[kalee20]

I've made bobbins and central tubes from 0.007" Nomex (synthetic aramid paper, OK to 220°C) but have also used cardboard from a cereal packet for a quick temporary job - which lasted 12 years before doing it 'properly! A couple of turns of adhesive Nomex tape covers the join.

The flimsier the bobbin or former, the better the mandrel needs to be when winding, else the pressure of the windings distorts the bobbin.

The open corners of this construction has advantages and drawbacks! Advantage is that it allows excellent varnish penetration. Down side is that insulation from laminations is not good - although the winding and the core will never overlap, there is minimal distance between them, so side cheeks may need to be considered. But it's a very flexible technique (helped by having a friendly machine shop nearby which makes central mandrel supports for a small fee).

The photos show an intervalve transformer, not a mains transformer, if anyone is worried about primary/secondary creepage distance!