there are a bunch of different toroids to choose from but so far i've only tried the plain black ones and had good success. winding a 1/2" OD toroid can be a real pain so next time i'll try E cores

don't worry about wire length for the primary and feedback coils, number of turns is what's important and since there are so few all you need is a short piece of wire. the wire for the secondary has to be first wound around a "shuttle" to make winding easy so they give a good starting point for wire length...


dave