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chaps,
I found the design details; it was posted by on the Joenet a few years ago.
Basically, it involved making a vertically sectioned bobbin; start with a 70mm diameter cylinder of whatever material (I used white nylon cos it was available and cheap; teflon or something like maple may be the way to go), 78mm length.

Then cut 17 grooves in, each 3mm wide, separated by about 1.6mm of material. Groove depth of a few mm.

Primary is 9 sections of 34 swg wire, about 200-250 turns each, secondary 8 sections, 22 gauge, wound to the desired turns ratio (about 18 for the original design). All sections wired in series, with alternate winding of Primary/Sec/Primary/Sec etc

Now this gave about 200mH of inductance, which is about right for a tweeter application with low Z tubes. DC resistance (DCR) of primary was about 200 ohms, which is probably on the high side for a 6C33, so may want to experiment with thicker windings and more/deeper sections, although more turns will degrade top end response. I think with the 6c33 you may get away with it though.

Now, given that the 6C33 has such a splendidly low internal impedance, it seems a shame to go and wind a high DCR transformer. Particularly for bass duty - which is why I think an enormous core with many turns (!) of thick wire is the way to go. You need the turns for the inductance and to keep the core operating at a low flux level at LF (less core and tube distortion).

The flux due to an ac signal component is proportional to (primary ac voltage) / (core cross sectional area * number of turns * frequency).

more later....

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