Vacuum tube

In electronics, a vacuum tube is a device generally used to amplify a signal. Once used in most electronic devices, vacuum tubes are now used only in specialized applications. For most purposes, the vacuum tube has been replaced by the much smaller and less expensive transistor, either as a discrete device or in an integrated circuit. At the start of the 21st century there has been renewed interest in the vacuum tube, this time in the form of the Integrated circuit vacuum tube.

Operation

Vacuum tubes, or thermionic valves are arrangements of electrodes in a vacuum within an insulating, temperature-resistant envelope. Although the envelope was classically glass, power tubes often use ceramics, and military tubes often use glass-lined metal.

Vacuum tubes resemble incandescent light bulbs, in that they have a filament sealed in a glass envelope, which has been evacuated of all air. When hot, the filament releases electrons into the vacuum, a process called thermionic emission. The resulting negatively-charged cloud of electrons is called a space charge. These electrons will be drawn to a positively charged metal plate, the anode. This results in a current of electrons flowing from filament to plate.

Development

Obviously this does not work the other way round, because the plate is not heated, so we have a diode, a device that conducts current only in one direction. This was invented in 1904 by John Ambrose Fleming, scientific adviser to the Marconi company, based on an observation by Thomas Edison.

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 applied to the grid, and that the current drawn by the grid was very low, being composed of the electrons which are intercepted by the grid. As the applied voltage of the grid varied from negative to positive, the current of electrons flowing from the filament to the plate would vary correspondingly. Thus the grid was said to "control" the plate current. 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.

The non-linear operating characteristic of the triode gave early valve audio amplifiers a distortion that became known as the valve sound. To remedy this 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". 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.

Batteries were designed to provide the various voltages required. "A Batteries" provided the filament voltage. B Batteries provided the plate voltage. To this day, plate voltage is referred to as "B+". C Batteries were used to provide grid bias, although many circuits used grid leak resistors or voltage dividers to provide proper bias.

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. 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.

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 parasitic anode to grid capacitance. Many complex 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 grid could solve these problems. A positive voltage slightly lower than the plate voltage was applied, 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. This two-grid tube is called a tetrode, meaning four active electrodes.

However the tetrode too had a problem, especially in higher-current applications. At high instantaneous plate currents, the plate would become negative with respect to the screen grid. 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 plate current and the amplification of the circuit. This effect was sometimes called "tetrode kink". Again the solution was to add another grid, called a suppressor grid. This third grid was biased at either ground or cathode voltage and its negative voltage (relative to the anode) electrostatically 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 additional grids were all "control grids" with different signals applied to each one. In combination with each other, they create a single, combined effect on the plate current (and thus the signal output) of the tube circuit. 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, an 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 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 in RCA in 1936. 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.

Reliability issues

The chief reliability problem of a tube is that the filament or cathode eventually "wears out", losing its ability to emit electrons. To increase life, tube designers try to run filaments at as low a temperature as possible while still sustaining sufficient thermionic emission. Large transmitting tubes have tungsten filaments containing a small trace of thorium oxide. A thin layer of thorium atoms forms on the outside of the wire when heated, serving as an efficient source of electrons.

To encourage electron emission at lower temperatures, filaments are coated with a mixture of barium and strontium oxides. Indirectly heated cathodes are coated with a similar mixture. To meet the reliability requirements of MIT's Whirlwind computer project, which evolved into the air defense computer system SAGE, it was necessary to build special "computer vacuum tubes" with extended cathode life. The problem of short lifetime was traced to evaporation of silicon used in the tungsten alloy to make the wire easier to draw. Elimination of the silicon from the heater 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. High-purity nickel tubing and cathode coatings free of materials that can poison emission (such as silicates and aluminum) also contribute to long cathode life. The first such "computer tube" was Sylvania's 7AK7 of 1948. By the late 1950s it was routine for special-quality small-signal tubes to last for hundreds of thousands of hours, if operated conservatively.

Another important reliability problem is that the tube fails when air leaks into the tube. Usually oxygen in the air reacts chemically with the hot filament or cathode, quickly ruining it. 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 (Cunife and Fernico) 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.

It is very important that the vacuum inside the envelope be as perfect as possible. Any gas atoms remaining will be ionized at operating voltages, and will conduct electricity between the elements in an uncontrolled manner. This can lead to erratic operation or even catastrophic destruction of the tube and associated circuitry. Unabsorbed free air sometimes ionizes and becomes visible as a pink-purple glow discharge between the tube elements.

To prevent any remaining gases from remaining in a free state in the tube, modern tubes are constructed with "getters", which are usually small, circular troughs filled with metals that oxidize quickly, with barium being the most common. Once the tube envelope is evacuated and sealed, the getter is heated to a high temperature (usually by means of RF induction heating) causing the material to evaporate, adsorbing/reacting with any residual gases and usually leaving a silver-colored metallic deposit on the inside of the envelope of the tube. The getter continues to absorb any gas molecules that leak into the tube during its working life. If a tube develops a crack in the envelope, this deposit turns a white color when it reacts with atmospheric oxygen. Large transmitting and specialized tubes often use more exotic getters.

Some special-purpose tubes are intentionally constructed with various gases in the envelope. For instance, voltage regulator tubes contain various inert gasses such as argon, helium or neon, and take advantage of the fact that these gases will ionize at predictable voltages.

Tubes usually have glass envelopes, but metal, fused quartz (silica), 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. Air is blown through an array of fins attached to the anode, thus cooling it.

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. Another WWII idea was to make very small and rugged glass tubes, originally for use in radio-frequency metal detectors built into artillery shells. These proximity fuses made artillery more effective. Tiny tubes were later known as "subminiature" types. They were widely used in 1950s military and aviation electronics.

Applications

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 power amplification for broadcasting.

Other vacuum tube electronic devices include the magnetron, klystron, and cathode ray tube. The magnetron is the most common type of tube in microwave ovens. Most televisions, oscilloscopes and computer monitors use cathode ray tubes.

Other vacuum tube devices

The fluorescent displays commonly used on VCRs and automotive dashboards are actually vacuum tubes, using phosphor-coated anodes to form the display characters, and a heated filamentary cathode as an electron source. These devices are properly called "VFDs", or Vacuum-Fluorescent Displays.

A tube in which electrons move through a vacuum (or gaseous medium) within a gas-tight envelope is called an electron tube.

Integrated circuit vacuum tubes

In the early years of the 21st century there has been renewed interest in vacuum tubes, this time in the form of integrated circuits.

Their advantages include greatly enhanced robustness combined with the ability to provide high power outputs at low power consumptions. Operating on the same principles as traditional tubes, prototype devices devices have been constructed with electrodes formed using nanotubes, and by etching electrodes as hinged flaps (similar to the technology used to create the microscopic mirrors used in Digital Light Processing) that are stood upright by a magnetic field.

It's expected that such integrated tubes will be valuable in microwave devices including mobile phones, for Bluetooth and Wi-Fi transmission, in radar and for satellite communication.

See also

• Irving Langmuir
• Gas filled tube

External links and References

• http://www.marconicalling.com/museum/html/events/events-i=39-s=0.html - The invention of the thermionic valve
• http://www.svetlana.com/docs/tubeworks.html - Plenty of interesting information about vacuum tubes
• http://www.radau5.ch/valves.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. 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 transconductance (durchgriff) as the central parameter, which is a little bit more abstract but since the transconductance is the most constant of all the parameters of the tube, it makes calculations more predictable and more precise.
• http://www.milbert.com/tstxt.htm - AES paper on audible differences in sound quality between vacuum tubes and transistors.