I bought a Peavey 400BH Mark III from 1978 today. I plugged it into my cab (2x15 600W 4 Ohms) and tried playing, it was quiet but really distorted, I tried the bass in the other channel, the same thing happened but it was a lot quieter. I tried plugging the cabinet into the other output and the same thing happened. I switched the amp off and back on again and it made the loudest noise ever. It was a really loud and low hum but the kind that actually made my trousers billow in the wind from the ports. It does this even when the bass isn't plugged in and does it on another cabinet as well. At the time that the hum started a wisp of smoke came out from the top of the amp as well. What have I done to it? Is it something that can easily be fixed? How much will it cost to repair it? Thanks in advance.