Inductive output tube

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The Inductive Output Tube was invented in 1938 by Andrew V. Haeff. A patent was later issued for the Inductive Output Tube to Andrew V. Haeff and assigned to the Radio Corporation of America. During the 1939 New York World's Fair the Inductive Output Tube was used in the transmission of the first television images from the Empire State Building to the fair grounds. RCA sold a small inductive output tube commercially for a short time, under the type number 825. It was soon obsoleted by newer developments, and the technology lay more or less dormant for years.

The Inductive Output Tube (IOT) has reemerged within the last twenty years after having been discovered to possess particularly suitable characteristics for the transmission of digital television and high definition digital television. The power output of the modern 21st Century Inductive Output Tube (IOT) is orders of magnitude higher than the first IOTs produced by the Radio Corporation of America (RCA) in 1940/1941 but the fundamental principle of operation basically remains the same.

The inductive output tube or IOT is a variety of vacuum tube which evolved in the 1980s to meet increasing efficiency requirements for high-power RF amplifiers. The primary commercial use of IOTs is in UHF television transmitters, where they have mostly replaced klystrons because of their higher efficiencies (35% to 40%) and smaller size. IOTs are also used in particle accelerators.

IOTs have been described as a cross between a klystron and a triode, hence one manufacturer's trade name for them, Klystrode. They have a cathode with a control grid 0.1 mm in front of it like a triode. They then use high voltage DC and a magnetic lens to focus a modulated high energy electron beam through a small drift tube like a klystron. This drift tube prevents backflow of electromagnetic radiation. The bunched electron beam passes through a resonant cavity, equivalent to the output cavity of a klystron. The electron bunches excite the cavity, and the electromagnetic energy of the beam is extracted by a coaxial transmission line.

The highest frequency achievable in an IOT is limited by the grid-to-cathode spacing. The electrons must be accelerated off the cathode and pass the grid before the RF electric field reverses direction. The upper limit on frequency is approximately 1300 MHz.

Heat radiation from the cathode heats the grid; as a result, low-work-function cathode material evaporates and condenses on the grid. This eventually leads to a short between cathode and grid, as the material accreting on the grid narrows the gap between it and the cathode. In addition, the emissive cathode material on the grid causes a negative grid current (electrons sent from the grid to the cathode) that may cause problems to the grid power supply if getting too high (causing the grid (bias) voltage to change). Today's IOTs are equipped with coated cathodes that work at very low operation temperatures, and hence have very low evaporation rates, preventing this effect. IOTs, like most linear beam tubes having external tuning cavities to achieve bandwidth, are equipped with arc detectors located in the output cavities, which trigger a crowbar circuit based on a hydrogen thyratron in the high-voltage supply. Latest versions of IOTs achieve even higher efficiencies (60%-70%) through the use of a Multistage Depressed Collector (MSDC) (one manufacturer's version is called Constant Efficiency Amplifier (CEA) or another manufacturer's version ESCIOT (Energy Saving Collector IOT)). The design difficulties of an MSDC on IOTs have been overcome by the use of transformer oil as a combined cooling and insulation media to avoid arcing and erosion between the collector stages and to provide maintenance free cooling operation for the life of the tube. Earlier MSDC versions had to be air cooled (limited power) or used de-inonized water that had to be filtered, regularly exchanged and provided no freezing and corrosion protection.