page_title: Vacuum tube
section_title: Beam power tubes
hierarchical_section_title: Vacuum tube / History and development / Beam power tubes
caption_attribution_description: by en:User:EricBarbour Pair of different 6L6GC tubes - (l) General Electric version from 1960s (r) currently made version by Svetlana Electron Devices, Russia en:Image:6L6tubespair.jpg
context_page_description: A vacuum tube, an electron tube, valve or tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
  The type known as a thermionic tube or thermionic valve uses the phenomenon of thermionic emission of electrons from a hot cathode and is used for a number of fundamental electronic functions such as signal amplification and current rectification. Non-thermionic types, such as a vacuum phototube however, achieve electron emission through the photoelectric effect, and are used for such purposes as the detection of light intensities. In both types, the electrons are accelerated from the cathode to the anode by the electric field in the tube.
  The simplest vacuum tube, the diode, invented in 1904 by John Ambrose Fleming, contains only a heated electron-emitting cathode and an anode. Electrons can only flow in one direction through the device—from the cathode to the anode. Adding one or more control grids within the tube allows the current between the cathode and anode to be controlled by the voltage on the grids.
context_section_description: The beam power tube is usually a tetrode with the addition of beam-forming electrodes, which take the place of the suppressor grid. These angled plates (not to be confused with the anode) focus the electron stream onto certain spots on the anode which can withstand the heat generated by the impact of massive numbers of electrons, while also providing pentode behavior. The positioning of the elements in a beam power tube uses a design called "critical-distance geometry", which minimizes the "tetrode kink", plate to control grid capacitance, screen grid current, and secondary emission from the anode, thus increasing power conversion efficiency. The control grid and screen grid are also wound with the same pitch, or number of wires per inch. The windings of the control and screen grid wires are aligned such that the screen grid is in the "shadow" of the control grid. The two grids are positioned so that the control grid creates "sheets" of electrons that pass between the screen-grid wires. 
  Aligning the grid wires also helps to reduce screen current, which represents wasted energy. This design helps to overcome some of the practical barriers to designing high-power, high-efficiency power tubes. EMI engineers Cabot Bull and Sidney Rodda developed the design which became the 6L6, the first popular beam power tube, introduced by RCA in 1936 and later corresponding tubes in Europe the KT66, KT77 and KT88 made by the Marconi-Osram Valve subsidiary of GEC (the KT standing for "Kinkless Tetrode").
  "Pentode operation" of beam power tubes is often described in manufacturers' handbooks and data sheets, resulting in some confusion in terminology. While they are not strictly pentodes, their overall electrical behavior is similar.
  Variations of the 6L6 design are still widely used in tube guitar amplifiers, making it one of the longest-lived electronic device families in history. Similar design strategies are used in the construction of large ceramic power tetrodes used in radio transmitters.
  Beam power tubes can be connected as triodes for improved audio tonal quality but in triode mode deliver significantly reduced power output.
caption: 6L6 tubes in glass envelopes
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