Frequency conversion can be accomplished by various methods in superheterodyne receivers. Tubes with 5 grids, called pentagrid converters, were generally used, although alternatives such as using a combination of a triode with a hexode were also used. Even octodes have been used for frequency conversion. The additional grids are either control grids, with different signals applied to each one, or screen grids. In many designs a special grid acted as a second 'leaky' plate to provide a built-in oscillator, which then coupled this signal with the incoming radio signal. These signals create a single, combined effect (equivalent to a crude analog multiplier) on the plate current (and thus the signal output) of the tube circuit. The useful component of the output was the difference frequency between that of the incoming signal and that of the oscillator.
The heptode, or pentagrid converter, was the most common of these. 6BE6 is an example of a heptode (note that the first number in the tube ID indicates the filament voltage). Octodes were rare in the US, the 7A8 was one example, but much more common in Europe particularly in battery operated radios where the lower power consumption was an advantage.
Toward the end of the tube era, precision control and screen grids, called frame grids, offered enhanced performance. Instead of the typically elliptical fine-gauge wire supported by two larger wires, a frame grid was a metal stamping with rectangular openings that surrounded the cathode. The grid wires were in a plane defined by the stamping, and the control grid was placed much closer to the cathode surface than traditional construction would permit.[7]
To reduce the cost and complexity of radio equipment, by 1940 it was common practice to combine more than one function, or more than one set of elements in the bulb of a single tube. The only constraint was where patents, and other licencing considerations required the use of multiple tubes. See British Valve Association.
For example, the RCA Type 55 was a double diode triode used as a detector, automatic gain control rectifier and audio preamp in early AC powered radios. The same set of tubes often included the 53 Dual Triode Audio Output.
Another early type of multi-section tube, the 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 12AX7 is a dual high-gain triode widely used in guitar amplifiers, audio preamps, and instruments.
The invention of the 9-pin miniature tube base, besides allowing the 12AX7 family, also allowed many other multi section tubes, such as the 6GH8 triode pentode. Along with a host of similar tubes, the 6GH8 was quite popular in television receivers. Some color TV sets used exotic types like the 6JH8 which had two plates and beam deflection electrodes (it was known as the 'sheet beam' tube). Vacuum tubes used like this were designed for demodulation of synchronous signals, an example of which is color demodulation for television receivers.
The desire to include many functions in one envelope resulted in the General Electric Compactron. A typical unit, the 6AG11 Compactron tube contained two triodes and two diodes, but many in the series had triple triodes.
An early example of multiple devices in one envelope was the Loewe 3NF. This 1920s device had 3 triodes in a single glass envelope together with all the fixed capacitors and resistors required to make a complete radio receiver. As the Loewe set had only one tube socket, it was able to substantially undercut the competition since, in Germany, state tax was levied by the number of sockets. However, reliability was compromised, and production costs for the tube were much greater. In a sense, these were akin to integrated circuits. In the US, Cleartron briefly produced the "Multivalve" triple triode for use in the Emerson Baby Grand receiver. This Emerson set also had a single tube socket, but because it used a four-pin base, the additional element connections were made on a "mezzanine" platform at the top of the tube base.
Loewe were to also offer the 2NF (two tetrodes plus passive components) and the WG38 (two pentodes, a triode and the passive components).
The beam power tube is usually a tetrode with the addition of beam-forming electrodes, which take the place of the suppressor grid. These angled plates focus the electron stream onto certain spots on the anode which can withstand the heat generated by the impact of massive numbers of electrons, while also providing pentode behavior. The positioning of the elements in a beam power tube uses a design called "critical-distance geometry", which minimizes the "tetrode kink", plate-grid capacitance, screen-grid current, and secondary emission effects from the anode, thus increasing power conversion efficiency. The control grid and screen grid are also wound with the same pitch, or number of wires per inch.
Aligning the grid wires also helps to reduce screen current, which represents wasted energy. This design helps to overcome some of the practical barriers to designing high-power, high-efficiency power tubes. 6L6 was the first popular beam power tube, introduced by RCA in 1936. Corresponding tubes in Europe were the KT66, KT77 and KT88 by GEC (the KT standing for "Kinkless Tetrode").
Variations of the 6L6 design are still widely used in guitar amplifiers, making it one of the longest lived electronic device families in history. Similar design strategies are used in the construction of large ceramic power tetrodes used in radio transmitters.
Special-purpose tubes
Some special-purpose tubes are constructed with particular gases in the envelope. For instance, voltage regulator tubes contain various inert gases such as argon, helium or neon, and take advantage of the fact that these gases will ionize at predictable voltages. The thyratron is a special-purpose tube filled with low-pressure gas or mercury, some of which vaporizes. Like other tubes, it contains a hot cathode and an anode, but also a control electrode, which behaves somewhat like the grid of a triode. When the control electrode starts conduction, the gas ionizes, and the control electrode no longer can stop the current; the tube "latches" into conduction. Removing plate (anode) voltage lets the gas de-ionize, restoring its non-conductive state. Some thyratrons can carry large currents for their physical size. One example is the miniature type 2D21, often seen in 1950s jukeboxes as control switches for relays. A cold-cathode version of the thyratron, which uses a pool of mercury for its cathode, is called an Ignitron (tm). It can switch thousands of amperes in its largest versions. Thyratrons containing hydrogen have a very consistent time delay between their turn-on pulse and full conduction, and have long been used in radar transmitters. Thyratrons behave much like silicon-controlled rectifiers, or to be more chronologically precise, silicon controlled rectifiers mimic some of the behaviours of Thyratrons.
Tubes usually have glass envelopes, but metal, fused quartz (silica), and ceramic are possible choices. The first version of the 6L6 used a metal envelope sealed with glass beads, while a glass disk fused to the metal was used in later versions. Metal and ceramic are used almost exclusively for power tubes above 2 kW dissipation. The nuvistor is a tiny tube made only of metal and ceramic. In some power tubes, the metal envelope is also the anode. The 4CX1000A is an external anode tube of this sort. Air is blown through an array of fins attached to the anode, thus cooling it. Power tubes using this cooling scheme are available up to 150 kW dissipation. Above that level, water or water-vapor cooling are used. The highest-power tube currently available is the Eimac 4CM2500KG, a forced water-cooled power tetrode capable of dissipating 2.5 megawatts. (By comparison, the largest power transistor can only dissipate about 1 kilowatt.) Another very high power tube is the Eimac 8974, a 1.25 megawatt tetrode used in military and commercial radio-frequency installations.
An extremely specialized tube is the Krytron, which is used for extremely precise, rapid high-voltage switching. Due to their intended purpose, the initiation of the precise sequence of detonations used to set off a nuclear weapon, they are heavily controlled at an international level.
Medical Imaging Equipment, such as Radiographic and Nuclear Imaging, use special vacuum tubes. Radiographic, Fluoroscopic, and CT X-ray imaging equipment use a specially designed vacuum tube diode, which has a rotating anode to dissipate the large amounts of heat developed during operation, and a focused cathode. They are housed in an aluminum housing which is filled with a dielectric oil. Nuclear Imaging Equipment use Photomultiplier Tube arrays to detect radiation.
Miniature tubes
The miniature vacuum tube made tubes smaller by eliminating the Bakelite base. It was invented in 1938.[8] Instead of the separate base, the pins are fused in the glass base of the envelope. This forces the sealing tip to the top of the envelope. Making tubes smaller reduced the voltage that they could work at, and also the power of the filament, so the older style continued to be used for high power rectifiers, valve amplifier output stages and certain transmitting tubes. Miniature tubes with a size roughly that of half a cigarette were used in hearing-aid amplifiers.
Development continued, and led to the sub miniature tubes and the "acorn" valve (named due to its shape).
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