Vacuum tube

In electronics, the vacuum tube was the predecessor of the transistor. Vacuum tubes are now only used in particular specialist applications, having been replaced completely by integrated circuits and discrete semiconductor devices for general-purpose electronics.

Vacuum tubes (also known as thermionic valves) are arrangements of electrodes surrounded by vacuum within a glass envelope, superficially similar to incandescent light bulbs. Like light bulbs, tubes have a filament through which current is passed, heating the filament. In its heated state it is ready to release electrons into the vacuum. These electrons are electrostatically drawn to a positively charged outer metal plate called the anode, or more commonly just the plate. Electrons do not flow from the plate back toward the filament, even if the charge on the plate is made negative, because the plate is not heated. The arrangement of a filament and plate is called a diode and invented in 1904 by John Ambrose Fleming scientific adviser to the Marconi company based on an observation by Thomas Edison.

The next innovation, due to Lee DeForest in 1907, was to place another electrode, the grid, between the filament and plate. The grid is a bent wire or screen, leaving plenty of room for electrons to get past it to complete their journey toward the plate. De Forest discovered that the current flow from filament to plate was highly dependent on the voltage of the grid, and that the current drawn by the grid was very low. The resulting three-electrode device, the triode, was therefore an excellent amplifying device.

Many further innovations followed. It became common to use the filament to heat a separate electrode called the cathode, and to use the cathode as the source of electron flow in the tube rather than the filament itself. It was discovered that additional grids could be used to lower the current draw of the triode's control grid, and to improve the linear response of the tube. The triode gave way to the tetrode and later the pentode.

Tubes with 4, 5, 6, or 7 grids, called hexodes, heptodes, octodes, and nonodes, are generally used for frequency conversion in superheterodyne receivers. The heptode, or pentagrid, is the most common of these. 6BE6 is an example of a heptode.

It was common practice in some tube types to include more than one group of elements in one bulb. For instance, type 6SN7 is a "dual triode" which, for most purposes, can perform the functions of two triode tubes, while taking up half as much space and costing less.

The beam power tube is usually a tetrode with the addition of "beam forming plates". These plates direct the electron stream to certain spots on the anode, and thus overcome some design barriers to designing high power, high perfromance power tubes. 6L6 is a beam power tube.

Tubes usually have glass envelopes, but metal and ceramic are possible choices. The nuvistor is a tiny tube made only of metal and ceramic. In some tubes, the metal envelope is also the anode. 4CX800 is an external anode tube of this sort.

Tubes were ubiquitous in the early generations of electronic devices, such as radios, televisions, and early computers. They are still used for specialised audio amplifiers, notably for electric guitar amplification, and for very high-powered applications such as signal amplification for broadcast radio.

Other vacuum tube electronic devices include the magnetron, klystron and cathode ray tube.

Other tube devices

Specialist low-pressure gas-filled tube devices include the nixie tube and the dekatron.