improved their electrodynamic performance. How it works Because of the every-household presence of the conventional television picture tube (Cathode-Ray Tube), it may be helpful to think of its principles of operation. Though the IOT does not produce a glowing phosphor output, internally many principles are the same. IOTs have been described as a cross between a klystron and a triode, hence Eimac's trade name for them, Klystrode. They have an electron gun like a klystron, but with a control grid in front of it like a triode, with a very close spacing of around 0.1 mm. The high frequency RF voltage on the grid allows the electrons through in bunches. High voltage DC on a cylindrical anode accelerates the modulated electron beam through a small drift tube like a klystron. This drift tube prevents backflow of electromagnetic radiation. The bunched electron beam passes through the hollow anode into a resonant cavity, similar to the output cavity of a klystron, and strikes a collector electrode. As in a klystron, each bunch passes into the cavity at a time when the electric field decelerates it, transforming the kinetic energy of the beam into potential energy of the RF field, amplifying the signal. The oscillating electromagnetic energy in the cavity is extracted by a coaxial transmission line. An axial magnetic field prevents space charge spreading of the beam. The collector electrode is at a lower potential than the anode (depressed collector) which recovers some of the energy from the beam, increasing efficiency. Two differences from the klystron give it a lower cost and higher efficiency. First, the klystron uses velocity modulation to create bunching; its beam current is constant. It requires a drift tube several feet in length to allow the electrons to bunch. 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. Secondly, since the klystron has beam current throughout the RF cycle, it can only operate as an inefficient class-A amplifier, while the grid of the IOT allows more versatile operating modes. 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 highest frequency achievable in an IOT is limited