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.
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.