Ground loops, poor grounds in cords, etc can all lead to extra noise too. Flourescent lighting in the area, or on the same supply circuit will induce noise into almost any system.
:Hi I built an AC30 and It sounds great only when I crank channel one or channel 2 with no instrument plugged in it hums faintly especially when I turn up the bass control on the top boost. I want to increase the filtering but wonder how far I can go before I'm over filtering. As it stands I have a GZ34 and 40uF - 10Henry Choke - 20uF. I have the Center tap of the output transformer and all the plate B+ feeds coming off of the 20uF after the choke. My question is can I add another resitor 5 - 10K? and then another 20uF and then maybee move my B+ leads for the preamp to the 20uF after the 5 or 10K Resistor? Would this be over doing it?