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Beam Switching Tubes

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Beam switching tubes are thermionic decimal counting devices used in high speed applications through the 1950s and 1960s. They feature counting speeds up to 10 MHz (versus 1 MHz for the fastest dekatrons) and high electrical efficiency. Many types are direct-drive, producing output signals which can be coupled directly to a Nixie tube with minimal intermediate components.

The magnetic beam switching tube, alternately referred to as a 'magnetron beam switching tube', 'crossed field counting tube' or 'trochotron', operates on the principle of crossed electrical and magnetic fields: an electron beam is emitted from a centrally heated cathode, spirals outward under the influence of a permanent magnet, and sequentially strikes a series of spade electrodes. The potential between the cathode and a selected spade causes the beam to lock onto that spade, which passes current to its adjacent target electrode. Adjacent to each spade/target pair is a switching grid electrode which controls the positional advancement of the electron beam. Early MBSTs have a large cylindrical permanent magnet which encircles the glass envelope, while later types have 10 discrete internal magnets which also function as the target electrodes.

In addition to the conventional trochotrons, National Union designed a number of 'radial beam' or 'commutator' tubes, a series of 18- and 25-position devices somewhat similar in construction to a trochotron, but with an external polyphase wound stator instead of a permanent magnet. Two electron beams are generated within the tube, spaced 180 ° from each other. Radial beam tubes have a much simpler internal construction than a conventional trochotron, as beam focusing and rotation functions are controlled by the external stator. Radial beam tubes were not widely used, and cannot achieve the MHz-range switching frequencies of trochotrons.

Another somewhat similar device is the electrostatic beam switching tube, of which only one model was produced: the E1T. The E1T uses no magnets, and has a side-viewing phosphor screen which indicates count position.

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