Has anyone replaced tubes in Aesthetix Rhea ?

Sounds like a tube (or tubes), Stew. Only way to know is to swap 'em and see. Let us know how you make out once you change them.
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Rushton, still don't have the new tubes but perhaps you can explain why I would be getting ringing in both channels on some days but not others. For example, last night I had a few friends over to listen to the system. Of course, the ringing was present when I turned the system on and stayed all night without abating. Tonight I'm alone and I turn on the system and other than the usual tube rush one might expect to hear, all is quiet. The night before last quiet too. Like you say, the only way we'll know if its the tubes is to swap 'em out. At the moment, I only plan to swap out the 12AX7s in the first gain stage. Any reason to think these might not be the culprits? This is just too strange.

Stew, I don't have an answer for you. Could be you're getting some sort of power line interference or RF interference. That would account for the intermittance and for hearing it simultaneously in both channels. The interference could be coming from some other appliance running, such as a furnace motor or freezer compressor. One way to check that would be to start turning off circuit breakers the next time you have an occurrence of the ringing.
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I have the Rhea plugged into a PS Audio P500 power regenerator, as I do virtually all of my components. I tried going directly into the wall receptacle, but that did not kill the noise either. I guess the Rhea, which is my only piece of tube equipment, might be more susceptible than the others to RF, but I have my RPTV plugged into the P500 and it plays fine. Hopefully the tubes come tommorow and we will know more. If its any help, the ringing when it comes is present even when I switch inputs, gain settings, load settings, mute, and when I power up/down the unit. Would you agree with me, since the unit performs flawlessly on certain nights, like tonight, it is something external to the unit itself?

Stew,

What does the manual say about plugging into a power regenerator? I know that the Io manual specifically warns against plugging into any kind of AC conditioner or regenerator and says to plug directly into the wall.

May not be true for the Rhea, but it's a thought.