['LIVEWIRE?']

Echoing barrymagrec, I would suggest connecting the feedback to the other end of the o/p transformer secondary winding. What is happening is almost certainly that you are getting positive instead of negative feedback. This will cause the amp. to become an oscillator. The way I was taught it is that NFB 'reduces gain and improves (increases) frequency response. Conversely, PFB, increases gain, and (almost certainly) reduces frequency response. edit; crossed with Refugee's post!