You can if you are carefull. If you aren't, you will fry the glowplug, or it just won't light up. I use several 10W 1Ohm resistors so that the power transistor in my supply does not get as hot. I use a 2n3055 in a to3 package with a big heat sink. The resistors drop the voltate across the transistor by several volts and dissapate a bit of heat - 10W resistors are designed to get quite hot. Otherwise the transistor gets *very* hot. However, the resistors alone are a poor way to regulate it. You would need a bank of them and the means to switch them in and out to adjust for the plug you are using. It's doable, might be a bit of work. If you understand electricity to the point of knowing basic Ohms law math, you can probably easily design it. I designed my own using an lm317 regulator, a 2n3055 general purpose transistor, and a few resistors and a potentiometer for adjusting the output. I can post the design if anyone is interested.