A11 CONTROLLER EXPERIENCED OPERATIONAL ERROR WHEN B737 LOST SEPARATION FROM PRECEDING HEAVY AIRCRAFT ON FINAL; CORRECTIVE VECTOR ACTION WAS TOO LATE.

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I WAS WORKING FINAL W WITH 3 PLANES. WE JUST SWAPPED RWYS FROM RWY 7L TO RWY 7R FOR SNOW REMOVAL. I CREATED A WORKABLE HOLE TO PUT ACR X BEHIND ACR Y AND AHEAD OF ANOTHER HVY. I ISSUED SPD CTL AFTER VECTORING THE B737 BEHIND THE FIRST HVY ON FINAL. FOR THE ENTIRE TIME ON THE FINAL; I HAD SEPARATION UNTIL COMPRESSION INDICATED I WOULD LOSE 5 MI BEHIND THE HVY ON FINAL SO I BROKE THE B737 OUT PRIOR TO LOSING 5 MI. AT THE POINT ON FINAL I'M ACCUSED OF LOSING SEPARATION; I HAD IN EXCESS OF 4 MI WHEN I NEEDED 3. WHEN THE B737 WAS ESTABLISHED ON FINAL BEHIND THE HVY; HE WAS MORE THAN 5 MI IN-TRAIL AND 30 KTS SLOWER. I GOT THIS SPACING BY SLOWING THE ACFT TO ENSURE IT WOULD HOLD. LATER; THE HVY SLOWED SO MUCH; I HAD TO BREAK THE B737 OUT. I CONTEND THERE WAS NO LOSS OF SEPARATION ON FINAL. TO MY SURPRISE; ANCHORAGE TWR CALLED THE OPS MGR TO LAUNCH AN INVESTIGATION OF THE SPACING BTWN THESE 2 ACFT AND AFTER REVIEWING THE PLAYBACK OF THE EVENT; LCL MGMNT DETERMINED THAT I HAD LESS THAN 5 MI BEHIND THE HVY BUT HE WAS NOT YET ESTABLISHED ON FINAL; SO I ONLY NEEDED 3. SINCE HE WAS SLOWER; WHEN THE B737 JOINED FINAL; HE WAS 5 MI PLUS BEHIND SO ALL ALONG I BELIEVED I HAD STANDARD SEPARATION. LATER; WHEN IT WAS APPARENT THE LEAD ACFT SLOWED TOO MUCH AND I WOULD LOSE SEPARATION; I CANCELED THE B737 APCH CLRNC; BROKE HIM OUT AND RE-SEQUENCED HIM. I TRIED TO MAKE IT WORK AND IT DIDN'T BUT I NEVER BELIEVED I LOST SEPARATION.