Sounds to me like you either have a differential chassis voltage, or a voltage imbalance on the tube. If you have a digital multimeter, measure between the chassis grounds of both amp and preamp, AC and DC voltage, and see if you have anything. I bet you do. I have grounded all my chassis grounds together at my preamp and have since never had any more hum problems (and my stereo uses 18 tubes).
I take it you are saying this preamp is a DIY project. If the tube has an imbalance on the anodes, you need some experience before proceeding. The fact the sound changes with heat would indicate a bias imbalance probably caused by an asymmetrical/incorrect power layout for the tube. You can email me and I can tell you all about how to figure it out if you feel you are up the task.
Otherwise, have you tried other pairs of interconnects? Some of them don't have proper grounding and this too can cause hum in certain situations....
Arthur
I take it you are saying this preamp is a DIY project. If the tube has an imbalance on the anodes, you need some experience before proceeding. The fact the sound changes with heat would indicate a bias imbalance probably caused by an asymmetrical/incorrect power layout for the tube. You can email me and I can tell you all about how to figure it out if you feel you are up the task.
Otherwise, have you tried other pairs of interconnects? Some of them don't have proper grounding and this too can cause hum in certain situations....
Arthur