I just purchased this Fender Princeton 65 amp today at a second hand shop for only $32. I just tested it and everything works except I didn't realize when I bought it that the treble knob was missing. I thought maybe I would just replace the knob but it looks to me like it's a bigger job then I thought. Is it as easy as just replacing the knob or do you have to take the amplifier apart because I wouldn't know how to do that. Any suggestions will be much appreciated.