Any tips on where to find an Ampeg-style baby bass magnetic diaphragm pickup?

[David King]

Well considering that it takes 2 minutes to try it both ways, I'm going to take the easy way out and suggest that you listen to them and tell us which sounds better. At the very least you might post diagrams so we can see what the substantive circuit differences are.

[Bruce Johnson]

How the coils are wired up may significantly affect the tone....or it may not! You won't know until you try it. I would start out with them wired like conventional P-Bass coils, but make up the wiring so you can play around with the phasing.

This has to do with the mechanical spring rates of your diaphragm assembly (or assemblies, if you made up two). There's a spring rate for the vertical up and down motion of the bridge, and there's a different spring rate for the rolling side-to-side motion of the bridge. When you pluck the string, the bridge goes into some combination of those movements. Whether the motion of the bridge on your bass is primarily vertical or primarily rolling will depend on the geometry of your mechanical components; the spacing of the diaphragms, mounting type, width of the bridge feet, etc. The proportion between these two is very important.

Here's how that affects the coils: If the coils are wired in phase, they will mostly be picking up the vertical component of the motion. If they are wired out of phase, they will mostly be picking up the rolling component.

This whole issue of the motion patterns of the bridge and how the coils detect them is where I spent the majority of my R & D effort on the pickup system for my Scroll Basses. I'm not going to give away all of my secrets, but this should give you a general idea of what's going on in there. You're just going to have to experiment with it yourself.