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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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  • \$\begingroup\$ Define close? tube amps in old style combos were often mounted with the tubes facing into the speaker cabinet somewhat close to the speaker (like this ), unless you are literally touching tube to speaker I wouldn't be too worried. \$\endgroup\$ – crasic Aug 13 '13 at 22:53
  • \$\begingroup\$ It may increase the possibility of ringing or feedback whine when driven loud, but this is often a desirable sound \$\endgroup\$ – crasic Aug 13 '13 at 22:55
  • \$\begingroup\$ @craisic That is hardly the case! Guitar feedback through the instrument body and strings is desirable, but not circuit board microphonics (tube, or otherwise). \$\endgroup\$ – Kaz Aug 13 '13 at 23:02
  • \$\begingroup\$ @Kaz Depends on your definition of desirable I guess, I made an amp for hardcore metal guitarist who asked for just that. \$\endgroup\$ – crasic Aug 13 '13 at 23:03
  • \$\begingroup\$ @Kaz, regardless, in my experience, preamp tubes will not tend to be the cause of volume dependent microphonics (i.e. microphonics/feedback that become audible only when the amp is cranked) because of the low gain in the preamp stage, power tubes are the more likely to behave that way. \$\endgroup\$ – crasic Aug 13 '13 at 23:09
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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.

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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?

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  • \$\begingroup\$ I didn't actually build the amp yet, but I think it will be about 10~15 cm of the speaker. My really concern is about the life cicle of the tubes, there is any effect? \$\endgroup\$ – user22230 Aug 13 '13 at 23:59

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