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?
\$\begingroup\$
\$\endgroup\$
7
-
\$\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\$– crasicAug 13, 2013 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\$– crasicAug 13, 2013 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\$– KazAug 13, 2013 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\$– crasicAug 13, 2013 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\$– crasicAug 13, 2013 at 23:09
|
Show 2 more comments
3 Answers
\$\begingroup\$
\$\endgroup\$
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.
\$\begingroup\$
\$\endgroup\$
1
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?
answered Aug 13, 2013 at 23:33
-
\$\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\$– user22230Aug 13, 2013 at 23:59
\$\begingroup\$
\$\endgroup\$
2
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.
-
\$\begingroup\$ The link you posted is to a file on your computer. Only you have access to it. \$\endgroup\$– JREAug 23, 2020 at 7:56
-
\$\begingroup\$ If that link doesn't work, try this one: The Bulletin of the British Wireless Society Vol 33 No. 2 The Beam Tetrode & click on The Beam Tetrode. \$\endgroup\$ Sep 2, 2020 at 1:13