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In Reply to: Re: Can a tube become magnetic with use? posted by Michael Samra on April 28, 2007 at 23:38:40:
in my experimentation, and I also tried passing a hand held degausser over the tubes in circuit, degaussing a tube greatly increases the top end. Usually in addition to the cathodes you mentioned, but the plates are often nickel and thus susceptible to magnetism.
I used a 2800 gauss unit, but passing it over some power tubes was a bit scary as I could hear the grids rattling. While the effect was quite nice, it was very transitory, and within a minute period I could hear the top end dropping back to normal. I've never tried a more powerful degausser.
Follow Ups:
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).
Jim:
I've wondered about this myself. Lots of guitar amplifiers have the power tubes an inch or less from the speaker magnet, which is very strong in some amps.Reason dictates that this would cause a hot spot on the plate in the direction of the magnet, but I have not seen that problem.
Now we are getting somewhere. I have had the tubes running for some 6 hours since demagnetised, & I don't think I have heard them this good before. The Seimens 6922 have a very sparkly top end, so I cannot say I noticed any difference in that department. I don't know how many gauss this unit is, but I had to put them in a plastic box in cotton wool or your hand would want to smack the side plate.
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