hi, i have a musical fidelity B1 amp. after turning the amp on there is a delay of approx. 2 mins after which there is a thudding clipping sound which continues to get faster for approx. 2 mins and then disappears. three transistors have been previously replaced by a repair shop. should all 8 have been replaced at the same time? how do you reset the bias? any help would be greatly appreciate cheers glyn