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Questions about tubes and gear that glows. FAQ

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Beam deflection

Posted by Jim Hagerman on April 30, 2007 at 22:30:58:
You get a similar problem with color CRTs (like we still use them). It seems unlikely a tube would get magnetized by ordinary use. However, something external could have programmed them with some magnetism. The coercivity of nickel (or was it remanence, I forget) is enough to hold a magnetic charge. That is, act like a permanent magnet itself.
The electron beam in a tube can get deflected in perhaps an undesirable fashion. This steering effect could alter tube operation in a subtle way. That is, the electrons do not follow their originally intended path.
With color CRTs, the effect is obvious. Putting a magnet near the screen deflects the trajectory of the electron beam, the misalignment causing a color shift by striking the wrong phosphors. Do NOT try this at home, unless you know you can degauss it. In fact, the Earth's magnetic field is strong enough to do the same thing. Just turn your TV sideways 90 degrees without turning it off. It doesn't take much. And the electron velocity is much lower in an audio vacuum tube, so the effect is greater (more time spent in field).
jh