Humbucker Magnet swaps - things that make you go ..eh?

SunDevil

I swapped magnets on a humbucker at the weekend - I swapped a 2.5" polished Alnico III for a rough cast Alnico IV which is probably 8th to 16th inch shorter.

The result - a fair bit less output

Pickup maintains the same DC resistance before and after swap (so I'm assuming I didn't break it - done a few of these now) and the pickup height is near as matters the same

So the magnet's wot dun it!

Best I can figure out, the reason for this is that the Alnico IV is no fully charged (it was bought from one of the pickup sellers as a charged magnet)?

Is there anything else it could be?