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CV,
Nicely put.... and thanks for the Rp information on the 6C33. Do you have any thoughts, or a curve trace source, about the Rl ratio? 

Now, since air core's are to be your weapon, here are some thoughts to explore.

1. Wind biffilar or trifillar in each receiving coil and do not be afraid to let the wires wander a little bit. This will help with the clumps where it is important.

2. Think about coupling in two planes. Means surround the receiver with drivers on both top and bottom and inside and outside. You can use polybutadene coated wire to accomplish this, as self supporting windings, made on ever so slightly tapered Alcatel plastic arbors are a breeze, except for the alcohol needed to activate the poly. Use 99.99% pure juice for this, please. The poly wire will give you a tiny bit more dielectric to work with and air around the wires, as opposed to wax dipping or the like.

3. Wind the receiver in a square cross section for this scenario and parallel connect all of the primaries and make them as shallow vis a vis the receiver as possible.

4. Do not overlap primary on primary. You may even need to make E Field shields to enforce their mutual exclusion, except through their connections.

If all of this is just too mad, do make sure your receiver is driven on two opposing sides, to ensure a linear flux gradient, and don't hesitate to extend the primary windings beyond the secondary by 6 to 8 mm, in this scenario only, to kill the bridging capacitance losses. Also, a wide thin winding, for the receiver, is good here.

On another path entirely I think a non saturable reactor style core construction might also be a beneficial exploration. This was pioneered by Peerless in the late 40's for Altec and JBL and just means you take the I's and put them on the bottom of the E core, rather than across the legs. No reason why this would not be valid at this low an impedance and DC offset current should not be a factor at all. We make single coil crossover inductors in this fashion. as replacements for hockey puck, dog bone and foil wound inductors to very good effect, though we do multifillar the windings and pay attention to the dielectric circuit.

Another very good possibility would come from non saturable permalloy powder core torroids. These are used mostly as common mode rejection chokes in switcher power supplies. I do not have any information about their audio behavior with a DC field polarizing current though, and they might need to be rather large.....

I like everything I have seen about Sowters work. They are not afraid to innovate and we have mutual thoughts about the problems gapped cores exhibit.

Bud

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