I built a tube preamp from a schematic and it has a 60hz hum problem. It works and is still usable. The hum comes out of both channels all the time with any of the four inputs. It even hums at the same level when not turned on once it is plugged into the AC socket (even though not turned on). I think my construction was not perfect. Can you help me out as far as what can cause a problem like this. What is the best way to troubleshoot this problem? Thanks.