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Electrons in Picture tubes
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The M14-170W is a delightful little picture tube that is representative of all modern television tubes. The neck is 20 mm in diameter and the bell flares out to a rectangular screen. This small example has 90 degree deflection and domestic television tubes flare out at 110 degrees.
Focus is by electromagnetic coils and so is deflection. The choice of magnetic over electrostatic is mainly due to the increased angle of deflection that is possible with the magnetic system. The choice was made over 50 years ago that television should adopt magnetic deflection. In those days a second argument was the degree of manufacturing precision required in electrostatic deflection plates was greater than the corresponding engineering required for magnetic deflection. It is also true that EMI had the technology for magnetic deflection in-house and so were comfortable in using it. The Iconoscope camera tube also used magnetic deflection as did the EMI version. In the intervening years development has taken place in circuit requirements for the magnetic system and it has remained supreme.
The deflection coil assembly is now fixed to the tube in manufacture. Originally the two were separate so that the tube could be replaced. Today the tube life is so great, that the life of the tube is probably greater than the equipment around it.
The screen is 125 mm across the diagonal, and the envelope is 170 mm long 93 mm high and 119 mm wide.
Reference: Observation.


Updated April 23, 2013.
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