Need some help fixing a faint hum

I first noticed it with my HD800's. Then I tried a few lower impedance headphones (portapros and IE600's) and it's more noticeable with those headphones.

When I first posted it didn't seem like the impedance or balanced switches had any effect, but now after resoldering a bunch of joints and sanding those ground points I am noticing a bit of a difference. It gets slightly louder when switching from low to high impedance and it almost doubles in volume (still faint though) if I switch from balanced to unbalanced while using the TRS input. Hum stays about the same volume when using the XLR output and changing from bal to unbalanced.
 
I would try taking the amp to a different location (work is usually easiest) just to gather a datapoint as to whether it's environmental or not.  Stuff like this can creep through your powerline and get into the signal path, and we can certainly help with ways to combat that if it ends up being the issue. 

 
Unfortunately no luck on moving the amp to a different house, noise is still there.

However I may have somewhat narrowed down what's going on by accident. I still had my portapros connected to the headphone jack while the amp was flipped upside down and I was about to desolder the regulator board to reflow those joints. I heard a loud hum out of the portapros when I moved the soldering iron close to the amp (even though it was unplugged and powered off obviously). Put the portapros on to hear the hum while moving the soldering iron over the internals of the amp and its the exact hum I'm hearing when the amp is on. No clue what that means but the hum seems to be the loudest when my soldering iron is close to the output transformers?
 
Your soldering iron puts out a strong AC field. That radiated field what is getting into the transformers. You might want to double check that the drain wires that connect from each transformer to the safety ground have been well connected to the safety ground screw, and that the insulators under the all transformers are properly installed to insulate the transformer from the chassis. If they are not there is the possibility that the radiated field from the power transformer (which is, by design, a very small magnitude for a transformer of this size) is getting into the output transformers via eddy currents generated in the chassis panel and up thru any mounting screws that are not isolated from the chassis.
 
I would also double check the DC resistance between the earth lug at the back of the chassis and all the headphone jack terminals with black wires on them (set to unbalanced).
 
Thanks Doc, I'll check the transformer mounts tonight an make sure everything is properly insulated.

I think the ground lugs on the output transformers are working because I'm getting continuity from the earth lug to the metal housing of the transformers.

Thanks as well PB, measuring the headphone output ground wires with the switch on unbalanced I'm getting .1-.2 ohms on all 3 headphone jack ground connections (which is what I get measuring any of the ground lugs).
 
Not sure if it matters or not but I just noticed that I get a little extra resistance measuring ground on the main transformer. If I clip one lead to E and then measure the top bell screws or the side of the transformer where it's not coated I get like 2-4 ohms instead of the usual .1-.2 for ground connections.
 
2-4 ohms should still be fine. You might just try tightening the mounting screws a little. It's difficult to know whether all the connections are as they should be from an ohm meter reading since they should read close to 0 ohms from the transformer to the safety ground if properly isolated and connected to the safety ground, and would also read close to 0 ohms if they happen to not be isolated and are touching the chassis right at the transformer. Because of this a visual inspection is probably best to make sure that all the transformer mounting washer hardware is installed in the right order so the transformers "float" where they are mounted and only connect to the chassis electrically at the safety ground.
 
Oh my gosh... Doc I think that was it! I pulled off the transformer and the fiber washers were completely deteriorated. A few of them were crushed as I had already tried tightening the transformer down and the metal posts holding the power supply board were touching the chassis. I just ordered a new set of washers from digikey and I'll put them in once they arrive. Thank you both for the quick replies especially over the weekend, this was driving me nuts and hopefully this fixes the problem!
 
Okay well scratch my last post... it's still there. I put in all new fiber washers on all the transformers and verified that they're isolated from the chassis by disconnecting the ground lug near E and they don't show continuity on any of the screws.

If anything the hum is actually louder now, I have no idea what else to check for here.
 
Any other suggestions for things to try/check for? I was doing a bit of googling and found a few instances where people had mentioned faint hum on their newly built Mainlines. One guy said it just went away after a few hundred hours of usage. If it's something that will may go away on it's own would it just be the tubes settling in or something else?
 
Just circling back on this with a follow up that resolved my issue. It looks like it turned out to be a noisy 6c45 tube. Originally I didn't think it was a bad tube because it didn't sound like microphonics. However, I think it was oscillating and making a hum in the low 100-200 hz range. Just swapped in some new reflectors I purchased off ebay and it's dead silent now. Thanks for all the help with tracking this down!
 
Thanks for coming back to let us know what resolved the issue.  With Russian tubes in particular, we have found that emission and noise performance will stabilize with some break in time (which may suggest they don't get much burn in after production), so it may be worth leaving the amp running with those tubes in for an extended time to let them cook before discarding them.
 
Thanks guys! Yeah I cleaned the pins with a q-tip on a dremel and some metal polish. I also put about 200-300 hours on them but it seems the one tube just doesn't want to quiet down. Out of the 4 6c45's I have now it is the most microphonic by far so I think it's just humming away no matter what.
 
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