AMP IN QUESTION: SF Fender Bassman with 1x12 4 ohm MODS: The output section is AA864, except the bias circuit wasn't fully rendered to AA864 specs, it still operates as a bias balance. The bias cap is a 47uf/100v. I'm going to have to sort this out, although it does bias fine, cold-ish, but fine. The bass channel has been modified to be guitar friendly, although I forgot what that was exactly. I can pull out the chassis and take pics if necessary. PROBLEM: Everytime I get close to the amp to attempt controlled feedback, all I get is whistling. At this point I assume there is something not right with the circuit because my old Bassman and Bandmaster were sublime at controlled feedback. I'm using either a VFE The Scream or Skreddy Lunar Module (or both) to help invoke feedback. MORE: Whatever it is you need to help me fix this, let me know! I will take pics, measure things, whatever.