Before the 1960s, vacuum tubes prevailed in nearly every electronic device you could find. Even in radio receivers and portable television sets, all of the amplifiers, oscillators, power supplies, and other circuits required tubes (called valves in England). A typical vacuum tube ranged from the size of your thumb to the size of your fist. An elaborate vacuum-tube radio was a major appliance, and some of them were as big and heavy as the dresser in your bedroom!
Vacuum tubes are still used in some radio-transmitter and audio power amplifiers, microwave oscillators, and video display units. Tubes work better in certain ways than semiconductor devices do, even today. Tubes can tolerate momentary voltage and current surges and transients better than
semiconductors can do. Some popular music bands claim that amplifiers built with vacuum tubes produce richer, truer sound than amplifiers built with semiconductor devices. But tubes have two big drawbacks: They need high voltages to operate, and they consume a lot of power for the actual work that they do.
Did You Know?
Back in the days when vacuum tubes prevailed in electronics, it took from 50 to a few hundred volts of DC to make a vacuum tube function, even in relatively small consumer
devices, such as clock radios and portable television sets. These voltages mandated the use of bulky power supplies, and created an electrical shock hazard.
A vacuum tube accelerates electrons to high speed, resulting in electric current. This current can be made more or less intense, or focused into a beam and guided in a certain direction. The intensity and/or beam direction can be adjusted with extreme rapidity, making possible a variety of different useful effects. In any vacuum tube, the charge carriers are free electrons, meaning that they don’t
“orbit” any particular atomic nucleus. Instead, they hurtle like submicroscopic bullets through the tube’s internal vacuum.
Before the start of the twentieth century, scientists knew that electrons could carry current through a vacuum. They also knew that hot electrodes would emit electrons more readily than cool ones
would. These phenomena went to practical use in the first electron tubes, known as diode tubes, for the purpose of rectification.
In any tube, the electron-emitting electrode is called the cathode. A wire filament that carries AC, similar to the glowing element in an incandescent bulb, heats the cathode. In some tubes, the filament also serves as the cathode. This type of electrode is called a directly heated cathode. In other tubes, the cathode is separate from, and surrounds, the filament. This arrangement is called an indirectly heated cathode. The electron-collecting electrode is known as the anode or plate. The cathode, and by extension the negative DC output of the power supply, is usually connected to a metal chassis that serves to support all the electronic components in the device. The chassis is connected to electrical ground.
Did You Know?
In either the directly heated or indirectly heated cathode type of vacuum tube, electrons get driven off the element by the heat of the filament. Because the electron emission depends on the filament or “heater,” tubes need a little while to “warm up.” This waiting period is (ironically) about as long as the boot-up time for a typical computer.
A tube’s anode comprises a metal cylinder concentric with the cathode and filament. The plate goes to the positive DC power supply terminal, usually through a coil or resistor. The output signal is
usually taken from the plate.
Tubes operate at voltages ranging from about 50 volts to several thousand volts. Because the plate readily attracts electrons but is not a good emitter of them, and because the opposite state of affairs prevails for the cathode, a diode tube works well as an AC rectifier or AC-to-DC converter. Although semiconductor diodes have replaced tubes for rectification in most applications, tubes are still used in power supplies that must deliver extreme voltages.
Voltages imposed deliberately on an electrode between the cathode and the plate can control the flow of current in a vacuum tube. This electrode, called the control grid, comprises a wire mesh that lets electrons pass through. The control grid interferes with the electrons if it is provided with a voltage that’s negative with respect to the cathode voltage. The greater this so-called negative grid bias, the more the grid impedes the flow of electrons, and the less current flows in the plate. In most tube-type amplifiers, the control grid receives the AC input signal.
Some vacuum tubes have more than one grid. The extra grids help amplifiers to boost the signals more (in other words, they allow for more gain). They also help the amplifier to operate in a more stable manner than a single-grid tube does. A screen grid can be added between the control grid and the plate. This grid carries a positive DC voltage, roughly 1/3 that of the plate voltage. The screen grid reduces the tendency of the amplifier to oscillate (generate a signal of its own because of feedback inside the tube). The screen grid can also serve as a second control grid, allowing for the injection of two different signals into a tube so that they can be mixed, producing new signals at different
frequencies or with different characteristics than the originals.
Tube performance can sometimes be improved even more by placing a third grid, called the suppressor grid, between the screen grid and the plate. The suppressor grid carries a negative charge with respect to the screen and the plate; usually it’s the same voltage as the cathode. The suppressor grid reduces the tendency of a tube to oscillate more than a screen grid alone can do. In addition, a suppressor grid “recovers” stray electrons that “bounce off” the plate upon impact, so that the tube can amplify better than it would without the suppressor grid.
Did You Know?
Old-time radio and TV receivers featured tubes with four or five grids in some circuits.
The usual function of such tubes was signal mixing. You’ll probably never hear about these devices in modern electronics because solid-state components do all the signal mixing nowadays.
The most common contemporary application of vacuum tubes is in massive amplifiers designed to deliver a great deal of signal output power, especially at high frequencies (in radio and television transmitters, for example) and in big audio systems (for popular music bands, for example).
Vacuum tubes prevail in antique TV receivers and old computer monitors. These devices, which can be large and heavy, are called cathode-ray tubes (CRTs). In a CRT, a device called an electron gun emits a high-intensity stream of electrons, something like a “flashlight” that “shines” a beam of
subatomic particles. This beam gets focused and accelerated as it passes through the holes in donut-shaped anodes that carry high-positive DC voltages. The anodes of a CRT work differently than the anodes of a conventional vacuum tube do. Rather than hitting the anodes, the electrons go right on through, gaining speed with each pass, until they strike a screen with a phosphorescent inner coating.
The phosphor glows visibly as seen from the face of the CRT. Internal coils or electrodes carry signals that deflect the electron beam back-and-forth and up-and-down in an intricate pattern at speeds faster than the eye can follow, creating a motion-picture image on the phosphorescent screen.
Some electronics hobbyists enjoy working with antique radios. Certain people like to have a radio broadcast receiver that takes up as much space and weighs as much as a small refrigerator. The big old
“boat anchor,” sitting on the living room floor, brings back memories of a time when drama shows came over local radio stations. Users had to employ their imaginations as the plots unfolded in the voices and music; video was nothing more than a few roguish inventors’ dreams-yet-to-come-true.
Did You Know?
In the days of radio before semiconductor devices took over most of the roles of vacuum
tubes, the high internal voltages (required for the tubes to work) caused dust and airborne oil droplets to accumulate on a radio’s circuit components as a result of
electrostatic precipitation. Over the years, the grit, grease, and grime would grow into a gray, fuzzy wax. You might even find a few dead and well-cooked insects in there!
Sometimes these insects would cause a circuit malfunction by shorting out something.
According to one legend, the term bug (in regards to flaws in all sorts of devices and systems, including computer software) arose from this phenomenon.
For Nerds Only
Perhaps you’d like to collect and operate antique radios, just as some people collect and drive vintage cars. However, if you get interested in that stuff, you should know that replacement vacuum tubes are hard to find, and they can also be costly. When your old toy breaks, you’ll need to become a spare-parts sleuth.