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In type, which have the internal gas pressure reduced as far as possible. Almost all depend on the thermal emission of electrons, hence Thermionic . Vacuum tubes were the critical devices that enabled the development of electronics technology, leading to the development and commercialization of such technologies as Radio broadcasting, Television , Radar , High Fidelity Sound Reproduction , large Telephone networks, modern types of digital Computer , and industrial Process Control . Many of these technologies pre-dated electronics, but it was electronics that made them widespread and practical; analogue computers such as slide-rules have become almost extinct due to electronics. For most purposes, the vacuum tube has been replaced by s are still used as display devices in Television sets, Video Monitor s, and Oscilloscope s, although they are being replaced at various rates by LCD s and other Flat-panel Display s. A specialized form of the electron tube, the Magnetron , is the source of microwave energy in Microwave Oven s and some radar systems. EXPLANATION A vacuum tube consists of arrangements of Electrode s in a Vacuum within an insulating, temperature-resistant envelope. Although the envelope is classically glass, power tubes often use ceramic and metal. The electrodes are attached to leads which pass through the envelope via an air tight seal. On most tubes, the leads are designed to plug into a Tube Socket for easy replacement. The simplest vacuum tubes resemble from plate (anode) to the filament (cathode); this is the opposite direction to the flow of electrons (called electron current). Vacuum tubes operate primarily on the function of the heat gradient difference between the hot cathode and the cold anode. Vacuum tubes are power-inefficient due to this operational requirement, since enclosing the tube within a heat-retaining envelope of insulation would allow the entire tube to reach the same temperature, resulting electron emission from the anode that counteracts the normal one-way current flow. Because the tube requires a vacuum to operate, convection cooling of the anode is typically not possible. Instead anode cooling occurs primarily through Black-body Radiation and conduction of heat to the outer glass envelope via the anode mounting frame. A high operating temperature is typically required to start the thermal emission from the anode that maintains the temperature gradient inside the evacuated tube. Cold Cathode tubes do exist but are used primarily in lighting systems, where unidirectional power regulation is not the functional purpose of the tube. The vacuum tube is a ''voltage-controlled device'', which means that the relationship between the input and output circuits is determined by a Transconductance function. The solid-state device most closely analogous to the vacuum tube is the JFET , although the vacuum tube typically operates at far higher voltage (and power) levels than the JFET. HISTORY OF DEVELOPMENT The 19th century saw increasing research with evacuated tubes, such as the Geissler and Crookes Tube s. Scientists who experimented with such tubes included Eugen Goldstein , Nikola Tesla , Johann Wilhelm Hittorf , Thomas Edison , 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 1883 investigation of the " Edison Effect " that is more often mentioned. Edison promptly patented what he found (), but he did not understand the process. Diodes and triodes The English physicist John Ambrose Fleming worked as an engineering consultant for many technology firms of his day, including Edison Telephone; in 1904, as a result of experiments conducted on Edison Effect bulbs imported from the USA and while working as scientific adviser to the Marconi company, he developed the "oscillation valve" or kenotron, which can also be used as part of a radio wave detector. Later known as the Fleming Valve and then the Diode , it allowed Electrical Current to flow in only one direction, enabling the Rectification of alternating current. Its operation is described in greater detail in the previous section. 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 therefore an excellent and very sensitive Amplifier of voltages. DeForest called his invention the " Audion ". In 1907, DeForest filed 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 Tigerstedt significantly improved on the original triode design in 1914, while working on his Sound-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. 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 overdrive 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 ''. type ranging from 2 to 12 volts (1-6 cells) with single, double and triple cells being most common. Because these batteries produced 2 V, 4 V or 6 V, tube heaters were designed to operate at those voltages—a scheme which continues to be followed today. In portable radios, flashlight (torch) batteries were sometimes used. The "B" Batteries (in North American English) provided the plate voltage. These were generally of Dry Cell construction, containing many small 1.5 volt cells in series. They typically came in ratings of 22.5, 45, 67.5, 90 or 135 volts and were made of series-connected zinc-carbon batteries. To this day, plate voltage is referred to as B+, but only in America. The rest of the English-speaking world calls this the HT (high tension) supply or battery. Some sets used "C" Batteries (North American English) to provide grid bias, although many circuits used Grid Leak Resistor s, Voltage Divider s or Cathode Bias to provide proper tube bias. Most of the English-speaking world calls this simply the 'grid bias battery'. Direct and indirect heating Many further innovations followed. It became common to use the filament to heat a separate electrode called the cathode, and to use this cathode as the source of electron flow in the tube rather than the filament itself. This minimized the introduction of hum when the filament was energized with Alternating Current . In such tubes, the filament is called a Heater to distinguish it as an inactive element. Tetrodes and pentodes |
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