I'm building my own guitar tube amp, and now I have a dilemma. The speaker magnet is so close to the preamp tubes that I'm worry about interference between both.
Should I be worried or that is not a problem at all?
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Since tubes make electrons traveling in vacuum between electrodes they are sensitive to magnetic fields. But in preamp tubes, their grids are so close and the electrons are so fast, a magnet can hardly disturb their path. Nevertheless, some power tubes like beam tetrodes use electrostatic deflectors to concentrate electrons into beams. These kind of tubes must be kept in some distance of transformers and speaker magnets.
The magnetic field of the speaker will have a negligible effect on the electrical performance of the tubes.
However, there are other considerations such as heat and vibration.
Is there still adequate airflow around the tubes so that they don't overheat, or heat the speaker excessively?
In addition to microphonic effects that other commenters have alluded to, is there any possibility that the speaker magnet can pull the tubes out of their sockets over time, perhaps aided by vibration of the chassis?
I came across a very interesting article by L L (Bill) Williams, published in The Bulletin of the British Wireless, about beam power tetrodes.
The beams are not those viewed from above, formed by the beam forming plates. That is a misnomer. The original name "beam confining" plates is more correct. The beams are actually the sheets of electrons formed as they go around the wires of G1, missing the G2 wires hidden behind, then converging into a dense pack, forming a virtual cathode near the anode, forcing the electrons onto the anode.