My Princeton has recently developed a 60hz hum when powering up. The hum "surges" shortly after the amp is turned on, then it quiets down to being barely audible. It isn't affected by the volume or tone controls; however, the hum is pulsing and can be altered by turning the intensity knob. Knowing the little that I do about the tremolo circuit in these amps, I'm wondering if there is a problem in the bias circuit or possibly a coupling capacitor. The tremolo seems a bit weak as well. Any help would be greatly appreciated. I'm new to the tube amp world, and trying to learn as much as I can about them. Also, I've measured the bias a few times over the last 2 years and noticed a trend. The resistance of the OT has been dropping evenly on both halves, and the plate voltage of the 6v6's is slowly on the rise. Not sure if this has anything to do with the recent hum or not.