Speaker magnet proximity to preamp tubes

LPV

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Before I cut my baffle, does anyone have an opinion on speaker magnet proximity to preamp tubes ? As it stands my cover for my weber alnico will be 3/4" away from the preamp tube covers. I believe the magnet itself is at least another 1/4" away under the cover so that would be 1".



I can't find any well documented issues on this. I did find something with proximity to beam power tubes but nothing about the preamp.
 

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ES350

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I have a similar situation with some N series alnicos in tweed amps and it hasn't been a problem...
 

hipofutura

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It's an interesting question, and I don't know the answer. My guess is that it doesn't matter. It doesn't seem reasonable that the magnetic shadow of a speaker would interfere with the preamp signal or tube operation.

I'm curious to learn the correct answer!
 

LPV

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This is the only thing I have found

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.
 

hipofutura

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Well, I stand corrected. It does make sense. Now you have a dilemma!
 

LPV

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I should be good. I'm no where near that close. I can sacrifice a tiny bit on the bottom of the speaker exposure and gain yet another 1/2" as well.
 

Cygnus X1

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Just grind off part of the magnet.

Problem solved!
 

Case24

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You almost had me worried about my Victoria 518. I replaced the original Jensen P8R re-issue with a Weber 8A125-O.

As you can see in the picture below, the Weber is considerably deeper due to its magnet cover:



However, the actual speaker is only 3/8ths of an inch deeper than the Jensen based on specs from each manufacturer's website.

Mounted in the cab, the Weber's magnet cover is 1/2 an inch away from the 6V6GT but the actual magnet is 1 1/4 inches away from the power tube.



I haven't noticed any reduction in power with this setup. But I certainly believe a large speaker magnet too close to a tube could be a problem.

Thanks for this post, LPV. :thumb:
 

Cygnus X1

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I've wondered about the GIANT covers on some of those Webers, but they address it by saying it's safe to just remove the cover.

Right?
 

Case24

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I've wondered about the GIANT covers on some of those Webers, but they address it by saying it's safe to just remove the cover.

Right?
Any articles I have read regarding magnet covers state that they are purely cosmetic and can be removed with no consequence.
 

hipofutura

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Any articles I have read regarding magnet covers state that they are purely cosmetic and can be removed with no consequence.

That's true. They also serve to keep years of accumulated crud away from the voice coil gap (for rear vented speakers).

Companies like Altec would sell their OEM speakers without the covers (lower cost).
 


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