powering a monotron

[nicktheknife]

I'm absolutely new to bending. Good, bad or indifferent, I'm trying to run a casio pt-100 into a monotron. I wanted to start by finding a way to power the monotron from the pt-100 board. PT - 7.5 volts, montron - 3 volts. I ran a ground from the power section of the board. I had no idea how to lower the voltage of the main power (tried a potentiometer, didn't cut the voltage back enough), so I found a spot on the board that shows 3.5 volts on the multimeter. I ran wires and it doesn't power up the monotron, it only kills the sound of the casio altogether. Any tips?