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from Wikipedia
The KT88 is a beam tetrode/kinkless tetrode (hence "KT") vacuum tube for audio amplification.
| KT88 | |
|---|---|
 KT88 labeled G.E.C. made by GEC/MOV in the U.K. | |
| Classification | Beam-power tetrode |
| Service | Class-A amplifier, (single-ended) class-AB amplifier, (push-pull) |
| Cathode | |
| Cathode type | Indirectly-heated |
| Heater voltage | 6.3 |
| Heater current | 1.6A |
| Anode | |
| Max dissipation Watts | 42W |
| Max voltage | 800 |
| Socket connections | |
| Octal base, (IO) Pin 1, Not used | |
| Typical class-A amplifier operation | |
| Anode voltage | 250V |
| Anode current | 140mA |
| Screen voltage | 250V |
| Bias voltage | -15V |
| Anode resistance | 12 kilohms |
| Typical class-AB amplifier operation (Values are for two tubes) | |
| Power output | 100W |
| Anode voltage | 600V |
| Anode current | 100mA |
| Screen voltage | 350V |
| Bias voltage | -45 (class AB2) |
| References | |
| Super Radiotron Tube Manual, Amalgamated Wireless Valve Co. Australia, June 1962 Radio Tube Data, Eighth Ed. Ilife Books Ltd., London, 1966 | |
Features
[edit]The KT88 fits a standard eight-pin octal socket and has similar pinout and applications as the 6L6 and EL34. Specifically designed for audio amplification, the KT88 has higher plate power and voltage ratings than the American 6550. It is one of the largest tubes in its class and can handle significantly higher plate voltages than similar tubes, up to 800 volts. A KT88 push-pull pair in class AB1 fixed bias is capable of 100 watts of output with 2.5% total harmonic distortion or up to about 50W at low distortion in hi-fi applications. The transmitting tubes TT21 and TT22 have almost identical transfer characteristics to KT88 but a different pinout, and by virtue of their anode being connected to the top cap have a higher plate voltage rating (1.25 kilovolt) and a higher power output capability of 200 watts in class AB1 push–pull.[1]
The screen grid is sometimes tied to the anode so that it becomes effectively a triode with a lower maximum power output.
History
[edit]The KT88 was introduced by GEC in 1956 as a larger variant of the KT66. It was manufactured in the U.K. by the MOV (Marconi-Osram Valve) subsidiary of G.E.C, also labelled as IEC/Mullard, and, in the U.S., Genalex Gold Lion.[2]
As of 2022, KT88 valves are produced by New Sensor Corporation (Genalex Gold Lion[3] and Electro-Harmonix brands) in Saratov, Russia, JJ Electronic[4] in Čadca, Slovakia and Hengyang Electronics[5] (Psvane brand) at former Guiguang factory in Foshan, China.
NOS examples in good condition are extremely rare. Due to its availability and characteristics, the KT88 is popular in hi-fi production amplifiers.
Historically, it has been far more popular with high fidelity stereo manufacturers than guitar amplifier builders, given its characteristics of high-power and low-distortion. Due to these characteristics, it is regularly used to replace 6550 tubes by end users seeking a guitar amplifier tone with less distortion.[6] Some of the amplifiers which shipped with the KT88 power tube include the Hiwatt, Marshall Major, and some Ampeg models.[7]
Characteristics
[edit]-
Anode characteristics
See also
[edit]References
[edit]- ^ https://frank.pocnet.net/sheets/086/k/KT88.pdf KT88 Datasheet
- ^ Barbour, Eric. "6550/KT88 Power Kings Part II" (PDF). worldradiohistory.com. Vacuum Tube Valley Quarterly, 2003, Issue 19. Retrieved 23 April 2021.
- ^ Genalex Gold Lion KT88,"New Sensor Corporation".
- ^ JJ KT88,"JJ Electronic s.r.o."
- ^ Classic Series KT88C,"Changsha Hengyang Electronics Co., Ltd".
- ^ Fliegler, Ritchie; Jon F. Eiche (1993). Amps!: the other half of rock 'n' roll. Hal Leonard. p. 15. ISBN 978-0-7935-2411-2.
- ^ Aspen Pittman (2003). The Tube Amp Book. Backbeat. pp. 125–. ISBN 978-0-87930-767-7.
External links
[edit]from Grokipedia
Overview and Design
Physical Construction
The KT88 utilizes an octal base, specifically a metal shell wafer design that provides robust mechanical support and electrical connectivity.[6] The pin configuration includes: pin 1 connected to the suppressor grid (internally tied to the cathode; externally no connection), pins 2 and 7 to the heater, pin 3 to the anode, pin 4 to the screen grid, pin 5 to the control grid, pin 6 no connection, and pin 8 to the cathode.[7] As a beam power tetrode, also known as a kinkless tetrode, the KT88 incorporates a suppressor grid internally connected to the cathode, which minimizes secondary electron emission from the anode and enhances beam focusing for efficient operation.[8] The tube features an indirectly heated oxide-coated cathode, with the heater operating at 6.3 volts and 1.6 amperes.[3] The glass envelope is tubular in shape, with a maximum overall length of 125 mm, a seated length of 110 mm, and a diameter of 52 mm, providing ample space for internal components while facilitating heat dissipation.[3] Key internal elements include a nickel-plated steel anode for durability and thermal management, and a copper screen grid to support high-power handling.[9] Mounting recommendations emphasize vertical orientation to optimize heat dissipation, with a minimum socket center-to-center spacing of 4 inches between tubes; for horizontal mounting, pins 4 and 8 should align to prevent mechanical stress and ensure even cooling.[4] Variations in construction exist between the original GEC design, which employed black-coated plate anodes for improved thermal properties, and later copies from manufacturers like JJ Electronic, which often use grey plate anodes while maintaining the core beam tetrode structure.[9]Basic Principles of Operation
The KT88 functions as a beam tetrode, in which the indirectly heated cathode emits electrons through thermionic emission when the filament is energized at 6.3 volts and 1.6 amperes, providing the primary source of electrons for amplification. The control grid, positioned closest to the cathode, modulates the intensity of this electron flow by varying its negative bias relative to the cathode, controlling the number of electrons that proceed toward the anode. The screen grid, held at a positive potential, accelerates these electrons past the control grid while electrostatically shielding it from the anode's varying field, maintaining stable gain characteristics.[10][11] To form a focused electron beam, beam-forming plates connected to the cathode potential are incorporated between the screen grid and anode, directing the electrons into dense, ribbon-like sheets rather than a diffuse cloud; this beam focusing minimizes partition noise by reducing the random division of electron current between the screen and anode, as most electrons are channeled directly to the plate. The precise alignment of the helical windings on the control and screen grids further compresses the electron stream, creating a region of high electron density that behaves like a virtual cathode between the screen and anode. This configuration also repels secondary electrons emitted from the anode back toward the plate, lowering screen grid current to a minimal fraction of the total.[10][12] The KT88's kinkless design eliminates the characteristic "kink" or negative resistance region in the tetrode's output characteristics—caused by secondary emission in standard tetrodes—through the virtual cathode's suppressive effect, which linearizes electron flow and reduces distortion while minimizing grid current draw. By concentrating electrons into beams, this design enhances overall efficiency and beam density without requiring a separate suppressor grid, as found in pentodes. Consequently, the KT88 achieves higher power output than triodes, which lack grid shielding for high voltages, and lower distortion than conventional pentodes, owing to the controlled beam dynamics that limit unwanted electron scattering.[12][10] For applications requiring triode-like performance, the KT88 can be strapped into triode mode by electrically connecting the screen grid to the anode, effectively neutralizing the screen's accelerating role and merging its potential with the plate; this alters the tube's characteristics to follow a three-halves power law at lower voltages, yielding smoother load lines with reduced higher-order harmonics compared to tetrode operation.[12]Historical Development
Origins and Introduction
The KT88 vacuum tube was developed by the General Electric Company (GEC) in the United Kingdom as a high-power successor to the earlier KT66 beam tetrode, addressing the need for greater output capability in audio applications. Introduced in 1956, it was manufactured by GEC's Marconi-Osram Valve (MOV) subsidiary and quickly gained recognition for its robust performance in push-pull configurations.[4][13] This development occurred amid the post-World War II surge in consumer interest in high-fidelity sound reproduction, as home hi-fi systems became increasingly popular in the 1950s, driven by advancements in phonograph records and amplifier technology. The KT88 was specifically engineered for high-fidelity audio amplification, enabling amplifiers to deliver up to 100 watts from a pair of tubes in Class AB1 operation with low distortion. Early marketing and datasheets from GEC emphasized its 42 W maximum plate dissipation rating, positioning it as a premium choice for audiophile-grade equipment.[14][4] The tube also received a military designation as the CV5220, reflecting its commercial adaptation from ruggedized designs suitable for reliable operation in demanding environments. Concurrently, the American firm Tung-Sol introduced the similar 6550 beam tetrode in 1955, which shared comparable electrical characteristics and became a direct equivalent, fostering interchangeability in international audio circuits. Physically, the KT88 retained a similar cylindrical envelope to the KT66 but with enhanced internal structures for higher power handling.[4]Manufacturers and Production Timeline
The KT88 vacuum tube was originally produced by the General Electric Company (GEC) in the United Kingdom, with manufacturing handled by its Marconi-Osram Valve (MOV) subsidiary starting in 1956.[2] Production continued through the 1960s and 1970s at facilities like the Hammersmith plant, where GEC/MOV created the tube under various brandings, including Genalex Gold Lion, which was essentially a rebranded version of the same UK-made product.[5] By the early 1980s, Western production had largely ceased due to the widespread adoption of solid-state technology in audio equipment, leading to a decline in demand for vacuum tubes; the last known GEC/MOV KT88s were made around 1986.[5] In Eastern Europe, production persisted beyond the Western discontinuation, with the Reflektor factory in Saratov, Russia—operated by New Sensor Corporation—resuming KT88 manufacturing in the 1990s under brands like Electro-Harmonix and a reissued Genalex Gold Lion.[15] This Russian output filled gaps left by the end of UK production, often rebranding Soviet-era designs for export. Meanwhile, JJ Electronic in Slovakia began KT88 production in the mid-1990s, initially as the KT88S variant developed in collaboration with the former Tesla factory, transitioning to full commercial output by the early 2000s.[16] Chinese manufacturing emerged in the late 1980s, with Shuguang Electron Tube Factory starting KT88 production in the late 1980s as part of its expansion to over 120 tube types.[17] Changsha Hengyang Electronics Co., Ltd. (associated with Psvane) also entered the market in the late 2000s, leveraging facilities in Hunan Province for high-volume KT88 output, including premium lines like the Treasure series.[17] The 2010s saw periodic shortages of reliable KT88s due to factory disruptions, which drove up prices for both NOS and new production tubes by 20-50% in some markets.[18] These issues were further exacerbated following the 2022 Russian invasion of Ukraine and subsequent Western sanctions, which disrupted supply from Russian manufacturers and increased costs. This scarcity prompted reissues, such as New Sensor's Genalex Gold Lion KT88 in the mid-2000s, aimed at recreating the original's specifications using modern Russian processes to meet audiophile demand.[19] Quality comparisons highlight differences between originals and contemporaries: GEC/MOV KT88s are prized for their warm, vintage tonal character but suffer from inconsistencies due to aging production methods, while modern Russian (New Sensor) and Slovakian (JJ) versions offer greater reliability and uniformity in output, though some users note a less nuanced "vintage tone."[16] Chinese KT88s from Shuguang and Psvane provide cost-effective alternatives with improved consistency over early batches, but they can exhibit brighter highs and variable longevity compared to European counterparts.[20] As of November 2025, KT88 production continues at three major facilities: New Sensor in Russia, JJ Electronic in Slovakia, and Shuguang/Psvane in China, ensuring steady supply for audio applications despite ongoing export challenges from sanctions.[21] Following the 2022 Russian invasion of Ukraine and subsequent Western sanctions, supply of Russian-made KT88s has faced disruptions, increasing reliance on Slovakian and Chinese production. Matched pairs of current-production KT88s typically retail for $100-300, depending on brand and testing standards, reflecting stable market availability.[22]Electrical Characteristics
Key Specifications
The KT88 is a beam power tetrode with key electrical ratings defined in its original datasheet, establishing it as a high-power output tube suitable for audio amplification. The maximum plate voltage is rated at 800 V, with a screen voltage limit of 600 V, allowing operation at elevated potentials while maintaining structural integrity. Plate dissipation is specified at an absolute maximum of 42 W, and the grid bias can reach -200 V under fixed bias conditions, though typical operating biases are less severe to prevent instability. These limits ensure safe operation within tetrode configurations, referencing the tube's inherent beam-forming structure for efficient electron flow.[23] In typical Class AB1 push-pull operation, a pair of KT88 tubes delivers up to 100 W of output power at 2.5% total harmonic distortion when driven with a 250 V RMS input signal, using fixed bias at plate voltages around 560 V. The filament requires 6.3 V at 1.6 A, supporting indirect heating for low noise. Transconductance measures 11.5 mA/V (11,500 µmhos) under standard conditions of 250 V plate and screen voltages, providing strong amplification gain. Cathode current is limited to 230 mA maximum, balancing power handling with thermal management.[23]| Parameter | Value | Notes/Source |
|---|---|---|
| Maximum Plate Voltage | 800 V | Absolute limit[23] |
| Maximum Screen Voltage | 600 V | Absolute limit[23] |
| Maximum Plate Dissipation | 42 W | Anode only; total with screen 46 W[23] |
| Maximum Grid Bias | -200 V | Fixed bias limit[23] |
| Filament Voltage/Current | 6.3 V / 1.6 A | Heater rating[23] |
| Transconductance | 11.5 mA/V | At 250 V plate/screen[23] |
| Output Power (Pair, AB1) | 100 W | At 2.5% THD, 560 V plate[23] |