Your less than traditional ground loop issue.

[TheWinterSnow]

I am a builder of an all in one tube amp load plus line signal conditioner and I have build many in the past that have worked, but now I have two where there is a ground loop issue that I cannot resolve.  I had a parts change, from open metal on the line output to all plastic jacks, so I made sure that the line out's ground (also earth through the computer/interface) to one of the potentiometers back side, which is indeed connected to the chassis.  Because the speaker out on the amp is connected to earth, the both the + and - inputs on the stompbox sit at about 22K from the DC supply's ground and subsequently the earth ground via line out.  The result is from the earth ground of the line out and the earth ground of the amp, which again is separated by a 22k load, has a 60Hz sine wave tearing up the noise floor.  Since I cannot rely on the DC supply to be grounded through the amp because there is that 22K load (the ground noise gets worse if I remove earth from the line out), I have no way of isolating the DC ground from the amp's input.  The weird thing is this is my 5th unit and the first time I had this problem, my first few units worked flawlessly.

Here is a picture depicting the overall ground circuit:



The red point is the DC+ and the black is DC-.  The green points are the two earth ground points that have a voltage difference of a 60Hz sine wave, about 2Vp to 3Vp.  The one thing I am going to have to test out, I tried to run a continuity test on my amp, the negative terminal  inside the stompbox is connected to the eath ground on the amp (chassis), but a continuity test from the chassis to the earth prong is showing an infinite impedance, I am about to take apart my amp to see if the amp is the issue, it itself has had a loud 60Hz noise from it, but I though it was a bad tube, as one which has a crack in the glass red plated when I was prototyping the first unit when the chasis of the box accidentally shorted out the load.  It could very well be that the ground connection on the amp went out, but if that is not the case, I need to find where the issue is occurring.

Figure two is the only possible way I could think of to use a small signal transformer to get the earth separation, if that will work then I will use that in my upcoming model, but right now I need to get the current build working with as little modification as possible.

[TheWinterSnow]

Ok so I quickly got my amp apart and it indeed does look like the earth connection of the plug has broken off inside the cable and the amp no longer has an earth.  Would that cause this issue?  I am sure it is as the amp would be connected to earth through the 16+22K+1K load and then to earth, which would mean that it is the amp that is picking up all the noise and not my unit?

[Jazznoise]

Dump the plug is the simplest solution. Other than that we'd need to identify another difference between these amps to spot a source for a problem. I'm guessing the tube voltages are roughly the same across the board?