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mu metal questions

6/4/1999 7:56 AM
Don Symes
mu metal questions
So, where do you get mu-metal, and does it need to be grounded to provide a magnetic shield?
6/4/1999 8:16 AM

"where do you get mu-metal"
Excellent question. I suspect it's only available on special order from magnetics materials companies. The easiest source would be to make friends with someone in an FCC qualification lab or a university. I don't know where I'd get any, and I've even had occasion to use the stuff. I got it from our EMC lab.  
"does it need to be grounded to provide a magnetic  
No. It's shielding is purely due to the extremely high  
permeability (mu...) which "shorts" any magnetic fields through the mumetal instead of letting it pass through. No electrical connection is needed for this. However, there ain't no such thing as a free lunch - mumetal saturates at very low flux densities, so you have to use it to shield things like input lines and sensitive circuits, because if you put it near a strong source of field, like a power transformer, it usually saturates and then is no shielding at all. The resistance of mumetal is high, so it's not much good as a Faraday shield. High isolation shields are usually a steel outer shield with alternating layers of copper and then either nickel or mumetal inside. The outer steel shunts a lot of the M-field around the mumetal, which then takes care of the residual in several layers.
6/4/1999 10:08 AM

Hi, Don. You may be able to find some mu metal around the CRT in an old oscilloscope. Also, it may be found in an old computer around the disk drives; we use the stuff at my work around floppy drives.
6/4/1999 9:30 PM
don symes

Thanks, guys.  
Part of my plan is to convert this SFDSR into something wild. Step one is to go from a 100W mono output to stereo 50W. The mu metal was to shield the two OTs from each other.  
I guess the thing to do, then is to get a pair of Angela's shielded, epoxied 60W ultralinear OTs and figger out how to mount 'em.  
Three 12A_7s will get me the stereo power sections, leaving three more tubes for the preamp - probably a 5F6-A/G40V sort of thing.  
I'll still need to cram in a super-low-power power stage, a red box (or something), and an FX loop.  
I must be knuts.
6/5/1999 6:44 PM

"The mu metal was to shield the two OTs from each other."
Oh. OK, you might just try getting some copper sheet from a crafts store and fabbing a soldered copper covering for the two OT's. Or fab up a box from soft sheet steel and then coat the inside of THAT with soldered copper sheet. Should be enough decoupling - probably get 20db shielding from that.  
OT's are run at lower flux densities than power transformers and so the leakage is lower. I thought you were shielding an input from a power transformer in tight quarters.
6/6/1999 9:32 AM
don symes

Thanks, I feel better.  
Should have been more specific in the first place.
6/7/1999 6:13 AM

Multimedia/Surround speakers designed to be used near computer monitors or TVs are magnetically shielded to avoid ruining the CRTs. Maybe you can get enough mu metal to shield a transformer with a pair of these speakers.
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