The bias resistor is not needed for the Heil microphones. You may need to increase the overall gain of the KSB2 as suggested in the notes by John, KI6WX: http://home.pacbell.net/johngreb/modifications_to_the_ksb2.pdf I have used the preamp from the KSB2 SSB Adapter Notes page for the past 3 years with my ProSet4 with excellent results. 73 Gary, N7IR DM43bi