[Ed_Dinning]

Hi, Melbourne was only a suggestion if you get over that way at all, I doubt we will be out in the Adelaide direction this year, too much DIY at son's.

Primary taps often give problems and take up space, so I personally try to avoid them unless necessary.
Try and get your primary as smooth and even as possible and use insulation between layers. If you really need to pack the turns in it is possible to insulate 1/2 layers (between wires with the greatest voltage stress), but be certain to insulate at the ends of a layer where a turn could slip down.

Double insulated wire is now used a lot in Europe, without interlayer insulation
(Grade 2) especially on secondaries (it is a moot point if the extra wire insulation is thicker or thinner than the additional paper thickness)

Note that there are a variety of insulation coatings available, only some of which are solderable and different types will have different thickness. Not much, but which can cause an upset in a "tight" transformer with many layers.

Some transformers use a safety screen/ EMC screen, this is a thin layer of copper foil in a single turn, but with ends insulated. It can provide safety from a pri/ sec short (foil thickness important here) or to act as an earthed screen for interference (usually thinner or can be a single layer of, say, 0.063 dia or similar, connected at one end only)

If you can only increase the wire dia on one winding, make it the pri, as being the innermost it heats the most.

If the core is Stalloy it is being worked hard at 1.25T and will have a highish mag current, adding to heating. If Unisil then well within its capabilities (1.5T). Possibly worth doing a mag curve on the completed transformer to see how close it is to the knee.

LV secs can be single insulated wire

T/V figure for the pri will be what is required to give the core flux density. T/V for the secs are sometimes higher to give compensating turns to allow for the I * R drop on load.
Especially on LV secs, a thinner wire is sometimes used than standards dictate, for economics as well as window area. The load is constant so the extra drop is easily compensated for

Bobbin winding is probably your best route, it is easier to do and gives added safety for wires slipping down at the ends.

Your figures for 50Hz and increased stack should compensate for the frequency difference, you will also win out slightly with the thinner primary as it has a better packing factor.

The nominal standard for winding is 3A/mm^2

Hope that helps

Ed