From what I can extract - Guarang wants a variable 0-120v output - I assume this is AC at the mains frequency. Michael covered the usual technique of phase control with a triac.Firstly (I don't know if this was mentioned elsewhere) that the triac was a poor choice of device for PWM - as you can't turn them on or off at will. The more common technique is one used by variable speed drives and that is rectifying the mains into a capacitor and using pwm to drive IGBTs or MOSFETs to regenerate the ac waveform with the required frequency and amplitude. Not something that is particularly easy. I would suggest trying the Internation Rectifier site for some app notes.