[Amps] Magnetic shielding

[Borislav Trifonov]

I had a 1.5 kW microwave oven transformer (MOT) that I took apart, 
removed the magnetic shunts, and when putting it back together, I 
interleaved the laminations (originally, all the Es were together as one 
E; likewise the Is).  The transformer hums quietly on its own, but when 
I put it in my project chassis, which is steel, I found out that it had 
huge leakage as it made the chassis hum very loudly.  Knowing that MOTs 
are made with the bare minimum of materials, I figured that the core was 
saturating and added 15% more turns to the primary (about as much as I 
could fit).  That helped a bit, but it was still pretty bad.  I also 
added a copper tape around the windings, outside the core, as the 'flux 
band' sometimes seen in transformers (especially in audio equipment). 
That made a small difference.  The chassis still hums too loudly.  I've 
put in too much work in the current project chassis to replace it with a 
non-steel one.  So I'm wondering about magnetic shielding.  Mu-metal has 
very high permeability but saturates easily and is only useful for weak 
fields.  Often I've seen tube audio amp transformers either completely 
encased in, or at least wrapped around the edges of the EI, with silicon 
steel or soft iron, which reduces leakage flux.  However, I actually was 
not able to find any appropriate material.  Any suggestions?