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In Reply to: Copper, .025 inch posted by drlowmu on April 22, 2007 at 06:54:08:
I do not doubt you heard a difference with the copper in place, but for the magnetic fields, the copper i essentially invisible. What you may be hearing is some sort of ES screening.
Follow Ups:
Dave,That materials information is helpful to me. Thanks. Would my aforementioned and prior favorite material, 10 gauge steel be a lot better?
You see, when I did my actual listening over this past month, I was using about 10 gauge cold rolled steel, all around the power trannie, which fed into the pair of EZ81s and EL84s.
I just made the copper pieces over this weekend, and I have as yet to hear them. I will test them, and I can sub in 10 gauge steel pieces and compare the two. I had actually intened to do this, but had not taken the time to do so.
In past years, after doing some reading on the subject, I had selected the 10 gauge steel. Your Forum-posted comment will remind me NOT to delay or overlook its use again. Thanks a whole lot.
It will be easy to do it in steel, the same way as I executed the copper. Thanks !
Hi Jeff,
there has been a lot of dicussion about this at another site. The conclusion was to use 1/4" thick aluminum when you need to shield low frequency magnetic fields. This gives you somewhere between 5 to 10db attenuation at 60Hz, depending on what formulation of aluminum you use. Its pretty much purely resistivity based, the higher the conductivity of the material and the thicker the sheet the more attenuation you get.Copper would work fine, but its cheaper to go with the thick aluminum rather than the thick copper.
The experience was that using the aluminum sounds better than the steel. The theory being that the steel relies on the BH curve which is not all that linear but the aluminum is using eddy currents which are far more linear.
I tried this in my preamp, I was getting some hum from the filament transformers (the BDT tubes are quite sensitive to external magnetic fields). I used some .375 aluminum plate between the tubes and the transformers and it made a significant improvement. I haven't tried the steel sheet yet though.
Jeff, I'm wondering whether when you executed it with steel you actually surrounded the transformer or just had flat plates as you do with the copper. With the flat plates I would think the magnetic field would go around the plates, which would mean a longer path length thus decreasing the strength, but not nearly as good as a full shield, which would mean encasing the tranny. Of course decreasing the field strength might be "good enough", kind of like effectively moving the transformer a bit farther away. If you're intent on shielding mu metal might be even better since that is specifically designed to shield magnetic fields. Can't comment on its sonic qualities per se, but it's used a lot to block hum fields.
Very nice job on the Eico! :)
I like a lot your idea of a removable screen.
I have grounded copper screens on my tubes, and it does make an audible difference if you live in an RF-heavy region. (Cell phone towers, computer in household...) I think your copper screen has a similar effect on the tubes.For magnetic shielding, you could place an iron sheet parallel to each copper sheet. If you bolt them together tightly, or glue them together with conductive glue, the effectiveness of the metal shield will be even greater, as the generated eddy currents will be dissipated by the copper sheet.
Recently I saw in a textbook that when you have two magnets, and place an iron plate between the two, they stop seeing each other, and they see the metal plate instead. You could use one metal plate on each side of the copper sheet, and that would be a very effective magnetic shield between the iron / tube.
Also try ferrite plates. I've recently started using them for magnetic shielding, and they are terrific - and pretty cheap, around 1$ for a square in piece. BTW, that will be all you will need per copper plate. Place the ferrite plate at the height of the tube's anode structure, and it will prevent the magnetic lines from crossing to the other side that would most interfere with th tube.
I have also tried using transformer laminations from gutted iron for magnetic shielding, and is very effective, too. :)Long live DIY!
Janos
PS: you can also use ferrite plates to boost transformers! take off the endbells, and glue a plate on the coil! Will keep lots of magnetic lines inside the transformer, making it more efficient, and also will reduce emitted magnetic field greatly.
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