The output (anode) transformer for the 400 W PA uses a 58 mm OD 3C85 core with a primary winding of 140 turns, centre-tapped; and a secondary winding of 9 turns to provide a 50 ohm secondary.

Before making the transformer, Finbar EI0CF alerted me to the problem of using high voltages with ferrite or dust-iron cores and suggested using "high voltage tape".  I think that what he meant was that the conductive core could become a short circuit path to the RF voltage developed across the primary winding.

Dave G3YMC suggested using PTFE tape and I knew I had a couple of reels of PTFE tape of the sort used by plumbers.  So I wound a few layers of PTFE tape around the toroid. I then held the PTFE tape in place by a few layers of white PVC tape.  Then I put on some more PTFE tape, then some more PVC tape.  I then wound the centre-tapped anode winding with 140 turns of 26 swg enamelled copper, anchored with nylon cable ties.  I left a spacing of about 14 mm between the start and finish points.  More PVC tape was used to wrap the centre (but not the extreme ends) of the primary winding.  The secondary winding used wire from stripped-down mains flex.  The turns were wound over the centre part of the winding which had previously been covered with PVC tape.  The transformer was mounted on some 3 mm brown board of the type that used to be used in the manufacture of tag strip (called resin bonded 'something', 'something', I believe).   The ends of the windings were sleeved with neoprene sleeving and terminated on terminal posts fitted to the board.