Controversy exists between the benefits of Klystrons, Gyrotrons, Magnetrons and solid state. In all modern klystrons, the number of cavities exceeds two. Some klystrons have cavities that are tunable. A contract for the new klystrons was sent to EMI near the end of 1957. No two klystrons are exactly identical (even when comparing like part/model number klystrons). Microwave tube designers took numerous steps to reduce microphonics in klystrons. These klystrons provide the phase coherent return transmission. These accelerators are typically powered by klystrons, which require a high-voltage supply. Some tubes, such as magnetrons, traveling-wave tubes, carcinotrons, and klystrons, combine magnetic and electrostatic effects. High-frequency electromagnetic waves are generated by oscillators (often by gyrotrons or klystrons) outside the torus. Ultimately, only four klystrons were fit instead of twelve. In wartime, all twelve klystrons would be used, three in each octave. But in older transmitters tetrodes or klystrons are also utilized. Eimac klystrons also has been chosen for NATO's tropospheric scatter communications network.