In general, the volume control in a preamplifer is ahead of the active line circuits. Thus, any noise or hum originating in the line circuits will be a constant, not affected by where the volume control is set. So changing tubes, as I would have expected, did not change the hum issue.
What is left?
Three possibilities:
First, the hum is being generated in a source connected to the preamp. Try disconnecting all source cables at the preamp and see if this eliminates the hum. Reconnect one at a time to isolate the culprit.
Second, external enemies such as dimmer switches, water bed heaters, or anything else nasty thereabouts. Move the equipment to a different location and try again. Connection of anything in the system to a cable video source can be the problem. A three prong AC plug on any of the componets can make a ground loop too.
Third, internal grounding issues. It is really easy to design in a tiny grounding error that is very hard to track down. We did this when we first introduced a mute protect circuit in our preamps. We had to tear our hair to trace down the simple fact that we had the mute ground and input ground to the mute board connected and that was the problem.
Hope this is helpful.
Frank Van Alstine
