A bias control circuit controlling the power transistor Q2 in a transistor amplifier circuit for supressing the arc formed across the contacts 12, 14, 16 in a glow plug controller 10 for diesel engines. The bias circuit generates a load line for the contacts 12, 14, 16 which has a high voltage, low current portion and a high current, low voltage portion. The arc is formed when the contact 14, which may be slow operating, is required to break a high current, high voltage line.