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| In ], a '''vacuum tube''' like a ] is generally used for ] of a ]. Once used in most electronic devices, vacuum tubes are now used only in specialized applications. For most purposes, especially use as a ] in a ], ] and discrete ] are less expensive, much smaller, longer-lasting and rugged. | In ], a '''vacuum tube''' like a ] is generally used for ] of a ]. Once used in most electronic devices, vacuum tubes are now used only in specialized applications. For most purposes, especially use as a ] in a ], ] and discrete ] are less expensive, much smaller, longer-lasting and more rugged. | ||
| Vacuum tubes, or '''tubes''' or '''thermionic valves''' are arrangements of electrodes surrounded by ] within an envelope. Although the envelope was classically glass, power tubes often use ceramics, and military tubes often use metal. | Vacuum tubes, or '''tubes''' or '''thermionic valves''' are arrangements of electrodes surrounded by ] within an envelope. Although the envelope was classically glass, power tubes often use ceramics, and military tubes often use metal. | ||
Revision as of 01:50, 1 March 2003
In electronics, a vacuum tube like a transistor is generally used for amplification of a signal. Once used in most electronic devices, vacuum tubes are now used only in specialized applications. For most purposes, especially use as a switch in a computer, integrated circuits and discrete semiconductor devices are less expensive, much smaller, longer-lasting and more rugged.
Vacuum tubes, or tubes or thermionic valves are arrangements of electrodes surrounded by vacuum within an envelope. Although the envelope was classically glass, power tubes often use ceramics, and military tubes often use metal.
Tubes resemble incandescent light bulbs. Tubes have a filament heated by an electric current. When hot, the filament releases electrons into the vacuum. These electrons are electrostatically drawn to a positively charged outer metal plate called the anode, or more commonly just the plate. Electrons do not flow from the plate back toward the filament, even if the charge on the plate is made negative, because the plate is not heated. The arrangement of a filament and plate is called a diode and was invented in 1904 by John Ambrose Fleming, scientific adviser to the Marconi company, based on an observation by Thomas Edison.
Diode
The next innovation, due to Lee DeForest in 1907, was to place another electrode, the grid, between the filament and plate. The grid is a bent wire or screen. De Forest discovered that the current flow from filament to plate depended on the voltage of the grid, and that the current drawn by the grid was very low. The resulting three-electrode device was therefore an excellent amplifier. DeForest called his invention the audion, but it is better known as a triode. The valve equivalent of a transistor, triodes were used in early valve amplifiers.
Triode
The non-linear operating characteristic of the triode gave early valve audio amplifiers a distortion that became known as the valve sound.
Many further innovations followed. It became common to use the filament to heat a separate electrode called the cathode, and to use the cathode as the source of electron flow in the tube rather than the filament itself.
When triodes were first used in radio transmitters and receivers, it was found that they were often unstable and had a tendency to oscillate due to anode to grid capacitance. Many complex circuits were developed to reduce this problem (e.g. the Neutrodyne amplifier), but the addition of a second grid biased near the anode voltage but bypassed to ground with a capacitor decoupled the anode and the first grid, completely eliminating the oscillation problem. This two-grid tube is called a tetrode, meaning four electrodes.
However the tetrode too had a problem, especially in higher-current applications. The positive voltage on the second grid accelerated the electrons, causing them to strike the anode hard enough to knock out secondary electrons which tended to be captured by the second grid, reducing the anode current and the amplification of the circuit. Again the solution was to add another grid. This third grid was biased at either ground or cathode voltage and its negative voltage (relative to the anode) suppressed the secondary electrons by repelling them back toward the anode. This three-grid tube is called a pentode, meaning five electrodes.
Tubes with 4, 5, 6, or 7 grids, called hexodes, heptodes, octodes, and nonodes, were generally used for frequency conversion in superheterodyne receivers. The heptode, or pentagrid converter, was the most common of these. 6BE6 is an example of a heptode.
It was common practice in some tube types (e.g. the Compactron) to include more than one group of elements in one bulb. For instance, type 6SN7 is a "dual triode" which, for most purposes, can perform the functions of two triode tubes, while taking up half as much space and costing less.
The beam power tube is usually a tetrode with the addition of "beam forming plates". These plates direct the electron stream to certain spots on the anode, and thus overcome some design barriers to designing high-power, high-performance power tubes. 6L6 is a beam power tube.
The chief reliability problem of a tube is that the filament burns out. To increase life, tube designers try to run filaments at lower temperatures. To encourage electron emission at lower temperatures, filaments are coated, usually with thorium. To meet the reliability requirements of the air defense computer system SAGE, it was necessary to build special "computer vacuum tubes" with extended filament life. The problem of early filament burnout was traced to evaporation of silicon used in the tungsten alloy to make the wire easier to draw. Elimination of the silicon from the filament wire alloy (and paying extra for more frequent replacement of the wire drawing dies) allowed production of tubes that met the reliability requirements of SAGE.
Another important reliability problem is that the tube fails when air leaks into the tube. Usually the air reacts chemically with the hot filament. Designers therefore worked hard to develop tube designs that sealed reliably. This was much of the reason why many tubes were constructed of glass. Metal alloys and glasses had been developed for light bulbs that expanded and contracted the same amounts when hot. These made it easy to construct an insulating envelope of glass, and pass wires through the glass to the electrodes and filament.
Tubes usually have glass envelopes, but metal and ceramic are possible choices. The nuvistor is a tiny tube made only of metal and ceramic. In some tubes, the metal envelope is also the anode. 4CX800 is an external anode tube of this sort.
Near the end of World War II, to make radios more rugged, some aircraft and army radios began to integrate the tube envelopes into the radio's molded aluminum or zinc chassis. The radio became just a printed circuit, with non-tube components, that was soldered to the chassis that contained all the tubes.
Tubes were ubiquitous in the early generations of electronic devices, such as radios, televisions, and early computers. They are still used for specialised audio amplifiers, notably for electric guitar amplification, and for very high-powered applications such as microwave ovens and signal amplification for broadcast radio.
Other vacuum tube electronic devices include the magnetron, klystron, traveling wave tube and cathode ray tube. The magnetron is the most common type of tube in microwave ovens.
Other tube devices
Specialist low-pressure gas-filled tube devices include the Nixie tube and the dekatron. These are used to display numerals.
One of the proposed designs for a fusion reactor is basically a tube, the Farnsworth-Hirsch Fusor.
See also: Irving Langmuir
External links and References
- http://www.marconicalling.com/museum/html/events/events-i=39-s=0.html
- Plenty of interesting information about vacuum tubes at http://www.svetlana.com/docs/tubeworks.html.
- A lot of very interesting technical information about vacuum tubes, with PDF files from old books in both English and German, with an outstanding theoretical discussion can be found at http://www.radau5.ch/valves.html. The difference between the American and the German techniques is interesting. The American technique usually uses the gain as central parameter in the calculation. The German technique uses the transparency (durchgriff) as the central parameter, which is a little bit more abstract but since the transparency is the most constant of all the parameters of the tube, it makes calculations more predictable and more precise.