My home built low power Twin is making a faint hum when powered up. It's not filament hum, the noise is the transformer itself humming. I can't feel any vibrations if I touch the transformer, but the sound is clearly coming from the PT unit. The hum is audible even on standby. When running, other than the faint mechanical hum if you stop and listen, it sounds great when playing through it. Never encountered this before. Any thoughts as to the cause?
Allow me to post my recent exploits in this thread.
1. Late '90s Marshall JMP-1 preamp. PT hum. Took the cover off and the hum was gone. I noticed the top of the PT comes in contact with the cover. So I applied a layer of this to to top of the PT - where it was coming in contact with the case.
The PT didn't get hot, any ways, but this is the stuff I used. It tamed the hum quite a lot.
2. '76 Marshall 1987. Bad PT hum. The PT mounting ears were bent on one side of the PT and the PT was loose on that side. I opened up the amp and loosened those 2 screws (they were a bit loose, any ways) and straightened out the mounting ears on the PT - then tightened the screws up again. I don't think they loosened on their own. The had lock-nuts underneath. Either the screws stretched in shipping (entirely possible) or they were never properly tightened - from the factory. It didn't look like anyone had been on the screws/nuts before. End result - the amp is quiet... RE: chassis hum. The reality is that amp is anything but quiet. 😜
3. 2012 Marshall DSL1H - 50th anniversary model. Bad chassis hum. They put the PT and the OPT under the chassis and the PT comes in very light contact with the head shell. So I put some of the above-linked silicone sheet on the top of the PT where it comes in contact with the head shell. The amp is almost completely silent at idle now. I tried one layer but it wasn't enough so I ended up using 2.
If your PT is snugly mounted to the chassis, how much of a gap is there between the winding cover and the chassis? That could be the source of your noise. Try some of that stuff I mentioned above in between the winding housing and the chassis. I bet that would fix this.
In fact, if you want some of this stuff for frees, PM me your address. 😁
This is more rare and typical of an old transformer laminations that make up the portion the coil wraps around can become loose from one another and vibrate with the A/C current passing through it. This should only be suspect if everything Leftee mentioned (which are typically the issue) has been eliminated. The issue of loose laminations typically require transformer change