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Klystrons and velocity modulated tubes
Klystrons are velocity modulated tubes designed to overcome the frequency limits of traditional
grid tubes. Pioneers in the development of the klystron were R.H. and S.F. Varian at the Stanford
University, which started from the Heil velocity modulation theory and from the ‘rumbatron’
doughnut resonators ideated by W.W.Hansen of the same university. In 1939 the Varian brothers
designed the device in which an electron beam could interact with a couple of rumbatron resonators.
The research was financed by Sperry Company.
Fig 1 – 1a shows the internal view of a 410R two-cavity klystron, which can be used either as amplifier or as oscillator.
1b shows the interior of a 2K25, a reflex klystron, used as X-band oscillator.
Referring to the twin resonator klystron of fig. 1a, the electron beam generated by the gun in its
travel toward the collector passes through the input gap of the buncher cavity. Under the effect of
the oscillating electric field in the gap, the velocity of some electrons is increased, while being
decreased for other electrons; subsequently there is a bunching effect on the electron flow in the
drift space. The bunched electrons induce oscillations in the catching cavity when passing through
the output gap. In the reflex klystron, fig 1b, there is just one cavity, which acts as buncher for
electrons emitted by the gun and attracted by the shell; bunched electrons are then forced back, by
the negative potential applied to the repeller (top cap), to cross again the resonator, now seen as
Other velocity modulated devices are TWTs and BWOs, shown in the last section. Here RF signal
travels along a helix surrounding a focused electron beam. These devices can be tuned over a wide
frequency range, adjusting the collector voltage.
Fig. 1
HP 1950-0004 tunes from 7 to 11GHz with external cavity; used in 620B generator.
2K25, a classic X-band reflex klystron. 25mW average out through the coaxial probe.
2K28 required an external cavity to generate 125mW average from 1200 to 3750MHz.
2K41 integral cavity reflex klystron tuned from 2260 to 3310MHz; 250mW out.
2K45 electronically tuned X-band reflex klystron. Thermal tuning was accomplished controlling
the current flowing in a diode whose plate was one wall of the resonator. The entire band was
swept in 6 seconds.
2K48 could be tuned from 4200 to 10750 MHz, depending on the mode and on the cavity.
417A integral cavity reflex klystron, tunable from 2650 to 3330MHz; 250mW out.
417C improved version of the above.
Fig. 2:
428A Western Electric tunes from 6.4 to 7.2GHz; designed for microwave relay systems.
5837 external cavity reflex klystron tunes from 550 to 3800MHz; 150mW out.
6236 reflex klystron; 3.8 to 7.6GHz, 125mW.
6390 external cavity reflex klystron; 6.7 to 11.05GHz, 60mW average out.
6BL6 reflex klystron; 1.4 to 6.5GHz, 250mW average out.
6BM6 external cavity klystron tunes from 550 to 3800MHz, 175mW.
6BM6A is similar to the 6BM6 above.
723A/B is a classic X-band design, followed by 2K25.
726B is similar to the 723A/B above, with tuning range from 3.2 to 3.4GHz.
Fig. 3
8RK8 from Elliot-Litton is a millimeter reflex klystron.
A160011A looks like a Polarad branded but Raytheon built klystron.
A160034A Polarad should be a selected 6390; 6.7 to 11 GHz.
CV2346 from EMI could operate from 8 to 10 GHz; 30 to 70mW output power.
CV67, one of the early British reflex klystrons, with external cavity factory assembled.
K-839A from Sylvania was used in microwave relay links.
EEV 367B is another klystron used in microwave communication equipment.
Fig. 4
TV222 from Thomson is a power klystron for communication relays; 7125 to 7245 MHz.
6975 reflex klystron is mechanically tunable over 8.5 to 9.6GHz range; 40mW.
QKK753 reflex klystron.
VA220 power klystron for communication relays; 1.2W out in the 5925 to 7425 MHz range.
VA244 power klystron for communication relays.
VX5029 reflex klystron from EMI cam operate from 1.8 to 4.5 GHz.
Fig. 5
SRL-17 power klystron from Sperry; 750 to 990MHz; 1.8W output power.
SRL-7C Sperry power klystron; 1890 to 2100MHz; 5 to 10W output power. Forced air cooled.
SRU-55A Sperry, Ka band, 14 to 17.5GHz; flanged waveguide out.
WL-417A Westinghouse equivalent for Sperry 417A
WL-417B Westinghouse improved version of the type above.
X-13 Varian, high stability oscillator; 8.2 to 12.4 GHz, micrometer tuning. 90mW out.
TWTs and BWOs
Top: Philips 55340 is a power TWT amplifier; 3.8 to 4.2 GHz, 8W output.
Bottom: EEV CV2393 BWO; 7GHz to 11.5Ghz electrically tunable; 120mW output power.