Hi Mike,
Was trying to find information that related to secondary transmitters of EMF based on alloy ferrous content, but was unable to locate it again. Somewhere in researching this last year, I visited the whole question of whether unabsorbed redirected EMF off a MuShield would pose additional emitter difficulties as various metal alloys in the Lenco (platter, motor shaft, top plate) were permeated to the degree of iron in their respective mixes.
I do remember that it was of enough concern with the unknown (and unknowable?) alloy contents in various exposed Lenco parts to solidify a decision for going up top of the platter. Even then, I had concerns that the record spindle itself, might act as some RKO tower and omni emitter of interference. Thankfully, this is not the case, and one hears only a small amount of hum when the Gauss detecting Grado is directly over the spindle. Whether this is due to chrome plating of the spindle, or the brassy elements of the bearing sump, I haven’t a clue.
But I did come across a couple of notes today that should be highlighted as REVISIONS to the Lenco MuMetal recipe. (Grant & Chuck be advised)
1- It DOES matter which layer should be glued to the platter first. The thinner, low permeable foil should be closer to the EMF source and get pressed in first.
2- Hammering MuMetal reduces its effectiveness because it changes the crystalline structure of the alloy. Put your energies into cutting the MuMetal foils with a hard surface and many, many utility knife blades. Ridging of the cut edge is the bane of this project and must be avoided. It will not promote a smooth meld to the platter and its removal is problematic. Shearing often causes its own ridging.
In my application, I was able to get away with minimal shearing of edge burrs and some very light tapping with a hammer at the spindle hole.