You can forget for the benefit of practical use the entire "impedance ratio" thing. The simple idea is that the modulator if it makes 100% modulation must produce a peak (+) plus swing and peak (-)minus phase swing of exactly = to the B+ on the finals!
So, if you have 750vdc B+ on some 6146s, then you need to swing peak-to-peak 1500vac of modulator signal.
Since generally one would use a Class B or AB modulator, that implies a 750vdc B+ on the CT of the primary of the mod iron.
After that, you need to be sure that the mod tubes will supply the requisite POWER to back up the intended voltage swing... which brings us back to proper impedance matching, but in practical terms IF the modulator produces the requisite swing without distortion, that's all you really need to know!
Fwiw, the reason that power transformers can be considered for this application is ONLY because AM phone generally does not require really good frequency response. Few power transformers will have flat response beyond 5-10kHz., if they get that far...
Being in VK land, you can still mail order some Hammond mod iron from Canada, or look at what is offered in Taiwan and Japan... many transformer companies there...
Also, consider hand winding your own! Given the simple ratio required for mod iron, you can likely use the same gauge wire for the primary and the secondary, simply "stack" short windings on the bobbin, and connect the wires aftwards to create the dual primary and single secondary! For raw material you buy the magnet wire, get quad insulated if you can... find a used large size power tranny with nice thin lams, hopefully from a mfr you can find out what alloy lam was used (best case), some appropriate "fish paper" or similar material, or resuse the existing bobbin, mylar tape for interwinding insulation layers... and for final finishing an old pressure cooker, varnish (go to a motor winding shop/company if you can to get this done) of the right type, a vacuum source (air compressor intake) to vacuum impregnate the tranny for insulation.
Done!
You won't do worse than the power tranny... maybe way better.
_-_-WBear2GCR
IF you happen to have a big hi-fi output transformer you can simply drive the "speaker side" (4-8ohms) with a power amp of appropriate power and use the secondary in a Heising type set up (no DC) on the secondary, to drive the finals... you can build in a suitable solid state amp should you desire that. Imho, that power tranny could likely be set up as a nice choke...
