As I understand it, the 7025 is built with slightly heavier internal structure to make it more rugged. The electrical specs are identical, but some folks claim that it sounds a bit sharper than a 12AX7A. Whatever...

You need to apply high voltages to a tube to check it out. That means testing it in-circuit. That means risking electrocution! :eek: I realise you have a lot of electronic training, but you need to do some research on the subject, and get some training on how to work on [u]high-voltage circuits[/u] before you do that.

The simplest troubleshooting technique is to swap some tubes around. You need a schematic to help you sort things out. Basically, if you swap a tube from a noisy channel for the same type of tube in a quiet channel, and the noisy channel gets quiet while the quiet channel becomes noisy, you know that the tube is the cause of the problem. If there is no change, the problem is not the tube you moved. Look for something else - either anther tube, or the circuit around the tube socket. The schematic should have some 'typical' DC voltage values for various points in the circuit. These values are for 'zero-signal' conditions, no audio signal applied.