[Al Skierkiewicz]

The output amplifier is a IOT (Inductive Output Tube) The tube runs with 32-35kV on the collector and makes about 2 amps of cathode current in normal operation. The tube couples output power by running the electron beam (magnetically focused) through a cavity tuned to the output frequency. This causes the electron beam to act as a 1/2 turn transformer. If anything happens to drive, output coupling, cooling water, high voltage, filament (I seem to remember we are running at 5 volts, 27 amps on the filaments), or active arc detection in the output cavities, the high voltage must be removed from the very expensive tube to prevent permanent damage. In addition to the motorized breakers, there is a crowbar protection circuit (a gas discharge tube and drive circuits) which shorts across the high voltage should anything in the life support systems signify an issue. To test the crowbar, a test fixture consisting of a one foot piece of #36 wire is placed across the high voltage inside the transmitter cabinet. The bottom of the test fixture is an air operated switch made from 1/2" capped copper pipe and a bladder. A test of the crowbar is determined by stomping the foot operated bladder, shorting out the high voltage with the #36 wire. The crowbar passes if the #36 wire doesn't vaporize. When the switch engages, circuitry tells the crowbar to fire, and the motorized breakers to pull out. The main three phase power cabinet is 4 feet wide and eight feet high. It contains the motorized breakers and the step start relays for the power supply plus the resistors used for step start. When you stomp the test fixture, the wiring bangs inside the conduit, the transmitter relays and warning alarms go off, and the breakers and all the step relays bang away. Drive power amps also go into protect during this time as well. If you don't know it's coming, the noise will knock you down. Everyone on the floor and the floor below know when you run the test. Each transmitter should get tested at least twice a year. The crowbar tube should get replaced if it fails or as part of normal maintenance at some interval.