You can have DC on the line and oddly enough, it can be passed through transformers. It could also be considered a 2nd harmonic as basically its caused by loading on 1/2 of the AC line and so the AC waveform can be offset slightly just as if it were riding on a slight amount of DC. The resulting asymmetry could be interpreted as a 2nd harmonic.
Its easy to correct with a DC blocker which is a simple and inexpensive device. DC on the line can cause the core of the transformer to saturate, causing it to rattle or hum. It can vary from one transformer to the next even with parts from the same construction run.
Grounding in the house has nothing to do with this problem even if its not DC on the line. So the age of the wiring isn’t the problem.
Transformers can be mechanically noisy on their own despite the best of intentions. Tightening a noisy part down won’t help and might make it worse. But sometime the part is causing something else in the chassis (like a cover) to resonate and it might be possible to damp that part. If you can only hear the problem when you are within 6 inches of the amp its not a problem. IME, most audiophiles listen to their gear considerably further from the amp they are playing :)
Getting quiet transformers is always an on-going challenge. Transformer manufacturers often don’t understand how important it is for the part to be silent and might sometimes skimp on a few core laminations or the like thinking they can save a buck. In the process they might loose the entire run as reject parts!
Good Luck!
Its easy to correct with a DC blocker which is a simple and inexpensive device. DC on the line can cause the core of the transformer to saturate, causing it to rattle or hum. It can vary from one transformer to the next even with parts from the same construction run.
Grounding in the house has nothing to do with this problem even if its not DC on the line. So the age of the wiring isn’t the problem.
Transformers can be mechanically noisy on their own despite the best of intentions. Tightening a noisy part down won’t help and might make it worse. But sometime the part is causing something else in the chassis (like a cover) to resonate and it might be possible to damp that part. If you can only hear the problem when you are within 6 inches of the amp its not a problem. IME, most audiophiles listen to their gear considerably further from the amp they are playing :)
Getting quiet transformers is always an on-going challenge. Transformer manufacturers often don’t understand how important it is for the part to be silent and might sometimes skimp on a few core laminations or the like thinking they can save a buck. In the process they might loose the entire run as reject parts!
Good Luck!