S-adenosyl-L-methionine-dependent methyltransferase involved in the biosynthesis of mycaminose, an essential structural component of the macrolide antibiotic tylosin. Involved in the last step in mycaminose biosynthesis by mediating dimethylation of the hexose C-3' amino group. dTDP-3-amino-3,6-dideoxy-alpha-D-glucose + 2 S-adenosyl-L-methionine = dTDP-alpha-D-mycaminose + 2 H(+) + 2 S-adenosyl-L-homocysteine kcat is 7.2 min(-1) with dTDP-3-amino-3,4,6-trideoxy-alpha-D-glucopyranose as substrate. kcat is 32.5 min(-1) with TDP-3-N-methylamino-3,6-dideoxy-R-D-glucopyranose as substrate. kcat is 9.9 min(-1) with dTDP-3-amino-3,6-dideoxy-alpha-D-glucopyranose as substrate. Antibiotic biosynthesis; tylosin biosynthesis. Homodimer. His-123 is a strong candicate for an active site that hydrogen bonds to a water molecule which in turn hydrogen bonds to the C-3' amino group. However, it is not conserved in related S.venezuelae DesVI methyltransferase and its mutagenesis does not completely abolish catalytic activity (PubMed:21142177). Belongs to the methyltransferase TylM1/DesVI family.