Tube PHONO preamp interference - RFI, EMI, bad grounding?

@filipm - I understand you are probably super frustrated by now - yes, you could go solid state, but the fact that the tubed units work fine elsewhere suggests the problem is not with your gear.

I suggest that if testing the power for DC shows that it is present, getting a Humdinger from Audio by Van Alstine. You can return it if it does not help. Just realize that atmasphere, who I am reasonably sure knows a lot more than me about this (I am just a hobbyist), is correct - transformer hum should not show itself in the audio signal, though he does say "extremely rare". Perhaps your situation is one of the more rare ones. As for damage, not sure -  I think I read somewhere that the power company will fix it if very bad "for safety reasons". 

I would also do what atmasphere suggested right away with your DMM - It would be a good idea to check the REF150 grounds as well, and like he said check for impedance between the ground and the chassis to ensure you do not have a grounding issue in the units. I would have thought that would present itself in all locations, but maybe it's worse at your place.

@ejb14 I am :)

I guess I’d have to get two of these Humdinger units - one for the power amp and one for the rest. Also, the question is how it restricts the dynamics of a power amp.

I have measured the impedance between the ground plug on the inlet and the grounding pin of the chassis (or any screw) - it’s 33.2 ohms.

REF150 - the impedance between the ground on the inlet and any screw on the chassis - 0.01 ohms (0.00-0.02)

Could there be a grounding issue on the AC line? It can, right? And it’s probably quite hard to solve I guess. There’s a dedicated line for the living (listening) room with about 5 sockets around the room. Maybe a lot easier than looking for a ground issue there could be to run another dedicated line that would power the audio stage only. (That would only exclude a router and few lights that don’t really affect anything - I have had them unplugged of course)

Just to be clear - the noise in the REF150 is happening inside the unit - on the transformer and does not affect the sound. Maybe the music gets lean and thin when the buzz gets stronger but that I’m not 100% sure of. Still, it could be another problem, not really related to the noise of the phono stage. The REF150 was a second hand purchase and even tho we listened to it for about an hour and it was fine, something could have gone wrong when plugged directly into the wall.

The noise of the phono stage comes out through the speakers.

Also - one thing I wanted to be sure of - with the use of shorting plugs I should eliminate any signal coming from the air, right? BUT when the chassis is not properly grounded the signal still comes through and gets picked up by the phono stage tubes, is that correct?

If the transformer itself is humming, try tightening down on the screws that hold it to the chassis, and on the screws that hold the laminations together. Sometimes the case of the transformer can be a little bit loose and this can allow the laminations to vibrate and produce a hum. Transformers by their nature do not transmit DC, but DC can cause the transformer to be noisy. Make sure the transformer laminations are tight and that the transformer itself is firmly affixed to the chassis, and then report back whether that did any good as regards this latest problem. Of course I do not expect this to cure your problem with RFI. 

This is definitely a fascinating post (at least for me), with so much detail explored. The greatest thing that was accomplished right off the bat was the fact that the OP (filipm) was able to record and post the noise or interference that he is experiencing and hearing. When trying to troubleshoot noise and interference, *that* is worth gold all by itself. With so many suggestions already being offered and tried, I'm not sure I have anything left in my arsenal to propose. I'm going to go back over all of the tips and suggestions, and see if I can offer anything. This is definitely one of the worst cases of noise (RFI & EMI) that I've ever heard. So far, it looks like finding a fix for this particular setup is quite the challenge. I completely understand your frustration filipm, but admire your tenacity.  

>>>very late at night or very early morning, do you still have this noise? If not, then I’d suspect someone in your building is the culprit.<<<

I was just wondering if this question was ever answered? Is the noise consistent at all times of day or night?

Do you have any other phono cartridge (already mounted in a headshell) besides your Nagaoka MP200? If you do, does the noise change (in any way) when using a different phono cartridge?