I used 18 gauge wire to wind (I was hoping to get the DCR between taps a little closer to the original iron core version)
I got the inductance at the taps pretty close to the original JBL LX5 autoformer (The tolerances of the autoformers was pretty sloppy back then)
I installed it in a rebuilt crossover, everything new with air core coil for the 6 mH at the low end.

I tested the crossover with the new air core autoformer and then swapped in the original iron core.

They both crossed over at about the same, the slopes were similar, both flattened out at about 1K.
The iron core only dropped about 1.25 DBV all the way to 20K.
The air core was down 3 DBV at 7K and then dropped another 5.5 DBV on it’s way to 20K.

DCR at all the testing points between the iron and air were almost double but the max was only 1.2 ohms from the start to the end of the air core autoformer vs. .64 ohms for the iron core. I can't see how a higher DCR would only alter the high end.

Any ideas? Why would it roll off at the high end like that?
I did wind a second autoformer and tested that. It had the same results

I can post data if anyone has ideas.

Thanks,

Eric