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Ring Hybrid
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Two klystrons operate in parallel as a balanced amplifier with a coax power divider at the input and a waveguide 90° hybrid combiner at the output. The input power divider is actually a ring hybrid and the divided outputs have the same phase. However, one of these outputs is delayed additional 90° by a remotely adjustable motor-driven transmission line "trombone." The isolated port on the output combiner is terminated with a "wasted load." Nominally there should be no power dissipated in the waster if the trombone section is adjusted correctly but, if the two klystrons do not have identical output power, some wasted power is inevitable. If the phase is wrong by 190°, all power will be diverted to the waster instead to the antenna. Directional couplers allow the operator to monitor the total output power, the waster power, and the individual drive and output powers.
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Ring Hybrid
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The klystron beam current is modulated, i.e. turned on only for the duration of each RF drive pulse. During the pulse, 35 % of the beam power (beam voltage × beam current) is converted to useful RF output power and 65 % is converted to wasted heat. The output power during the pulse is 2.5 MW and the input power is 2.5/0.35 = 7.14 MW. The average powers are 150 kW and 429 kW, respectively. When there is no RF drive, 100 % of the beam power is converted to heat. If the beam were not turned off between pulses, the input power to the transmitter would be 7.14 MW/0.06 = 119 MW. Pulsing the beam at a 6 % RF duty factor reduces the input power to 429 kW, a considerable saving in power! The klystrons, of course, cannot dissipate enough heat to run with the beam on continuously; their maximum duty factor is 6 %.
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Klystron A on the Klystron Vault
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Beam pulsing is done by means of a "mod anode" control element built into each klystron. When the mod anode is biased about halfway between the cathode and anode voltages, the beam current is turned on. When the mod anode is biased slightly (5 kV) more negative than the cathode voltage, the beam is completely turned off. The complete turn off is needed, even in the absence of RF drive, to prevent the klystron from generating noise, some of which would leak through the vacuum tube switching circuit that connects the mod anodes (which are connected in parallel) to a -55 kV "half voltage" tap on the beam power supply or to the chassis of the "buffer deck" which is at a potential 5 kV more negative than the cathodes. For high voltage insulation, a fiber optic link to the floating deck is used to turn on the beam. A second fiber optic link, to the buffer deck, pulses the buffer for 2 microseconds following the beam pulse to bring the mod anodes back down to cathode potential, turning off the beam.