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.
Tubes were heavily used in the early generations of electronic devices, such as radios, televisions, and early computers such as the Colossus which used 2000 tubes, the ENIAC which used nearly 18,000 tubes, and the IBM 700 series.
For most purposes, the vacuum tube has been replaced by solid-state devices such as transistors and solid-state diodes.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 in high-power radio frequency transmitters; or for their aesthetic appeal and distinct sound signature, as in audio amplification. Cathode ray tubes are still used as display devices in television sets, video monitors, and oscilloscopes, although they are 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.
Vacuum tubes are less susceptible than corresponding solid-state components to the electromagnetic pulse effect of nuclear explosions. This property kept them in use for certain military applications long after transistors had replaced them elsewhere. Vacuum tubes are still used for very high-powered applications such as industrial radio-frequency heating, generating large amounts of RF energy for particle accelerators, and power amplification for broadcasting. In microwave ovens, cost-engineered magnetrons efficiently generate microwave power on the order of hundreds of watts.
Many audiophiles, professional audio engineers, and musicians prefer the tube sound of audio equipment based on vacuum tubes over electronics based on transistors. There are companies which still make specialized audio hardware featuring tube technology. A common usage is in the high-end microphone preamplifiers preferred by professional music recording studios, and in electric guitar amplification. The sound produced by a tube based amplifier with the tubes overloaded (overdriven) has defined the texture of some genres of music such as classic rock and blues. Guitarists often prefer tube amplifiers for the warmth of their tone and the natural compression effect they can apply to an input signal.
Cathode-ray tubes (CRTs) are a highly-evolved type of vacuum tube, described elsewhere.
In 2002, computer motherboard maker AOpen brought back the vacuum tube for modern computer use by releasing the AX4GE Tube-G motherboard. This motherboard uses a Sovtek 6922 vacuum tube (a version of the 6DJ8) as part of AOpen’s TubeSound Technology. AOpen claims that the vacuum tube brings superior sound.
Cooling
Like any electronic device, vacuum tubes produce heat while operating. This waste heat is one of the principal factors that affect tube life [13]. The majority of this waste heat originates in the anode though some grids may also require cooling to remove excess heat. For example, the cooling of the screen grid in an EL34 is facilitated by the addition of two small radiators or "wings," located near the top of the tube. The heater (filament) also contributes to the total waste heat. A tube's data sheet will normally identify the maximum amount of heat each element may dissipate.
The method of anode cooling is dependent on the construction of the tube itself. For tubes with internal anodes such as the 12AX7 or EL34, the cooling occurs by radiating the heat by black body radiation from the anode to the glass envelope [14]. Natural air circulation, convection, then removes the heat from the envelope. Tube shields that aided heat dispersal could be retrofitted on certain select types of tubes. These shields act by improving heat conduction from the surface of the tube to the shield itself by means of tens of copper tongues in contact with the glass tube, and have an opaque, black outside finish for improved heat radiation. The ability to remove heat may be further increased by implementing forced air cooling, adding fins to the anode, and operating the anode at red hot temperatures. All of these measures are implemented in the 4-1000A transmitting tube.
The amount of heat that may be removed from a tube with an internal anode is limited [14]. Tubes with external anodes may be cooled using forced air, water, vapor, and multiphase. The 3CX10,000A7 is an example of a tube with an external anode cooled by forced air. The water, vapor, and multiphase cooling techniques all depend on the high specific heat and latent heat of water. The 8974 is an example of a water cooled tube and is among the largest commercial tube available today.
In a water cooled tube, the anode voltage appears directly on the cooling water surface, thus requiring the water to be an electrical insulator. Otherwise the high voltage can be conducted through the cooling water to the radiator system; hence the need for deionized water. Such systems usually have a built-in water-conductance monitor which will shut down the high-tension supply (often tens of kilovolts) if the conductance becomes too high.
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