So I have a GK800rb that I was using as a power amp with an ADA MB1 preamp plugged into the effects return. After a while of using this configuration I unplugged the preamp and tried to use the 800rb on its own, but it does not work. It powers up, it works fine as a power amp but the preamp section is not working and I cannot use it on its own. Is this a common thing? What could have happened to the preamp? I don't have the cash to drop it off at an amp repair place and possibly have to pay a couple hundred bucks to repair it. It is not just a loose input jack. I did not blow it up. I have no idea why the preamp section is not functioning now and would really appreciate any ideas on what went wrong as I love the 800rb and have not been able to use it for a long time.