Need some help fixing a faint hum

[MikeSattler]

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!

[MikeSattler]

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.

[MikeSattler]

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?

[Paul Birkeland]

There is a possibility that a couple hundred hours of run time could help.

[MikeSattler]

Okay thanks I'll give that a shot

[MikeSattler]

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!

[Paul Birkeland]

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.

[2wo]

Also worth giving the pins of the offending tubes a good cleaning.. John

[MikeSattler]

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.