pentode

thumb|upright|Graphic symbol representing a pentode of the indirectly heated cathode class Electrodes, listed from top to bottom: anode, suppressor grid, screen grid, control grid, cathode A pentode is an electronic device having five electrodes. The term most commonly applies to a three-grid amplifying vacuum tube or thermionic valve that was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode (called a triple-grid amplifier in some literature) was developed from the screen-grid tube or shield-grid tube (a type of tetrode tube) by the addition of a grid between the screen grid and the plate. The screen-grid tube was limited in performance as an amplifier due to secondary emission of electrons from the plate. The additional grid is called the suppressor grid. The suppressor grid is usually operated at or near the potential of the cathode and prevents secondary emission electrons from the plate from reaching the screen grid. The addition of the suppressor grid permits much greater output signal amplitude to be obtained from the plate of the pentode in amplifier operation than from the plate of the screen-grid tube at the same plate supply voltage. Pentodes were widely manufactured and used in electronic equipment until the 1960s to 1970s, during which time transistors replaced tubes in new designs. During the first quarter of the 21st century, a few pentode tubes have been in production for high power radio frequency applications, musical instrument amplifiers (especially guitars), home audio and niche markets.

== Types of pentodes == Ordinary pentodes are referred to as sharp-cutoff or high-slope pentodes and have uniform aperture size in the control grid. The uniform construction of the control grid results in the amplification factor (mu or μ) and transconductance changing very little with increasingly negative grid voltage, resulting in fairly abrupt cutoff of plate current. These pentodes are suitable for application in amplifier designs that operate over limited ranges of signal and bias on the control grid. Examples include: EF37A, EF86/6267, 1N5GT, 6AU6A, 6J7GT. Often, but not always, in the European valve naming scheme for pentodes an even number indicated a sharp-cutoff device while odd indicated remote-cutoff; the EF37 was an exception to this general trend, perhaps due to its history as an update to the EF36 ("The Mullard EF36, EF37 and EF37A" at the National Valve Museum). Remote-cutoff, variable-mu, super-control or variable slope pentodes handle much greater signal and bias voltages on the control grid than ordinary pentodes, without cutting off the anode current. The control grid of the variable-mu pentode is constructed so as to result in a given incremental change of control grid voltage having less effect on change of anode current as the control grid voltage increases negatively relative to the cathode. The control grid often has the form of a helix of varying pitch. As the control grid voltage becomes more negative, the amplification factor of the tube becomes smaller. Variable-mu pentodes reduce distortion and cross-modulation (intermodulation) and permit much larger amplifier dynamic range than ordinary pentodes. Variable-mu pentodes were first applied in radio frequency amplifier stages of radio receivers, typically with automatic volume control, and are applied in other applications requiring the ability to operate over large variations of signal and control voltages. The first commercially available variable-mu pentodes were the RCA 239 in 1932 and the Mullard VP4 in 1933. Power pentodes, output pentodes or power-amplifier pentodes. Power pentodes are designed to operate at higher currents, higher temperatures and higher voltages than ordinary pentodes. The cathode of the power pentode is designed to be capable of sufficient electron emission to give the required current through the tube to produce the desired power in the load impedance. The plate or anode of a power pentode is designed to be capable of dissipating more power than that of an ordinary pentode. The EL34, EL84, 6CL6, 6F6, 6G6, SY4307A and 6K6GT are some examples of pentodes designed for power amplification. Some power pentodes for specific television requirements were: video output pentodes, e.g. 15A6/PL83, PL802 frame output or vertical deflection pentodes, such as the PL84 and the pentode sections of the 18GV8/PCL85. line output or horizontal deflection pentodes, such as the PL36, 27GB5/PL500, PL505 etc. A "triode-pentode" is a single envelope containing both a triode and a pentode, such as an ECF80 or ECL86. thumb|Image of a type GU-81 power pentode, a Russian electron tube used in military radio stations in the 70s and 80s