Vacuum tubes and valves explained

Introduction

In electronics, a vacuum tube (American English) or (thermionic) valve (British English) 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 For most purposes, the vacuum tube has been replaced by the much smaller and less expensive transistor, either as a discrete device or in an integrated circuit.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 Field-emitter microtube.

Advertisement

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 ceramic and metal. The electrodes were attached to leads which passed through the envelope, in a way that was sealed air tight. On most tubes, the leads were designed to plug into tube sockets for easy replacement.

The simplest 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 metal plate inside the envelope if the plate, also called the anode, is positively charged. This results in a current of electrons flowing from filament to plate. 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.

Development

John Ambrose Fleming, scientific adviser to the Marconi company invented the diode in 1904 based on an observation by Thomas Edison. The unidirectional current and construction of the orignial diode led to Lee DeForest placing another electrode, a bent wire or screen called the grid, between the filament and plate in 1907. 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 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, 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. This distortion was an undesirable distortion at low volume, not to be confused with the desirable distortion that tube amplifiers exhibit at high volume levels (the latter known as the valve sound). To remedy this low volume distortion 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.

A two-valve home-made radio from 1958. The valves are the two glass columns with the dark tops. The leads at the bottom connect to the low-voltage filament supply and to the high-voltage anode supplyHowever the tetrode 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. A special grid acted as a second plate to provide a built-in oscillator, which mixed with the incoming radio signal. These signals 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 6AG11 Compactron tube contained two triodes and two diodes. Currently the world's most popular vacuum tube is the 12AX7, with estimated annual worldwide sales of greater than 2 million units. The 12AX7 is a dual high-gain triode widely used in guitar amplifiers.

An RCA 12AX7 dual-triode tube (1947)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. 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

The chief reliability problem of a tube is that the filament or cathode is slowly "poisoned" by atoms from other elements in the tube, which damage its ability to emit electrons. Trapped gases or slow gas leaks can also damage the cathode or cause plate-current runaway due to ionization of free gas molecules. Vacuum hardness and proper selection of construction materials are the major influences on tube lifetime. (Tube cathodes do not "wear out" as lamp filaments do; this is a common misconception among laymen.)

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. The thorium slowly evaporates from the wire surface, while new thorium atoms diffuse to the surface to replace them. Such thoriated tungsten cathodes routinely deliver lifetimes in the tens of thousands of hours. The record is held by an Eimac power tetrode used in a Los Angeles radio station's transmitter, which was removed from service after 80,000 hours of uneventful operation.

Cathodes in small "receiving" tubes are coated with a mixture of barium and strontium oxides. An electric heater is inserted into the cathode sleeve, and insulated from it electrically. This complex construction causes barium and strontium atoms to diffuse to the surface of the cathode when heated to about 780 degrees Celsius, thus emitting electrons.

To meet the unique reliability requirements of the early digital computer Whirlwind, it was found 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 were reliable enough for the Whirlwind project. The tubes developed for Whirlwind later found their way into the giant SAGE air-defense computer system. 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 why most tubes were constructed of glass. Metal alloys (Cunife and Fernico) and glasses had been developed for light bulbs that expanded and contracted in similar amounts, as temperature changed. These made it easy to construct an insulating envelope of glass, and pass wires through the glass to the electrodes.

It is very important that the vacuum inside the envelope be as perfect, or "hard", 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 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, for use as a high-speed electronic switch.

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, later a glass disk fused to the metal was used. Metal and ceramic are used almost exclusively for power tubes above 2kW dissipation. The nuvistor is a tiny tube made only of metal and ceramic. In some power tubes, the metal envelope is also the anode. 4CX800A 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 8974, a water-cooled tetrode capable of dissipating 1.5 MILLION WATTS. (By comparison, the largest power transistor can only dissipate about 1000 watts.) A pair of 8974s is capable of producing 2 million watts of audio power. The 8974 is used only in exotic military and commercial radio-frequency installations.

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 cast aluminum or zinc chassis. The radio became just a printed circuit with non-tube components, 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 such as the Colossus which used 2000 tubes, the ENIAC which used nearly 18,000 tubes and the IBM 700 series. Unlike transistors, vacuum tubes are inherently immune to the electromagnetic pulse effect of nuclear explosions. This property kept them in use for military applications long after transistors had replaced them elsewhere. Vacuum tubes are still used for specialised audio amplifiers, notably for electric guitar amplification, and for very high-powered applications such as microwave ovens, industrial radio-frequency heating, and power amplification for broadcasting.

Other vacuum tube devices

A vast array of devices were built during the 1920-1960 period using vacuum-tube techniques. Most such tubes were rendered obsolete by semiconductors. Vacuum-tube electronic devices still in common use include the magnetron, klystron, photomultiplier and cathode ray tube. The magnetron is the type of tube used in all microwave ovens. In spite of the advancing state of the art in power semiconductor technology, the vacuum tube still has reliability and cost advantages for high-frequency RF power generation. Photomultipliers are still the most sensitive detectors of light. Many televisions, oscilloscopes and computer monitors presently use cathode ray tubes, though flat panel displays are becoming more popular as prices drop.

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.

Some tubes, like magnetrons, traveling-wave tubes, and klystrons, combine magnetic and electrostatic effects. These are efficient (usually narrow-band) RF producers and still find use in radar, microwave ovens and industrial heating.

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

Field emitter 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. The most common design uses a cold cathode field emitter, with electrons emitted from a number of sharp nano-scale tips formed on the surface of a metal cathode.

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 device cathodes have been constructed with emitter tips 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.

Such integrated microtubes may find application in microwave devices including mobile phones, for Bluetooth and Wi-Fi transmission, in radar and for satellite communication. Presently they are being studied for possible application to flat-panel display construction.

Vacuum tube solar heaters

The term vacuum tube has recently been used to refer to the tubular elements of solar panels used for heating water. Vacuum tube solar heaters are becoming increasingly popular.