Hello Burkuagh,
Feedback is a very powerfull feature. But you must handle it with some care. You should start to set feedback->gain to zero and then slowly raise it till you reached the desired effect.
Beside of that we already included a feature to protect your ears from feebacks which get out of control. Just click on the 'clip' knob in the delay section and it will be limited.
Best Regards,
Markus Feil