Electrons at Work (Sep, 1946)
Electrons at Work
How the busy family of vacuum tubes serves industry as valves, triggers and throttles of electric power.
By WILLIAM P. VOGEL, JR.
WHEN you snap your radio on; when you cross the path of an electric eye and untended doors jerk open to let you pass; when you hear your train called with strident clarity above the clamor of a vast terminal, electrons have been put to work.
Electrons are controlled by vacuum tubes. And vacuum tubes in the last 15 years have gone far afield from their original uses in communications to become the valves, triggers, and throttles of modern industry.
All vacuum tubes, from the tiny nutlike affairs in hearing-aid devices to the six-foot water-cooled giants used to handle hundreds of kilowatts in radio broadcasting and power conversion, depend upon the same principle. This principle is that electrons— those ultimate bits of electrical energy— can be rapidly and automatically controlled when they are freed from a metal conductor and jump across the empty space inside the tube. Their behavior during those microseconds can be changed in almost any way that human intelligence wants it to be changed.
Attempts to classify vacuum tubes began almost immediately after Dr. Lee DeForest invented the basic three-element tube in 1906, which he termed the “audion.” The late Dr. Michael Pupin, of Columbia University, objected to the name, saying it was a “mongrel … a Latin word with a Greek ending.” Soon afterward, Dr. Irving Langmuir, of the General Electric Com- pany at Schenectady, began to refer to two-element tubes as “kenotrons” and three-element tubes as “pliotrons.” Dr. DeForest scoffingly termed this jargon “Graeco-Schenectady.” Yet, today the electronic scene is full of ignitrons, phanotrons, thyratrons, magnetrons and such trade-marked items as Radiotrons. But don’t let the long words scare you: what electrons-at-work do is fairly simple, even if we don’t always know how they do it.
No matter how big or how small, whether for microwave work or to control the split-second cycle in resistance welding, all vacuum tubes must have cathodes—the source of the electron stream. There are many different types of cathodes. Some are simply filaments, very much like the glowing wire in the ordinary incandescent lamp. When an electric current heats the filament to incandescence, electrons are boiled out of it much as steam is boiled out of water. Another type of hot cathode is built like a tiny electric stove, with an electric heating coil inside the cathode sleeve. Some vacuum tubes have cold cathodes, in which the electrons are literally pulled out of the cathode material by a high voltage across the tube. An important class of tubes, known as ignitrons, have pools of mercury as cathodes, from which the electronic stream is started by an auxiliary electrode called the ignitor. Phototubes, widely used to measure paper thickness, to keep color plates in register on the printing press, and to open and close doors, have yet another type of cathode—one from which the elec- trons dart out in proportion to the intensity of the light falling upon the cathode.
Another essential for vacuum-tube operation is the anode—the collector or receiver of the electrons emitted by the cathode. The anodes are always positive with respect to the cathodes—ranging from a mere 90 volts or so on the anode (familiarly known as the “plate”) in a small household radio tube to many thousands of volts on the big water-cooled anode of an industrial or transmitting tube. The anode can never emit electrons—that is the special privilege of the cathode.
Because of the unique behavior of these two electrodes, vacuum tubes acquire one of their most important characteristics: they act as valves. The current flow through the tube can be in one direction only, from cathode to anode. This makes the two-element tube—the kenotron—a natural for rectifying alternating current into direct current. And in radio applications, kenotrons, commonly called diodes, apply their valve action as well to such jobs as detection and automatic volume control.
But the kenotron has its limitations. Be- cause the negative electrons tend to pile Up around their source, the cathode, they get in the way of other electrons traveling to the anode. This is known as; the space charge. To overcome the crowding, a tiny quantity of some gas such as mercury vapor, argon or xenon is placed in the tube. The gas overcomes the crowding action of the electrons and hustles them on to the anode. The gas acts in this way because it becomes ionized—by losing electrons its molecules become positive and attract new electrons from the cathode, which in turn are knocked out by others as the rate of flow increases. Such gas-filled tubes are called phanotrons; they have the ability of handling greater quantities of current than kenotrons.
A third class of rectifiers is the ignitrons. Because their cathodes are pools of mercury they have tremendous power-handling ability. Despite the ignitor, which starts the action of the tube, they are basically two-element tubes like kenotrons and phanotrons. As the ignitor fires the cathode, vast quantities of electrons pour out of the pool toward the anode. At the same time quantities of positive mercury-vapor ions are created, which, as in the phanotron, serve as traffic cops to help the electrons on their pell-mell course to the anode. The biggest ignitrons can deliver 200 amperes of current at 20,000 volts—a power output of 4,000 kilowatts, more than enough to make a typical small town shine like Times Square on New Year’s Eve.
Tubes That Control and Amplify The space charge can be put to work by adding a third element, called the grid, to a vacuum tube. When a grid, deliberately made negative, is placed between cathode and anode, it will hold up current flow just the way the space charge does. But if the grid becomes more positive, it will aid the flow of electrons to the plate the way the positive ions in the gas-filled tubes do. With the introduction of the grid, vacuum tubes acquire two more characteristics: control and amplification. If two-element tubes act like one-way check valves, then three-element tubes act like triggers or throttles.
Tubes with grids belong to two families: the pliotrons and the thyratrons. The pliotrons are high-vacuum tubes, like the kenotrons. The thyratrons, like the phanotrons, contain minute quantities of easily ionized gas. Most radio-receiving tubes, except rectifiers, belong to the pliotron family, while the thyratrons find their biggest application in industry.
With the pliotrons, the vacuum tube assumes its function as a throttle. Suppose the grid is made quite negative. The electrons, crowding away from the cathode, are stopped dead in their tracks by the negative grid, very much the way a closed Venetian blind keeps out light. Now, as the signal varies and the grid becomes less negative, it permits more and yet more electrons to flow to the plate. Plate current increases. The Venetian blind is now open, admitting full sunlight.
In the thyratrons the action of the grid is more like a trigger than a throttle. Like all gas-filled tubes, thyratrons can handle larger current flows. When the grid is negative the current is halted, but once the tube has started to pass current the grid becomes ineffective, because of the ions of gas that jostle the electrons on toward the anode. The only way a thyratron, once it has been “fired,” will cease to pass current is to make the anode negative. This permits the gas inside the tube to “deionize,” and the grid once more takes command of the electron stream.
“Seesaw” Effect on Volume In both types of tubes any degree of amplification can be obtained, depending upon tube design. If the grid is placed very near the cathode, as it usually is, it becomes possible for a small change in grid potential to control a large change in plate current. Suppose that a change of one volt in the grid potential causes a change of 15 volts in the plate potential. Then the tube is said to have an amplification factor (designated by the Greek letter mu) of 15.
In certain classes of pliotrons and thyratrons voltage gain is sacrificed in favor of power-handling abilities. Again the designers space the electrodes in such a way that while the grid voltage does not produce a significant change in the plate voltage, the tube permits a large number of electrons to pour into the anode. Such tubes are called power amplifiers and their most familiar use is in the amplifiers used to drive a loudspeaker of a radio installation or a public-address system.
Because of the control and amplifying action of pliotrons and thyratrons you can obtain whatever effect you want. All you have to do is to choose your tubes properly. You can magnify the tiny impulses of electricity produced by your brain or your blood stream to power a stylus that draws an encephalogram or a cardiogram—or you can automatically drive heavy switchgear or work the loudspeakers in Madison Square Garden.
It Helps to Add a Screen Like all other vacuum tubes, tubes with grids have certain limitations. The anode and grid oppose each other—for that reason, when we want maximum power in the load, we have to make the anode least able to attract electrons by making it less positive. Likewise, when we want less power in the load, we have to make the anode more positive, just at the time when we want reduced current flow. The best way to overcome this is to insert a second grid, called the screen, between the control grid and the anode. The screen is made positive. Thus the twin functions of the anode are split between two electrodes: the screen attracts and accelerates the electrons while the anode collects them. Even the screen has its limitations. Some of the slower electrons fail to get to the cathode and tend to return to the positive screen. To prevent the return of these electrons, still another grid, known as the suppressor, is inserted in the tube between the screen and the anode. Carrying a negative charge, the suppressor repels electrons that would like to go to the screen and hustles them on to the anode.
And On Up to “Hoptoads”
Tubes with three elements are commonly called triodes, those with four are called tetrodes, and those with five are called pentodes. Special radio and industrial tubes, in which several different tube types are enclosed in the same container, have as many as six, seven and eight elements and are called hexodes, heptodes and octodes. Among vacuum-tube designers there is frequent joking about a mythical tube known as the “hoptoad”—which, if it could ever be built, would have an Infinite number of electrodes and the ability to perform any function whatsoever without loss or limitation.
In general, all of the ordinary tubes used in radio, telephony, and industry are of the types described. There are many varieties of tubes, each with different characteristics and different circuit applications. The electronic engineer simply has to pick the right tube for his purpose. For example, a radio set could be built using only diodes and triodes. But the diodes used to rectify house current would be far larger tubes than those used to detect the signal. And the triodes used for voltage amplification would be far different from the triodes used to drive and supply power to the loudspeaker system.
As the electronic industry challenged the unknown region in the higher reaches of the radio spectrum—where the frequency is 300 megacycles and more—it became evident that a totally new kind of thinking about vacuum tubes was necessary. No longer could the tube be considered merely the electronic component of a circuit composed of conventional inductances, capaci- tors and resistances. As the wave lengths in this microwave region became comparable with the size of the tube, such new factors as the transit time of electrons from cathode to anode, the radiation through the tube itself, and the losses from leads became vitally important to performance. With the tremendous research on radar during the war, such problems were faced and successfully solved. Philosophically, vacuum tubes for this microwave range began to be considered as simply a walled-off section of a complete electrical system. Such tubes as the velocity modulators, of which the klystron is the best-known example; the disk-seal tubes, of which the lighthouse tubes are the most familiar; and the magnetrons were developed. In these tubes the whole oscillating system is usually built into and around the tube. In certain cases the tube and its system can be used as a plug into the radio-frequency source, much the way a lamp cord can be plugged into a house-current receptacle.
These strides have been important—but they have by no means exhausted the possibilities of vacuum tubes. As new uses are demanded, new tubes will be forthcoming. Even as the modern radio tubes of today are a far cry from Dr. DeForest’s first audion, so are the tubes on the drawing boards far different from the most outlandish magnetrons and klystrons of the war period. Because the principles of vacuum tubes are now so well understood, there is no challenge that the tube designers feel they cannot accept.