What are the diodes for? I've never connected a bunch of power transistors in parallel like that, but I understand it's not a very good idea becuase of current hogging. Use low value emitter resistors to mitigate that effect. Also, you're switching 12Vdirectly into the bases without anything to limit current. The transistors will just go *poof*. Why are you using obsolete 3055's? Also, the fast swiching might mean very high output voltage from your transformer, as well as high sipkes on the primary side.
Is this an experiment or a product?