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== Types of pentodes ==
* Ordinary pentodes are referred to as '''Sharpsharp-cutoff''' ("highor '''high-slope" or ordinary)''' pentodes and have uniform aperture size in the morecontrol grid. ordinaryThe uniform spacingconstruction of the control grid wires,results andin sothe mutualamplification conductancefactor decreases(mu inor anμ} essentiallyand uniformtransconductance changing very mannerlittle with increasingincreasingly negative biasgrid voltage, andresulting hasin a morefairly abrupt cutoff of plate current. These pentodes are more suitable for audioapplication amplifiersin 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 [[Mullard–Philips tube designation|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 ([http://www.r-type.org/addtext/add008.htm "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.<ref name="TM11662">Departments of the Army and the Air Force (1952, rev. 1958). [https://archive.org/details/TM11-662/page/n115/mode/2up?view=theater TM 11-662 ''Basic Theory and Application of Electron Tubes''.] Washington DC: USGPO. pp. 104 - 105.</ref> The control grid often has the form of a helix of varying pitch.<ref>Departments of the Army and the Air Force (1952, rev. 1958). [https://archive.org/details/TM11-662/page/n53/mode/1up?view=theater p. 41.]</ref> As the control grid voltage becomes more negative, the amplification factor of the tube becomes smaller.<ref name="TM11662"/><ref>Ballantine, Stuart and Snow, H.A. (Dec. 1930). [https://worldradiohistory.com/Archive-IRE/30s/IRE-1930-12.pdf "Reduction of Distortion and Cross-talk in Radio Receivers by Mean of Variable-mu Tetrodes"]. ''Proc. IRE''. p. 2122.</ref> Variable-mu pentodes reduce distortion and cross-modulation (intermodulation) and permit much larger amplifier dynamic range than ordinary pentodes.<ref>Rider, John F. (1936) [https://worldradiohistory.com/BOOKSHELF-ARH/Rider-Books/Rider-Automatic-Volume-Control-1936-Hour-a-Day.pdf ''Automatic Volume Control'']. New York: John F. Rider, Publisher. pp. 12 - 17.</ref> Variable-mu pentodes have been applied in radio frequency amplifier stages of radio receivers and 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.<ref>Stokes, John W. (1982). ''70 Years of Radio Tubes and Valves''. Vestal, NY: Vestal Publishers Ltd. p. 57.</ref><ref>Thrower, Keith R. (2009). ''British Radio Valves, The Classic Years: 1926-1946''. Reading, England: Speedwell. p. 5.</ref>
* '''Variable transconductance''' ("'''variable-mu'''", "'''remote-cutoff'''" or "'''super-control'''") tubes in general are those with a non-uniform grid wire spacing to allow them to handle a wide range of input signal levels without excessive cross-modulation [[distortion]], and so useful in [[radio frequency]] stages where [[automatic gain control]] is applied to the pentode. The first commercial variable-mu tubes were the 550 and 551 developed by Stuart and Snow around 1929.<ref>Stuart, B & Snow, H. A. (1930). Reduction of Distortion and Cross-Talk in Radio Receivers by Mean of Variable-Mu Tetrodes. Proc IRE, Vol 18, Issue 12, pp. 2102 - 2127</ref> Other examples include: C-335 (1931), [[EF50]], 1T4, 6K7, 6BA6, and the EF83 (while perhaps the EF85/6BY7, and certainly the 6JH6, could be described as "'''semiremote-cutoff'''" pentodes).<ref name="Evans">{{cite book|author=Grayson Evans|title=Hollow-State Design 2nd Edition|date=23 July 2014|url=https://books.google.com/books?id=Iv9UBQAAQBAJ&q=semi+remote+cutoff+pentode&pg=PA45|publisher=Lulu.com|isbn=978-1-300-96521-3|pages=45–}}</ref>
 
* '''Power outputpentodes''' or '''power-amplifier pentodes''',. 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 true pentodes useddesigned for power amplification, using a suppressor grid rather than the higher efficiency beam-forming plates used in beam tetrodes. (Beam tetrodes were also sometimes referred to as "beam pentodes",<ref>"Sylvania Receiving Tubes Technical Manual, 14th Edition" p 143</ref> and include the Sylvania (and possibly GE) 6CA7 version of EL34, 6V6GT and the GEC KT66 and KT88. Other sources preferred names such as "beam power amplifier" or "beam power tube", possibly due to caution regarding the [[Philips]] pentode patent). Some power output pentodes for specific television requirements were:
** '''video output''' pentodes, e.g. 15A6/PL83, PL802
** '''frame output''' or '''vertical deflection''' pentodes, such as the [[EL84#PL84, UL84 and EL86|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.
[[File:GU-81.jpg|thumb|Image of a type GU-81 power pentode, russiana Russian electron tube used in military radio stations in the 70s and 80s]]
 
== Advantages over the tetrode ==
The simple [[tetrode]] or ''screen-grid tube'' offered a larger amplification factor, more power and a higher frequency capability than the earlier [[triode]]. However, in the tetrode ''secondary electrons'' knocked out of the anode (plate) by the electrons from the cathode striking it (a process called [[secondary emission]]) can flow to the screen grid due to its relatively high potential. This current of electrons leaving the anode reduces the net anode current ''I''<sub>a</sub>. As the anode voltage ''V''<sub>a</sub> is increased, the electrons from the cathode hit the anode with more energy, knocking out more secondary electrons, increasing this current of electrons leaving the anode. The result is that in the tetrode the anode current ''I''<sub>a</sub> is found to ''decrease'' with increasing anode voltage ''V''<sub>a</sub>, over part of the [[Current–voltage characteristic|characteristic curve]]. This property (Δ''V''<sub>a</sub>/Δ''I''<sub>a</sub> < 0) is called [[negative resistance]]. It can cause the tetrode to become unstable, leading to [[parasitic oscillation]]s in the output, called [[Dynatron oscillator|dynatron oscillations]] in some circumstances.
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