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

@filipm 

Yeah so I can see that both the ground post and circuit board are all earthed through the 33ohm resistor. If the chassis and earth post measures 0 ohms then the chassis is lifted above ground ( not what you want ).

The chassis could be connected to the circuit board ground anywhere - it could be the binding post;  ?you would need to look around - are there any wires to the chassis ? It could be through circuit board legs ?

I would do the following test -

If you desolder the 2 green and yellow wires from the grounding post but leave them soldered together - then the circuit board would is still grounded through the 33 ohm resistor ( assuming you have reattached it ).

Now measure from the green wires to the chassis. If that measures open circuit then you would know that the chassis is earthed through the ground post mount.

Double check by measuring the chassis to mains earth tag - should be either 33ohms or open circuit. If it is 33 ohms the chassis must be earthed via the board somewhere else. If it is open circuit then we now know the chassis is earthed through the ground post mount only.

Depending on your test results I could suggest a very easy fix for you to the earthing issue. This may or may not fix your noise issue though, but in my view would be worth trying.

@dover the chassis and the board are connected by those green wires. 

board > right green wire > binding post < left green wire - 33ohm resistor - ground pin of the IEC. (which means the left and right wire are meeting in the binding post)

also, there are screws holding the board but they are not touching the circuit of course. 

There is a possibility that the joint of the board ground (where the circuit board and the grounding wire meet) needs to be resoldered. 

If I desolder the 2 wires it only means the chassis would be completely lifted from the ground. The chassis and these wires wouldn't be even electrically connected. 

What's your suggestion?

Let's not forget the error may not be in the preamp. It's working fine anywhere else but my place. Maybe it just needs better grounding from the binding post. But where to attach it if not the socket ground pin? I have no idea. 

(It's possible the grounding problem leads further - maybe the sockets are poorly grounded. Maybe there's a problem in the main switch. We still haven't measured that, if it's even possible. I'm waiting for the electrician to show up.)

@filipm 

If I desolder the 2 wires it only means the chassis would be completely lifted

Thats likely correct - but it would be prudent to verify that. I say  this because most of the time ground posts are insulated from the chassis, and there may be a different path we cant see.

If proven to be correct, and the chassis is now floating, then you would be able to earth the chassis to the incoming mains earth pin before the 33ohm resistor and achieve much better shielding - as per @atmasphere suggestion in his earlier post.

@dover I think If I wanted achieve the best shielding possible I'd have to leave the two green wires as they are but to cut out the 33ohm resistor. Then everything (the board, the chassis) would be grounded through the IEC with no resistance. I may be wrong, please take in account I'm a noob at this and I'm often missing what for others is common knowledge. So maybe if I cut out the resistor, it will lead to a ground loop.
I think if I unsoldered the green wires from the pin and soldered them together, I'd just ground the board while the cage would be left absolutely unshielded. 

@filipm

I think if I unsoldered the green wires from the pin and soldered them together, I’d just ground the board while the cage would be left absolutely unshielded.

Yes, but I’m not suggesting you leave it like this.

We want to get to a place where the chassis is grounded (no 33ohm resistor ) but leave the board floating with the 33ohm resistor. This I believe is the optimum solution.

If you can prove the chassis ground can be detached from the board ( by unsoldering the ground post earth and checking that there is no continuity between chassis and circuit board ground ) then you can safely ground the chassis and ground post to the mains ground pin ahead of the 33 ohm resistor with an additional wire. In other words ground the chassis and grounding post without the 33ohm resistor but still run the 33ohm resistor to the circuit board.

You could do as you suggest and just eliminate the 33 ohm resistor and leave everything grounded at 0. Only downside is it may be suboptimal for the circuit itself in terms of noise floor.