[QUOTE=rockon1;199876]I would like to use a trim pot in place of the 120K resistor in the bias supply in the schematic below to make my bias adjustable. At the moment I have a 100K in the amp and it works great. The trim pot is only rated for 300vdc max. The voltage before the diode is 330vac. Thanks Bo[QUOTE]
Put a resistor, say 60K ohms in series with the trim pot. That will recuce the voltage across the pot by about half.