electron tube definition of electron tube in the Free Online Encyclopedia.

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 diode (dī`ōd), two-terminal electronic device that permits current flow predominantly in only one direction.
..... Click the link for more information. , 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 cathode-ray tube, special-purpose electron tube in which electrons are accelerated by high-voltage anodes, formed into a beam by focusing electrodes, and projected toward a phosphorescent screen that forms one face of the tube.
..... Click the link for more information. can produce electron beams that strike a screen to produce pictures as in oscilloscopes and video 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 transistor, three-terminal, solid-state electronic device used for amplification and switching. It is the solid-state analog to the triode electron tube ; the transistor has replaced the electron tube for virtually all common applications.
..... Click the link for more information. , for most applications. However they are still widely used in high-power transmitters, some television cameras, specialty audio equipment, and as oscilloscope and video displays. A klystron is a special kind of vacuum tube that is a powerful microwave microwave, electromagnetic wave having a frequency range from 1,000 megahertz (MHz) to 300,000 MHz, corresponding to a wavelength range from 300 mm (about 12 in.) to 1 mm (about 0.04 in.). Like light waves, microwaves travel essentially in straight lines.
..... Click the link for more information. amplifier; it is used to generate signals for radar and television stations.

See also magnetron magnetron (măg`nĭtrŏn')
..... Click the link for more information. ; photoelectric cell photoelectric cell or photocell, device whose electrical characteristics (e.g., current, voltage, or resistance) vary when light is incident upon it.
..... Click the link for more information. .

Same as vacuum tube.

(electronics)

electron tube - (Or tube, vacuum tube, UK: valve, electron valve, thermionic valve, firebottle, glassfet) An electronic component consisting of a space exhausted of gas to such an extent that electrons may move about freely, and two or more electrodes with external connections. Nearly all tubes are of the thermionic type where one electrode, called the cathode, is heated, and electrons are emitted from its surface with a small energy (typically a Volt or less). A second electrode, called the anode (plate) will attract the electrons when it is positive with respect to the cathode, allowing current in one direction but not the other.

In types which are used for amplification of signals, additional electrodes, called grids, beam-forming electrodes, focussing electrodes and so on according to their purpose, are introduced between cathode and plate and modify the flow of electrons by electrostatic attraction or (usually) repulsion. A voltage change on a grid can control a substantially greater change in that between cathode and anode.

Unlike semiconductors, except perhaps for FETs, the movement of electrons is simply a function of electrostatic field within the active region of the tube, and as a consequence of the very low mass of the electron, the currents can be changed quickly. Moreover, there is no limit to the current density in the space, and the electrodes which do dissapate power are usually metal and can be cooled with forced air, water, or other refrigerants. Today these features cause tubes to be the active device of choice when the signals to be amplified are a power levels of more than about 500 watts.

The first electronic digital computers used hundreds of vacuum tubes as their active components which, given the reliability of these devices, meant the computers needed frequent repairs to keep them operating. The chief causes of unreliability are the heater used to heat the cathode and the connector into which the tube was plugged.

Vacuum tube manufacturers in the US are nearly a thing of the past, with the exception of the special purpose types used in broadcast and image sensing and displays. Eimac, GE, RCA, and the like would probably refer to specific types such as "Beam Power Tetrode" and the like, and rarely use the generic terms.

The cathode ray tube is a special purpose type based on these principles which is used for the visual display in television and computers. X-ray tubes are diodes (two element tubes) used at high voltage; a tungsten anode emits the energetic photons when the energetic electrons hit it. Magnetrons use magnetic fields to constrain the electrons; they provide very simple, high power, ultra-high frequency signals for radar, microwave ovens, and the like. Klystrons amplify signals at high power and microwave frequencies.