Join Date: Feb 2007
Location: upper left crust, united snakes
Back when I was being "forced" to learn how to design outputs for tubes, in 1974 when tubes were dead, I was told to add the Plate resistance, either one I calculated, or one given in mhos and shown as transconductance, in parallel with the load line. This, added to the DCR of the primary and the square of the turns ratio times the secondary DCR, would give me a figure for primary reactance. Dividing that number by two pi and the frequency of interest would give me the primary inductance I needed, out in the power band, to support a - 3db power output at that frequency of interest. Multiplying that number by 2.76 provides a -0.5 db power output.
In SE OPT's there can be resistors in the plate to ground path that occasionally swamp the Rp and then they become the Rp. Please understand that my education was at the hands of practical transformer engineers who wanted something close to an ideal answer, but due to the +/- 25% nature of E/I core permeability, worrying about the last bit of refinement was not acceptable.
I am always willing to learn better methods.