View source for KT66

{{Short description|Audio frequency beam tetrode}}
{{Infobox vacuum tube
|image = KT66 tube.jpg
|image_size = 150px
|caption = Original G.E.C./M.O.V. KT66 Beam-Power Tetrode tube; this is an example from late production
|classification = Beam-Power Tetrode <!-- Mandatory -->
|service = [[Class-A amplifier]], (single-ended)<br />
[[Class-AB amplifier]], (push–pull)
|height_in = <!-- inches -->
|diameter_in = <!-- inches -->
|weight_lb = <!-- pounds -->

<!-- Cathode -->
|cathode_type = Indirectly heated <!-- Mandatory -->
|heater_voltage = 6.3
|heater_current = 1.27A
|filament_voltage = 
|filament_current =

<!-- Anode -->
|max_dissipation = 25 <!-- Mandatory -->
|max_voltage = 500

<!-- Screen 
|max_dissipation = 3.5
|max_voltage = 400
-->
<!-- Socket Connections -->
|pinout = {{plainlist|
Octal Base, (IO)
*Pin 1, Not Used
*Pin 2, Heater
*Pin-3, Anode/Plate
*Pin-4, Screen Grid, g2
*Pin-5, Control Grid, g1
*Pin-6, Not Used
*Pin-7, Heater
*Pin-8, Cathode-Beam Plates
}}
<!-- Class A amplifier ops -->
|class_a_amplification_factor =
|class_a_anode_voltage = 250
|class_a_anode_current = 85mA
|class_a_screen_voltage = 250
|class_a_bias_voltage = −15
|class_a_anode_resistance = 22K Ohms

<!-- Class C amplifier ops -->
<!-- Recommend the use of highest rated CCS specification -->
|class_c_power_output =
|class_c_anode_voltage =
|class_c_anode_current =
|class_c_screen_voltage =
|class_c_bias_voltage =

<!-- Class AB amplifier ops -->
<!-- Recommend the use of highest rated CCS specification -->
|class_ab_power_output =
|class_ab_anode_resistance =
|class_ab_anode_voltage = 400
|class_ab_anode_current = 62.5mA (Includes Screen Current)
|class_ab_screen_voltage = 400
|class_ab_bias_voltage = −38, (Class AB1)
|reference = Super Radiotron Valve Manual, Amalgamated Wireless Valve Co. Australia, June 1962<br />
Radio Valve Data, Eighth Ed. Ilife Books Ltd., London, 1966
}}
'''KT66''' is the designator for a [[vacuum tube#Beam power tubes|beam power tube]] introduced by Marconi-Osram Valve Co. Ltd. (M-OV) of [[United Kingdom|Britain]] in 1937 and marketed for application as a power amplifier for audio frequencies and driver for radio frequencies.<ref name="KT66intro">Editors, [https://worldradiohistory.com/UK/Wireless-World/30s/Wireless-World-1937-08.pdf  "The New Valves"], ''Wireless World'', August 20th 1937, p. 178</ref><ref name=OsramKT66>General Electric Co., Ltd., [https://worldradiohistory.com/hd2/IDX-UK/Technology/Technology-Early/Television-IDX/IDX/Television-and-Short-Wave-World-1939-09-S-OCR%20ver%202-IDX-14.pdf#search=%22kt66%22 "Osram Valves"], ''Television and Short-Wave World'', Sept. 1939, p. 524</ref>
The KT66 is a [[beam tetrode]] that utilizes [[collimated beam|partially collimated electron beams]] to form a low potential [[space charge]] region between the anode and screen grid to return anode [[secondary emission]] electrons to the anode and offers significant performance improvements over comparable power pentodes.<ref>G.E.C. Valves, [https://frank.pocnet.net/sheets/084/k/KT66_GEC.pdf ''Beam Tetrode KT66''], Apr. 1963</ref><ref name=dreyer>Dreyer, J. F. Jr., [https://worldradiohistory.com/Archive-Electronics/30s/Electronics-1936-04.pdf "The Beam Power Output Tube"], ''Electronics'', Vol. 9, No. 4, April 1936, pp. 18 - 21, 35</ref> In the 21st century, the KT66 is manufactured and used in some high fidelity audio amplifiers and musical instrument amplifiers.

== Overview ==
Although the [[RCA Corporation|RCA]] [[6L6]] of 1936 (the result of a license agreement between RCA and [[EMI]]) was the first successful beam power tube on the market, the KT66 of 1937 became almost equally famous, at least in Europe.

Because the beam tetrode design eliminated the ''tetrode kink'' in the lower parts of the tetrode's voltage-current characteristic curves, M-OV marketed this tube family as the "KT" series, standing for ''kinkless tetrode''.

The KT66 was one of the "International series" introduced in 1937. This series utilized the "American Octal" base and had characteristics equivalent to tubes by U.S. manufacturers.<ref name="KT66intro"/> A number of different KT tubes were later marketed by M-OV. Some, but not all, were versions of existing American [[beam tetrode]] tubes or European power [[pentode]]s, such as the KT66 (6L6GC similar), KT77 (EL34 and 6CA7 similar), [[KT88]] (6550), and KT63 (6F6, pentode but almost identical characteristics).

[[File:Sovtek kt66 power tetrode.jpg|200px|left]]The KT66 was very popular in British [[radio]]s and audio [[amplifier]]s. It was the standard output tube in the classic [[Quad Electroacoustics|Quad]] II (1952, a version of which is still being manufactured today) and in the [[LEAK]] Type 15 (1945) and TL/12 (1948), both among the earliest British [[hi-fi]] amplifiers. Because of their excellent electrical characteristics and overload tolerance, KT66s are preferred by some guitar players for use in [[guitar amp]]s in place of 6L6GC. However, the plate dissipation of the 6L6GC, at 30W, exceeds the KT66's 25W, and adjustment of the amplifier's [[Grid bias|bias]] is necessary.

M-OV ceased glass vacuum tube manufacturing in 1988; their old audio tube types became valuable collectibles.{{Citation needed|date=March 2012}} In 2004 original M-OV KT66 tubes (bearing the official "Genalex" marketing brand that M-OV used outside the UK), unused in original carton, sold for US$250. {{As of|2022}} KT66 tubes continued to be manufactured at EkspoPUL<ref>Reflektor Saratov - 50 years {{cite web |url= https://www.itweek.ru/themes/detail.php?ID=64652 |title= itWeek}}</ref> in Saratov, Russia ([[Genalex Gold Lion]]<ref>Genalex Gold Lion KT66,{{cite web |url=https://shop.ehx.com/item/kt66%20genalex/tubes-vacuum-genalex-gold-lion/ |title= New Sensor Corp.}}</ref> brand), JJ Electronic<ref>JJ KT66,{{cite web |url=https://www.jj-electronic.com/en/kt66 |title = JJ Electronic s.r.o.}}</ref> in Slovakia, and by [[Changsha Hengyang Electronics Company|Hengyang Electronics]]<ref>Legend Series KT66GE,{{cite web |url=http://www.psvane.com/fkjdxl/738.html |title= Changsha Hengyang Electronics Co., Ltd.}}</ref> at former Guiguang<ref>Rachel Zhang, Who was Guiguang? {{cite web |url=https://premiumvacuumtubes.com/who-was-guiguang/ |title= Premium Vacuum Tubes}}</ref> factory in Foshan city, southern China.

Some modern Russian manufacture Sovtek KT66 tubes are actually 6L6GC tubes in a KT66 style bottle. While these tubes have the same pinout and minimum tolerances required of a KT66 tube, they do not have the performance characteristics of a true kinkless tetrode KT66 tube.

By contrast the very latest Russian manufactured tubes (2012) not only carry the same internal electrode structure as the original KT66 (they now look the same) they also have the same rugged electrical characteristics and can withstand a high voltage on grid 2 comparable to the anode voltage rating, allowing greater power output afforded by higher voltage capability when run in ultralinear connection.

==See also==
* [[KT88]]
* [[6L6]]
* [[6CA7]] / [[EL34]]
* [[6V6]]
* [[807 (vacuum tube)|807]]

==References==
{{reflist}}
* Barbour, Eric. "History of the 6L6" in ''Vacuum Tube Valley'', issue 4 (1996), p.&nbsp;3.
* [[Otto H. Schade|Schade, O. H.]] "Beam Power Tubes" in ''[[Proceedings of the IRE]]'', February 1938.
* Stokes, John. ''70 Years of Radio Tubes and Valves''. Vestal Press, NY, 1982.
* Thrower, Keith. ''History of the British Radio Valve to 1940''. MMA International, 1982, p.&nbsp;59.

==External links==
* [http://tubedata.milbert.com/sheetsK.html Several tube datasheets]
* [http://tdsl.duncanamps.com/show.php?des=kt66 TDSL Tube data[KT66]

{{DEFAULTSORT:Kt66}}
[[Category:Vacuum tubes]]
[[Category:Guitar amplification tubes]]
[[Category:Telecommunications-related introductions in 1937]]