In
electronics, a
vacuum tube,
electron tube (in North America), or
thermionic valve (elsewhere, especially in Britain) is a device used to
amplify,
switch, otherwise modify, or create an electrical
signal by controlling the movement of
electrons in a low-pressure space. Some special function vacuum tubes are filled with low-pressure gas: these are so-called soft tubes as distinct from the hard
vacuum type which have the internal gas pressure reduced as far as possible. Almost all tubes depend on the
thermionic emission of electrons.
Vacuum tubes were critical to the development of electronic technology, which drove the expansion and commercialization of
radio broadcasting,
television,
radar,
sound reproduction, large
telephone networks, analog and digital
computers, and industrial
process control. Some of these applications pre-dated electronics, but it was the vacuum tube that made them widespread and practical.
For most purposes, the vacuum tube has been replaced by
solid-state devices such as
transistors and solid-state
diodes. Solid-state devices last much longer, are smaller, more efficient, more reliable, and cheaper than equivalent vacuum tube devices. However, tubes are still used in specialized applications: for engineering reasons, as replacements in older, non-solid-state, high-power radio frequency transmitters; or for their aesthetic appeal and distinct sound signature, as in
audio amplification.
Cathode ray tubes until very recently were the primary display devices in
television sets,
video monitors, and
oscilloscopes, although they are now being replaced by
LCDs and other
flat-panel displays. A specialized form of the electron tube, the
magnetron, is the source of microwave energy in
microwave ovens and some radar systems. The
klystron, a powerful but narrow-band radio-frequency amplifier, is commonly deployed by broadcasters as a high-power
UHF television transmitter.
Different types of vacuum tubes
At the time Thomson was doing his research, inventor Thomas A. Edison had observed a bluish glow in some of the early light bulbs under certain conditions. he found that a current would pass from the cathode to the anode. Further experiments done by the English engineer John Ambrose Fleming and Thomson's students revealed that the so called Edison effect was the result of the emission of electrons from the cathode.
The 19th century saw increasing research with evacuated tubes, such as the
Geissler and
Crookes tubes. Scientists who experimented with such tubes included
Thomas Edison,
Eugen Goldstein,
Nikola Tesla,
Johann Wilhelm Hittorf and many others. These tubes were mostly for specialized scientific applications, or were novelties, with the exception of the
light bulb. The groundwork laid by these scientists and inventors, however, was critical to the development of vacuum tube technology.
Though the
thermionic emission effect was originally reported in 1873 by
Frederick Guthrie, it is Thomas Edison's 1884 investigation of the
Edison Effect that is more often mentioned. Edison patented what he found,
[1] but he did not understand the underlying physics, or the potential value of the discovery. It wasn't until the early 20th century that this effect was put to use, in applications such as
John Ambrose Fleming's
diode used as a radio detector, and
Lee De Forest's 1906 "
audion" (soon improved by others as the
triode in 1908) used in the first telephone amplifiers. These developments led to great improvements in telecommunications technology, particularly the first coast-to-coast telephone line in the US, and the birth of
broadcast radio.
Diodes and Triodes
The English physicist
John Ambrose Fleming worked as an engineering consultant for firms, including Edison Telephone and the
Marconi Company. In 1904, as a result of experiments conducted on Edison effect bulbs imported from the USA, he developed a device he called an "oscillation valve" (because it passes current in only one direction).
In 1906
Robert von Lieben filed
[2] for a three-electrode amplifying vacuum tube. His invention also included a beam-focusing electromagnet.
In 1907
Lee De Forest placed a bent wire serving as a screen, later known as the
"grid" electrode, between the filament and
plate electrode. As the voltage applied to the grid was varied from negative to positive, the number of electrons flowing from the filament to the plate would vary accordingly. Thus the grid was said to electrostatically "control" the plate current. The resulting three-electrode device was an excellent sensitive
amplifier of voltages. De Forest called his invention the "
Audion". In 1907, De Forest filed
[3] for a three-electrode version of the Audion for use in radio communications. The device is now known as the
triode. De Forest's device was not strictly a vacuum tube, but clearly depended for its action on ionisation of the relatively high levels of gas remaining after evacuation. The De Forest company, in its Audion leaflets, warned against operation which might cause the vacuum to become too hard. The Finnish inventor
Eric Tigerstedtsound-on-film process in Berlin, Germany. The first true vacuum triodes were the Pliotrons developed by
Irving Langmuir at the
General Electric research laboratory (
Schenectady, New York) in 1915. Langmuir was one of the first scientists to realize that a harder vacuum would improve the amplifying behaviour of the triode. Pliotrons were closely followed by the French 'R' Type which was in widespread use by the allied military by 1916. These two types were the first true
vacuum tubes. Historically, vacuum levels in production vacuum tubes typically ranged between 10
µPa to 10 nPa. significantly improved on the original triode design in 1914, while working on his
The non-linear operating characteristic of the triode caused early tube audio amplifiers to exhibit harmonic distortions at low volumes. This is not to be confused with the
overdrive that tube amplifiers exhibit at high volume levels (known as the
tube sound). To remedy the low-volume distortion problem, engineers plotted curves of the applied grid voltage and resulting plate currents, and discovered that there was a range of relatively linear operation. In order to use this range, a negative voltage had to be applied to the grid to place the tube in the "middle" of the linear area with no signal applied. This was called the idle condition, and the plate current at this point the "idle current". Today this current would be called the quiescent or
standing current. The controlling voltage was superimposed onto this fixed voltage, resulting in linear swings of plate current for both positive and negative swings of the input voltage. This concept was called
grid bias.
Tetrodes and pentodes
A two-tube homemade radio from 1958. The tubes are the two columns with the dark tops. The flying leads connect to the low-voltage filament and high-voltage anode supplies.
When triodes were first used in radio transmitters and receivers, it was found that they had a tendency to oscillate due to parasitic anode-to-grid capacitance. Many circuits were developed to reduce this problem (e.g. the
Neutrodyne amplifier), but proved unsatisfactory over wide ranges of frequencies. It was discovered that the addition of a second grid, located between the control grid and the plate and called a
screen gridMiller capacitance is also reduced, which improves gain at high frequency. This two-grid tube is called a
tetrode, meaning four active electrodes could solve these problems. ("Screen" implies shielding, not physical construction.) A positive voltage slightly lower than the plate voltage was applied to it, and the screen grid was bypassed (for high frequencies) to ground with a capacitor. This arrangement decoupled the anode and the first grid, completely eliminating the oscillation problem. An additional side effect of this second grid is that the , and was common by 1926.
) is used to show the EOR operation. 
Example
