Does the amp hum when there is no cable plugged in? If not, the problem lies in the cable, guitar or surrounding electrical interefence devices like CRT's, TV's, Fluorescent lights, etc.
If it hums with no cable plugged in, you need to do some checking. Start by pulling one tube at a time and then powering the amp back up to see if the hum goes away. Start with the rightmost tube as viewed from the rear, V1.
Take it out, power it up. If the hum goes away, the problem is in that tube or in that circuit. Always try tube replacement first, because it's easiest. The problem could be a bad electrolytic capacitor or it could be a shielding issue, as well.
Post your question on
www.fenderformum.com under the Amp Mods and Repairs forum for help.