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===Use with SB-200 / SB-221 Amplifier===So, theoretically, you can use the IC-7600 with your old Heathkit amplifier without the Harbach Soft-Key interface as long as you use the MOS-FET ACC mode. However, you are better off if you go ahead and install the Soft-Key interface. If you have the Soft-Key interface installed, thus reducing the key line to approximately 0.7VDC at 1.5mA, then you will want to use the Lead ACC mode.The recommended configuration: Use Harbach Electronics SK-220 Soft-Key Keying Interface and on the Icom ic7600 ACC set mode = Lead.