Inductive output tube

[1] The primary commercial use of IOTs is in UHF television transmitters,[2] where they have mostly replaced klystrons because of their higher efficiencies (35% to 40%) and smaller size.

In research undertaken prior to the transition from analog to digital television broadcasting, it was discovered that electromagnetic interference from lightning, high voltage AC power transmission, AC rectifiers, and ballasts used in fluorescent lighting, greatly affected low-band VHF channels (In North America, channels 2,3,4,5, & 6) making it difficult to impossible to use them for digital television.

At the other end of the tube the beam does not produce a glowing phosphor picture as in a CRT, but passes through a resonant cavity which extracts its energy, then strikes a positive electrode and is absorbed.

High voltage DC on a cylindrical anode accelerates the modulated electron beam through a small drift tube like a klystron.

In contrast the IOT uses current modulation like an ordinary triode; most of the bunching is done by the grid, so the tube can be much shorter, making it less expensive to build and mount, and less bulky.

The grid can be biased so the beam current can be cut off during part of the cycle, enabling it to operate in the more efficient class B or AB mode.

The initial design difficulties of MSDCIOTs were overcome through the use of recirculating high dielectric transformer oil as a combined coolant and insulation medium to prevent arcing and erosion between the closely spaced collector stages and to provide reliable low-maintenance collector cooling for the life of the tube.

Earlier MSDC versions had to be air cooled (limited power) or used de-ionized water that had to be filtered, regularly exchanged and provided no freezing or corrosion protection.

Today's IOTs are equipped with coated cathodes that work at relatively low operating temperatures, and hence have slower evaporation rates, minimizing this effect.

[1] The purpose of the crowbar circuit is to instantly dump the massive electrical charge stored in the high voltage beam supply before this energy can damage the tube assembly during an uncontrolled cavity, collector or cathode arc.