an improved version with high vacuum, the triode, developed by Irving Langmuir of General Electric, and then others.
It had been known since the mid-19th century that gas flames were electrically conductive. Later, it was observed that this
conductivity is affected by radio waves. De Forest concluded that the gas in a partial vacuum heated by a filament of the classic lamp
behaved in the same way. And therefore that by winding a wire around the glass tube (fig.1), the device could
serve as a radio signal detector. In its first version, the tube was equipped with a heating filament and a polarized plate
with a 22V battery placed in series with headphones. The negative pole being connected to one side of the lamp filament. When
RF signals are applied to the wire wound around the outside of the glass tube, they disturb the current passing through the headphones.
Later, the wire wound around the tube passed inside it. It would be called the grid. The sensitivity of the detector was significantly increased.
In radio systems based on older detectors (galena, ...), the power allowing the headphones to vibrate was drawn from the RF carrier wave. For the first time, this energy came from the battery. This allows the station tuning circuit to operate with maximum selectivity.
The commercial stakes were high, and a patent race began.
Later, the wire wound around the tube passed inside it. It would be called the grid. The sensitivity of the detector was significantly increased.
In radio systems based on older detectors (galena, ...), the power allowing the headphones to vibrate was drawn from the RF carrier wave. For the first time, this energy came from the battery. This allows the station tuning circuit to operate with maximum selectivity.
The commercial stakes were high, and a patent race began.
The first triodes
As well as De Forest himself, many researchers attempted to find ways to improve the reliability of the device
stabilizing the low pressure of Audion tubes. Much of this research that led to the development of
triodes was carried out by Irving Langmuir for the General Electric Research Laboratories.
One of the main weaknesses of De Forest's claims was that vacuum triodes cannot function without a trace of gas left in the envelope. Another weakness is that in none of his Audion schematics is a grid biasing device mentioned, an essential characteristic for the implementation of a vacuum triode.
Unlike the Audion, the vacuum triode could not demodulate radio signals directly (although Langmuir and other researchers quickly found ways to do so), but it was capable of linear amplification, which proved to be a much more useful property. It is ironic to note that many 'defective' Audions, which had lost their ability to demodulate radio signals due to the absorption of gas by the electrodes, had in fact been transformed into crude linear amplifiers, but no one understood this at first.
One of the main weaknesses of De Forest's claims was that vacuum triodes cannot function without a trace of gas left in the envelope. Another weakness is that in none of his Audion schematics is a grid biasing device mentioned, an essential characteristic for the implementation of a vacuum triode.
Unlike the Audion, the vacuum triode could not demodulate radio signals directly (although Langmuir and other researchers quickly found ways to do so), but it was capable of linear amplification, which proved to be a much more useful property. It is ironic to note that many 'defective' Audions, which had lost their ability to demodulate radio signals due to the absorption of gas by the electrodes, had in fact been transformed into crude linear amplifiers, but no one understood this at first.
Sources and references
[1] Paul BERCHE, "Pratique et théorie de la TSF", Librairie de la Radio, Paris, 1937, reviewed by Roger RAFFIN, 1958.
[2] Lucien CHRETIEN, "Théorie et Pratique de la Radioélectricité", Editions Chiron, Paris, 1933.
[3] F. E. TERMAN, "Radio Engineer's Handbook", McGraw-Hill, New York, 1943.
[4] M. ADAM, "La lampe radio", Librairie de la Radio, Paris, 1943.
[5] Mike Schultz, Site UV201
[6] The Electrical Experimenter magazine; Aug. 1916 (p255 et 288) The Vacuum Detector and How It Works
[7] Scientific American Supplement No. 1665, November 30, 1907, pages 348-350: Lee Forest
[8] Scientific American Supplement No. 1666, December 7, 1907, page 354-356: Lee Forest
