Catalyzes hydroxylation of the anthracycline structure at position C-6 during the biosynthesis of oxytetracyline. anhydrotetracycline + H(+) + NADPH + O2 = 5a,11a-dehydrotetracycline + H2O + NADP(+) Antibiotic biosynthesis; oxytetracycline biosynthesis. Mutants fail to produce oxytetracyline but instead synthesize novel polyketides of shorter chain length. Belongs to the PheA/TfdB FAD monooxygenase family.