Editing Tetrode (section)

==Screen grid valve==
[[File:S23 sg valve.JPG|thumb|right|View of the interior of an Osram S23 screen grid valve. In this valve the anode is in the form of two flat plates. The wires of the screen grid can also be seen. The anode connection is at the top of the envelope to minimise anode-grid capacitance]]
[[File:RCA 24-A Screen Grid tube anode characteristics.jpg|thumb|right|At anode voltages less than that of the screen grid, the tetrode characteristic curves are kinked due to [[secondary emission]] from the anode. In the normal range of anode voltages, the anode current is substantially constant with respect to anode voltage. Both features are quite unlike the corresponding curves for a triode, for which anode current increases continuously with increasing slope throughout.]]
[[Image:S625 screen grid tube.png|thumb|The Marconi-Osram S625, the first commercially produced screen grid tube. The screen is a cylinder with a metal gauze face that completely surrounds the anode, and the tube is double-ended, with the anode terminal at one end and the grid at the other, to improve isolation between the electrodes.]]
The screen grid tube provides much smaller control grid to anode capacitance and much greater amplification factor than a triode. Radio frequency amplifier circuits using triodes were prone to oscillation due to the grid to anode capacitance of the triode.<ref name="Henney6">Henney, K., Richardson, G. A. (1952) [https://archive.org/details/in.ernet.dli.2015.126478/page/n285/mode/2up ''Principles of Radio'', 6th ed.] New York: John Wiley & Sons. pp. 279 - 282</ref> In the screen grid tube, a grid referred to as the ''screen grid'', ''shield grid'' or sometimes ''accelerating grid'' is inserted between the control grid and the anode. The screen grid provides an electrostatic shield between the control grid and the anode, reducing the capacitance between them to a very small amount.<ref name="Henney6" /><ref name="ZEP_01">Zepler, E. E. (1943) [https://archive.org/stream/Zepler_1943_The_Technique_of_Radio_Design#page/n191/mode/2up ''The Technique of Radio Design''], New York: John Wiley and Sons. pp. 183–187, 219-221. Retrieved 13 Oct. 2021</ref> To reduce the influence of the anode's electric field on the cathode space charge and on the control grid, during 1915 - 1916 physicist [[Walter H. Schottky]] developed the first tubes having a grid positioned between the anode and the control grid to provide an electrostatic shield.<ref>Tapan, Sarkar, Mailloux, Oliner, [[Magdalena Salazar Palma|Salazar-Palma]], Sengupta (2006) [https://books.google.com/books?id=NBLEAA6QKYkC&q=schottky ''History of Wireless'']. New Jersey: John Wiley & Sons Inc. pp. 108 - 109, 344.</ref><ref name="ExpWireless">Editors (Oct. 1927) [https://worldradiohistory.com/UK/Experimental-Wireless/20s/Experimental%20Wireless-1927-10.pdf "Screened Valves"] ''Experimental Wireless & The Wireless Engineer'' pp. 585-586.</ref> Schottky patented these screen grid tubes in Germany in 1916 and in the U.S. in 1919.<ref>Ballantine, Cobb (Mar. 1930) [https://worldradiohistory.com/Archive-IRE/30s/IRE-1930-03.pdf "Power Output Characteristics of the Pentode"]. ''Proc. IRE''. p. 451.</ref><ref>H. J. Reich (1944) [https://archive.org/details/Reich_-_Theory_and_Applications_of_Electron_Tubes_1944 ''Theory and Applications of Electron Tubes'', 2nd ed.],. New York: McGraw-Hill Book Co. p. 56.</ref> These tubes were produced in Germany and known as Siemens-Schottky tubes.<ref name="ExpWireless" /> In Japan, Hiroshi Ando patented improvements to the construction of the screen grid in 1919.<ref name="Brown">{{cite book
  | last = Brown
  | first = L.
  | title = Technical and Military Imperatives: A Radar History of World War 2
  | publisher = CRC Press
  | year = 1999
  | pages = 35–36
  | url = https://books.google.com/books?id=uYgsr3exvS4C&pg=PA36
  | isbn = 9781107636187}} (Brown incorrectly gives Ando as first screen grid patent and gives incorrect account of Schottky).</ref> During the latter half of the 1920s, [[Neal H. Williams]] and [[Albert Hull]] at [[General Electric]], H. J. Round at MOV and [[Bernard Tellegen]] at Phillips developed improved screen grid tubes. These improved screen grid tubes were first marketed in 1927.<ref>Editors (Sept. 21, 1927) [https://worldradiohistory.com/UK/Wireless-World/20s/Wireless-World-1927-09.pdf "Guide to the Show Olympia 1927"]. ''Wireless World''. p. 375. Retrieved Oct. 12 2021</ref>

Feedback through the anode to grid capacitance (Miller effect) of the triode could cause oscillation, especially when both anode and grid were connected to tuned resonant circuits as is usual in a radio frequency (RF) amplifier.<ref>[https://archive.org/details/in.ernet.dli.2015.6382/page/n303/mode/2up Turner, L.B. (1931)] p. 257</ref> For frequencies above about 100&nbsp;kHz, neutralizing circuitry was necessary. A typical triode used for small-signal amplification had a grid to anode capacitance of 8&nbsp;[[farad|pF]], while the corresponding figure for a typical screen grid valve was 0.025&nbsp;[[farad|pF]].<ref name="C10">E. T. Cunningham, Inc. (1932) [http://nj7p.info/Manuals/PDFs/Tubes/Cunningham-C-10-1932.pdf ''The Cunningham Radio Tubes Manual, Technical Series No. C-10''], Harrison, NJ: E. T. Cunningham, Inc. pp. 22, 28</ref> Neutralizing circuits were not required for a well designed screen grid tube RF amplifier stage.<ref name="Henney01">Henney, K. (1938) [https://worldradiohistory.com/BOOKSHELF-ARH/Technology-Radio/Principles-of-Radio-Henney-3rd-1938.pdf ''Principles of Radio'', 3rd ed.]. New York: John Wiley & Sons, Inc. pp. 327 - 328. Retrieved 14 Oct. 2021</ref><ref>Hull, Albert W. (April 1926) "Measurements of High Frequency Amplification with Shielded-Grid Pliotrons". ''Physical Review'' Vol. 27. pp. 439 - 454.</ref>

The screen grid is connected to a positive DC voltage and at AC ground as insured by a [[bypass capacitor]] to ground.<ref name="Henney6" /> The useful region of operation of the screen grid tube as an amplifier is limited to anode voltages greater than the screen grid voltage. At anode voltages greater than the screen grid voltage some electrons from the cathode will hit the screen grid, producing screen current, but most will pass through the open spaces of the screen and continue to the anode.<ref name="Henney6" /> As the anode voltage approaches and falls below that of the screen grid, screen current will increase as shown in the plate characteristics image.

An additional advantage of the screen grid became apparent when it was added. The anode current becomes almost completely independent of the anode voltage, as long as the anode voltage is greater than the screen voltage. This corresponds to a very high anode dynamic resistance, thus allowing for a much larger voltage gain when the anode load impedance is large.<ref name="rider03">Rider, John F. (1945). [https://archive.org/details/Rider_-_Inside_the_Vacuum_Tube_1945/page/n301/mode/2up ''Inside the Vacuum Tube'']. New York: John F. Rider Publisher Inc. p. 286. Retrieved 10 June 2021</ref> The anode current is controlled by the control grid and screen grid voltages. Consequently, tetrodes are mainly characterized by their [[transconductance]] (change in anode current relative to control grid voltage) whereas triodes are characterized by their amplification factor (''mu''), their maximum possible voltage gain. At the time of the introduction of screen grid valves, a typical triode used in radio receivers had an anode dynamic resistance of 20&nbsp;kΩ or less while the corresponding figure for a typical screen grid valve was 500&nbsp;kΩ. A typical triode medium wave RF amplifier stage produced voltage gain of around 14, but screen grid tube RF amplifier stages produced voltage gains of 30 to 60.<ref>Henney (1938) pp. 317, 328</ref>

[[File:S23 screen grid valves in music magnet.jpg|thumb|right|Two S23 screen grid valves in a 1929 Osram Music Magnet receiver]]
To take full advantage of the very low grid-anode capacitance, the shielding between anode and grid circuits was observed in the construction of the radio. The S625 valve was mounted in a grounded, plane, metal shield aligned to correspond with the position of the internal screen grid. The input, or control-grid circuit was on one side of the shield, while the anode, or output circuit was on the other. In the receiver shown using S23 tubes, each entire stage of the 2-stage rf amplifier, as well as the tuned detector stage, was enclosed in an individual large metallic box for [[Electromagnetic shielding|electrostatic shielding]]. These boxes have been removed in the illustration, but the up-turned edges of the bases of the boxes can be seen.

Thus screen grid valves permitted better radio frequency amplification in the medium and high frequency ranges in radio equipment. They were commonly used in the design of radio-frequency amplification stage(s) of radio receivers from late 1927 through 1931, then were superseded by the [[pentode]] tube.