To me this sounds like a mechanical hum. First thing to check would be the screws that hold the circuit boards and transformer are snugged. Then I would call AR tech support and ask if it’s okay to remove one or all of the tubes and then power it on. This way you could remove one tube at a time and see if the hum goes away -- and you know it’s the tubes. (I should mention power off and unplug the unit each time before you remove any tube).
Tubes trouble?
Need help and advice.
Have an ARC PH5 Riaa where the tubes were changed two years ago. Sometimes when I play there is a humming sound that changes character when I put my hand on the cabinet and all of a sudden I am completely free of interference again. Where do I start? Guess it's a tubes that's bad. How do I find which one? Can I change just one tubes or do I have to change all of them? Is there any placement of the four that is less critical if a pipe is in worse condition? Thanks fore comments and tips. // Niklas