Humbucker Sounds Like Single Coil

I need some serious help. I just recently bought a 1974 Gibson SG Standard and when I put the bridge Humbucker pickup on, it's noticeably more quiet and sounds exactly as if my Fender Stratocaster was in bridge position. When I was playing the guitar on the sellers amp, I didn't notice it, but when I play through my H&K 36 watt, I can hear it. I have no idea how guitar wiring works (Yes I should learn someday) and I'm really concerned about losing the original pickup that's been in there since 74. Please help! :(