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== Types of pentodes ==
* Ordinary pentodes are referred to as '''
* '''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>
* '''Power
** '''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,
== 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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