by erichayes » Wed Jun 20, 2007 8:00 pm
The 12AX7 hum is probably from heater-cathode leakage in the tube. Since the two triodes are completely independent of one another, one side could have slightly more leakage and the hum was exacerbated by the failing rectifier. Normally, when a 5AR4 dies, it lets you know both visually and aurally in no uncertain terms.
Because the 12AX7 is the stage with the most gain, things like H-K leakage are going to be more noticeable than leakage in the driver or output stages. A heater-cathode short, on the other hand, will be noticeable in any stage.
Eric in the Jefferson State