It may simply be a loose transformer mounting. Try adjusting the mounting screws.
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In addition to kr...'s comments, I can add that the power line influence is likely act on tubes but under certain conditions can act on transistors as well. So make also sure that your amp is not standing next to the AC outlet; check AC outlet for proper and tight connection.
It's a good practice to clean the contacts that are probably greased over the time and than reatach them back. BE CAUTIOUS in performing ANY work with AC powerlines and turn-off the circuit breaker or fuse before doing something with the outlet(s).
Also you should check your powercord if it's stiff enough. Stock powercords(especially non-detachable) have a tendency to get loose and soft which means that powercord might even be the reason. Please note that improper connection has a non-negligable capacitance that can cause a "cavity transfers" i.e. through the air that inflict a large amount of parasite magnetic waves to the near-standing equipment.
Even having non-perfect AC outlet or AC line quality the high-quality powercords can act to eliminate the problem due to increased conductance and tightness of the contacts.
A humming noise from an amp is quite common, my Creek amp emits a hum. The main culprit is normally related to your AC line that your amp in plugged into. For me, I have these halogen lights on the same circuit. With the lights on the "low" setting, you can hear my amp hum from across the room. With them on "high", it is barely noticeable. When my fridge kicks on, the hum goes away completely. Go figure.
I've got the same issue with some very large torodial transformers. It's worse when the oven or microwave are on (even with dedicated lines!). The manufacturer told me it was dc voltage in the lines and there's nothing I could do. Anyone know if some of the line conditioners would help (e.g., the Noise Hound)?
For a transformer to hum, the gapping in a standard (i.e. square) transformer is off by some very small amount, whether in the gap itself or in the lamination spacing. When the gapping is off, the transformer suffers from eddy current losses that emit audible harmonics (i.e. a hum). In other words, the transformer is either loose as mentioned above, or the lamination has separated (both often the same problem). It is possible that there is a frequency problem that can induce such noise but if so, the problem lies in the rectifier and not the transformer. If a toroidal transformer makes noise, the design is most likely bad (that means get a better amp). I would check the transformer to make sure its mount is tight since the hum is actually a vibration that can make the problem worse with time by loosening it. If you put your hand firmly on the transformer, the problem should disappear in this case. Otherwise, take it to a pro for a close look at the rectifier circuit. Good luck! Arthur
I had the Big Rotel and the amplifier hum drove me crazy. The solution was the AH Offsett Killer. You can read reviews on it on Audio Review under the "others" section. I have since upgraded to a Clase thats dead quiet so I do not use it anymore. I would be willng to lend it to you to see if does the trick.
Thanks for your responses. You have all provided advice to consider.
Matty -- perhaps you need an exorcist.
Gooddomino -- have you ever tried correcting the problem? Do you know if the transformer is a torroidal design?
Aball -- someone else said something about the laminations coming apart. I assume it is safe to touch the transformer while turned on?
Darryl -- what is an AH Offset? I be interested in researching it!