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Thread: tube/speaker magnet proximity

  1. #1
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    tube/speaker magnet proximity

    Are there any problems with having speaker magnets too close to the tubes of an amplifier? I built a cabinet where the tubes are positioned about an 1 1/2" to 2" away from the speaker, and can't help but wonder if there might be some effect on the electrons in the tubes.

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    Supporting Member tubeswell's Avatar
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    I remember asking this same question a few years back and the answers I got at the time pretty much amounted to 'no effect worth worrying about' IIRC

    I have since built a number of combos where the tubes are sitting right next to the speaker magnets, and I feel reasonably confident in hindsight that those earlier answers I got were right on the money.
    Building a better world (one tube amp at a time)

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    I beg to differ here. I have seen problems first-hand. The first one drove me crazy. It was about 10 years or so ago with a Music Man amp in which the customer had retrofitted another speaker (I think it was an E-V). Anyway, the power tubes were just a hair away from the speaker magnet. The customer was complaining that the amp just didn't sound right, and he was correct... it DIDN'T! There was something ratty about the output that I couldn't put my finger on. I removed the chassis from the cabinet, placed it on a load and scope and saw nothing but clean audio, but I put it back in the cab on the internal speaker and YUK!!! I did this a few times and then decided to see what the waveform looked like on the speaker itself. I didn't want to extend the cables, so I just placed the chassis in the box and hung my scope leads across the speaker terminals. BINGO! There was one oddly distorted sine wave, the likes of which I had never seen in a tube amp. There was also a slight power reduction. I inched the chassis out to go probe for voltages near the power tubes, and the distortion went away. Needless to say it only took another second to figure out the issue. Since then, I have seen this happen to varying degrees on other amps. Some were audible, so weren't.

    Electron fields can be deflected by magnetic fields. It's the same principle that make CRT's work in scopes and TV's. If the speakers' magnetic field is strong enough and close enough to a tube, it WILL affect it in some way. This is where a scope comes in handy for waveform monitoring.
    John R. Frondelli
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    I asked this on another forum someone said that in Morgan Jones' book
    on building valve amplifiers using a beam tetrode too close to the magnetic field would have an affect. hmmm ... maybe over worrying about this but perhaps I'll experiment with some wood between the magnet and tubes. I'm using 6550's and they are basically the same as 6L6 but bigger and are called beam pentodes but really are tetrodes.

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    Well if its 'true' that there is an audible effect, wood won't stop it from happening. You'd need fairly thick grounded steel I would say. BUt I can't say I noticed any effect myself - I guess it might depend on the particulars of the circuit you are dealing with. The ones I have built like this - tweed type combos - may not get into a funky enough frequency range to trigger anything noticeable in this regard. I certainly can't hear any problems in these amps (if I may say that about my own amps )
    Building a better world (one tube amp at a time)

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    Quote Originally Posted by pontiacpete View Post
    I asked this on another forum someone said that in Morgan Jones' book
    on building valve amplifiers using a beam tetrode too close to the magnetic field would have an affect. hmmm ... maybe over worrying about this but perhaps I'll experiment with some wood between the magnet and tubes. I'm using 6550's and they are basically the same as 6L6 but bigger and are called beam pentodes but really are tetrodes.
    I would think that beam tubes are more prone to this because of the concentrated electron field formed by the deflection plates. In my mind, the field would bend off-target or disperse somewhat.

    In any case, wood is not the answer, only distance. If you wanted to provide magnetic shielding between the magnet and tubes, you'd need a piece of Mu Metal, which is the same material that is used to shield the magnets of A/V speakers.
    John R. Frondelli
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    Hey guys,

    I've posted this pic before:
    http://scopeboy.com/scopeblog/wp-con...a-innards1.jpg

    Does anyone think my tubes are too close to the magnet? I had to take the magnet cover off to get that speaker in there.

    It seems to work OK, and I measured the cathode voltage on those two preamp tubes with the chassis in and out, and never noticed any great difference. There's maybe about 1" of space between them and the magnet.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    As I posted in another thread, I had a weird problem when I put an EV 12L in my modded Princeton. (the magnet in close proximity to the PI tube). The amp hummed, and after changing filter caps noticed it only hummed when in the cab. The Princeton was modded for a LTPI and I discovered that certain types of tubes would hum and others wouldnt, none of them hummed if the amp was away from the speaker. (No problems at all now that I dont have that speaker in the amp)

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    I think this is one subject lacking a lot of empirical testing. In some cases the close proximity of a magnet seems to affect the tube's operation, in some cases not. I think there are several variables that could explain varying results...

    - the tube's architecture
    - distance to magnet
    - tube's orientation in relation to magnetic field
    - strenght of magnetic field

    I've never experienced this thing myself but I don't doubt that it exist - after all, CRT's pretty much work on principle of deflecting or focusing the electron beam with magnetic fields. From what I can interprete from photos the distance seems to be a very important factor. The tubes need to be nearly in touch with the magnet and it seems from a very large portion of the envelope too. Centimeter or half away and the whole problem might go away completely.

  11. #11
    daz
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    I've used a 12L in every combo i've had for years, and i only use combos. Even had a couple where the tube touched and never had any issues. But i always have wondered about it. just never had a problem so i figured it was OK.

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    Quote Originally Posted by teemuk View Post
    The tubes need to be nearly in touch with the magnet and it seems from a very large portion of the envelope too. Centimeter or half away and the whole problem might go away completely.
    In my amp mentioned above the speaker almost touched the tube shield. Not all tubes had problems.

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    It probably also depends on WHERE it is in the magnetic field. Magnetic fields have nodes, and do not radiate symmetrically around the magnet. I think that this must be evaluated on a case-by-case basis.
    John R. Frondelli
    dBm Pro Audio Services, New York, NY

    "Mediocre is the new 'Good' "

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    Another idea would be to put a small "bucking" magnet on the speaker. (Poles of the magnet opposing the main magnet.) This technique is commonly used in magnetically shielded speakers (which were popular back in the days of CRT TV sets.)

    The bucking magnet does not make a significant difference in the flux strength in the voice coil gap.

    The only problem I can forsee is that if you have enough room to fit a bucking magnet, you might have enough room not to have a problem in the first place.

    Nathan

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