Has anyone replaced tubes in Aesthetix Rhea ?

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.

I wouldn't assume the power regenerator eliminates the possibility of interference from some other electrical equipment. If I were you, I'd still explore the possibility.

I agree it seems unlikely, but it is possible that some component within the Rhea is failing and causing intermittant problems (e.g., some component within the power regulation circuitry). If replacing the tubes does not resolve this, I'd recommend contacting Jim White at Aesthetix directly to see if he has any ideas.
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Also... I don't know what an RPTV is, but if it's a TV, that could be causing tube interference too if it's turned on?

Patrick