Problem with MPF-102 preamp

[flp2mm]

I glued a piezo element inside my ukulele and then built this http://www.buildmyelectricguitar.com/electric-guitar/misc-electric-guitar/piezo-jfet-preamplifier-mpf102.html 
preamp (and several others) to help boost the sound. They all work but they all cause my Fender Champ to hum. If I plug the ukulele directly to the Fender Champ, no hum. If I plug the ukulele into the preamp and that into the Fender, it hums. If I plug my electric guitar into the preamp and that into the Fender, no hum.
I hope someone can solve this problem for me. Thanks.

[caspercody]

Did you try a different MPF-102, battery, or even a wall wart supply?