Positions A and B on the 77Dx/Sx amps are prewired for 1.8-2.0 MHz. The B position has extra loading capacitance. The other bands are: 3-5 MHz 5-8 MHz 8-12 MHz 12-18 MHz 24-30 MHz (this band not available on the 77SX, because the output cap of the two tubes is too high) 73, Jim W8ZR