Vacuum Tubes

A vacuum tube is an electronic device used for the processing of electrical signals. It consists of two or more electrodes inside a metal or glass tube which has been evacuated, hence the name.

In the mid-1800s Sir William Crookes (1832–1919) performed early experiments with passing electric current through an evacuated glass tube. In 1883 Thomas Edison (1847–1931) noticed that current would flow between two electrodes inside a light bulb if the negative electrode was heated. John Ambrose Fleming (1849–1945) constructed the first practical diode tube, containing two electrodes. When the heated electrode, called the cathode, was at a negative voltage compared to the other electrode, called the anode or plate, electrons flowed from the cathode to the anode. When the voltages were reversed, electron flow was prevented.

This type of action is called rectification; it is used to change alternating current into direct current. This is a basic operation needed in radio receivers to demodulate a radio frequency signal into audio.

Later, Lee De Forest (1873–1961) developed a tube with three electrodes, called a triode. The third electrode was called a grid. It was a fine mesh placed between the cathode and the anode. De Forest discovered that a small change in voltage on the grid produced a large change in current flow between the cathode and anode. A positive voltage attracts the electrons from the cathode toward the anode and produces a larger current. A negative voltage repels the electrons and produces less current. Thus, the current flow is proportional to the voltage of the grid. Called amplification, this discovery was central to the growth of the electronics industry.

Further developments in vacuum tube technology led to the development of the tetrode, which contained four electrodes; the pentode with five electrodes; and others. The additional electrodes are used to enhance the amplification action of the basic triode: extending power, availability, frequency, efficiency, or fidelity.

The cathode of all vacuum tubes must be heated. The heat is supplied by passing a high direct current through the cathode or, more commonly, by providing another element, called a filament, near the cathode and passing high current through it. The filament is not considered an electrode since it is electrically isolated from the other elements and its sole purpose is to heat the cathode. It is the filament that produces the characteristic glow of the vacuum tube.

The filament represents many of the disadvantages of the vacuum tube. It requires a lot of power, which is essentially wasted energy since it does not add to the output power of the device. In a large device, a cooling system is needed to remove the heat generated by this process. Also, filament burn-out is the most common failure mechanism of most vacuum tubes.

Vacuum Tubes in Computers

The first practical electronic digital computer was the Electronic Numerical Integrator and Computer (ENIAC), built in 1946 at the University of Pennsylvania's Moore School of Electrical Engineering. It weighed 33 metric tons (60,000 pounds), contained 18,000 vacuum tubes, and consumed 150,000 watts of electricity. It was originally designed to be able to compute artillery ballistics tables for the U.S. Ballistics Research Lab. However, it was a general purpose computer that could be programmed by connecting the machine's modules with cables.

Prior to ENIAC, mathematicians computed ballistics tables with mechanical adding machines, a process that took up to twenty hours for each one. Because ENIAC was electronic—rather than mechanical—and programmable, it was able to perform about 5,000 integer additions per second, reducing the time required to generate a ballistics table to about thirty seconds. (By comparison, a modern supercomputer can perform more than one billion floating point operations per second.)

Of course, the reliability of ENIAC was a real concern. As noted earlier, a vacuum tube filament is susceptible to burn out. Some felt that a device with so many tubes would never work long enough to produce useful results. By de-rating the tubes (running them with less than full rated voltage and current), the ENIAC team managed to keep the system running for several days without failure. This was a significant accomplishment.

Still, reliability was a continual problem for ENIAC and other vacuum tube-based computers. Another significant cause of failure with these early computers was large insects that crawled between vacuum tube electrodes and caused short circuits. The process of finding and fixing these short circuits was called debugging. The term has endured; it is used today to refer to the process of finding and fixing errors in the computer's program, or software.

Vacuum Tubes Today

Although they have been replaced with transistors and other integrated circuits in many low-power applications, vacuum tubes are still used in many high-power applications, including specialized sensors and television and computer display devices.

The only type of vacuum tube used in modern computer systems is the cathode ray tube (CRT), which is the main component in a computer display monitor. The cathode assembly in a cathode ray tube is called an electron gun. Located in the narrow tube neck, it generates a very narrow beam of electrons that are accelerated at high speed toward the anode. The anode is a large rectangular screen coated with phosphors that glow when struck by the beam.

The electron beam is guided by a strong magnetic field induced by deflection coils around the tube neck. The electron beam traces a raster scan pattern that covers the entire surface of the screen at a speed higher than the human eye can detect. The intensity of the beam determines the brightness of the spot.

Colors seen on the computer screen are produced by different phosphors that glow red, green, or blue. A color tube has three electron guns, one for each color. Each gun can only "see" spots on the tube corresponding to its color. Other colors are made by combinations of varying intensities of the three primary colors. The cathode ray tube has become so common in computer display devices that the acronym CRT has come to be synonymous with the entire display unit.

Donald M. McIver

Bibliography

Bell, David. Electronic Devices and Circuits. Englewood Cliffs, NJ: Prentice Hall, 1986.

Benedict, R. Ralph. Electronics for Scientists and Engineers, 2nd ed. Englewood Cliffs, NJ: Prentice Hall, 1976.

Internet Resources

"ENIAC-on-a-Chip." Moore School of Electrical Engineering, University of Pennsylvania. <http://www.ee.upenn.edu/~jan/eniacproj.html>

"John W. Mauchly and the Development of the ENIAC Computer." Van Pelt Library, University of Pennsylvania. <http://www.library.upenn.edu/special/gallery/mauchly/jwmintro.html>