tubes are 6sf5 6sn7 6h6 6sn7 two ob2 and a 6x5. I got it working today. The unit always in comp mode ,but the meter switches between input bypassed and comp. Inside is a threashold adjustment . The input wont take muck more than -35 db input and it puts out +20 or more .Frequency response rolls off at 4k. Output impedance is 600 ohms i assume? It has split coil centertap . Input is 25 ohms or 600? Id like to find a quieter tube for the 6sf5 spot.