I was not primarly intersted in the heating. Although I do agree it could be loss in the core in addition to the coil. The core was not getting that hot, if I remember correctly. Overall, it was not getting that hot.

I think the magnetizing inductance might be around 2H. On my circuit simulator I have trouble getting 10 Watts ac across a one ohm load. That is the transformed load. I know the transistor can handle 115 Watts, so its a mystery why so little power is getting to the load.

When there is such a heavy load, its kind of hard to turn on the transitor completly. The base voltage has to start approaching the 6 V absolute maxinum rating.