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Related to pentode: triode, tetrode

electron tube

electron tube, device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube). The two principal electrodes of an electron tube are the cathode and the anode or plate. The simplest vacuum tube, the diode, has only those two electrodes. When the cathode is heated, it emits a cloud of electrons, which are attracted by the positive electric polarity of the anode and constitute the current through the tube. If the cathode is charged positively with respect to the anode, the electrons are drawn back to the cathode. However, the anode is not capable of emitting electrons, so no current can exist; thus the diode acts as a rectifier, i.e., it allows current to flow in only one direction. In the vacuum triode a third electrode, the grid, usually made of a fine wire mesh or similar material, is placed between the cathode and anode. Small voltage fluctuations, or signals, applied to the grid can result in large fluctuations in the current between the cathode and the anode. Thus the triode can act as a signal amplifier, producing output signals some 20 times greater than input. For even greater amplification, additional grids can be added. Tetrodes, with 2 grids, produce output signals about 600 times greater than input, and pentodes, with 3 grids, 1,500 times. X-ray tubes maintain a high voltage between a cathode and an anode. This enables electrons from the cathode to strike the anode at velocities high enough to produce X rays. A cathode-ray tube can produce electron beams that strike a screen to produce pictures, as in some oscilloscopes and older television displays. Gas tubes behave similarly to vacuum tubes but are designed to handle larger currents or to produce luminous discharges. In some gas tubes the cathode is not designed as an electron emitter; conduction occurs when a voltage sufficient to ionize the gas exists between the anode and the cathode. In these cases the ions and electrons formed from the gas molecules constitute the current. Electron tubes have been replaced by solid-state devices, such as transistors, for most applications. However they are still used in high-power transmitters, specialty audio equipment, and some oscilloscopes. A klystron is a special kind of vacuum tube that is a powerful microwave amplifier; it is used to generate signals for radar and television stations.

See also magnetron; photoelectric cell.

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a five-electrode electron tube consisting of a cathode, a control grid, a screen grid, a suppressor grid, and an anode. Low-power pentodes (up to several watts of power) are used chiefly as receiving tubes; high-power pentodes (several tens of watts or more) are used as oscillator tubes. The pentode was developed from another electron tube—the tetrode. The introduction of the suppressor grid near the anode eliminated the distortion of the tetrode’s anode characteristics that was caused by the dynatron effect—secondary electron emission from the anode or the screen grid. Through the use of pentodes, electrical oscillations with frequencies of up to several tens of megahertz are generated and their voltage and power are amplified by factors of up to several hundred times.

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A five-electrode electron tube containing an anode, a cathode, a control electrode, and two additional electrodes that are ordinarily grids.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


A type of vacuum tube used in high-end audio preamplifiers, ham radios and a variety of other electronic circuits. A pentode is like a tetrode with the addition of a "suppressor grid" between the screen grid and the plate. Typically biased at or near the cathode voltage, the suppressor grid provides additional isolation between the control grid and plate. See tetrode, triode and diode.

The Pentode Uses Three Grids
In a pentode, the addition of a screen grid and suppressor grid between the control grid and plate controls feedback and oscillation.
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References in periodicals archive ?
Golden Tube Audio SE-40 amplifiers (pair; $1098 each) Single-ended Parallel Pentode; 40 Watt Stereo Class A tube design.
The A205 is a very small, very plain looking little amplifier using EL84 pentodes in single-ended mode.
Vacuum electronics technology embraces a wide range of devices, including the traditional multiterminal vacuum tubes, such as triodes, pentodes and beam power tubes, as well as many linear-beam and crossed-field devices, which employ either slow- or fast-wave structures, such as TWTs, klystrons, backward-wave amplifiers and oscillators, gyrotrons, free electron lasers and Cerenkov devices.