Catalyzes the transfer of a dimethylallyl group onto the adenine at position 37 in tRNAs that read codons beginning with uridine, leading to the formation of N6-(dimethylallyl)adenosine (i(6)A). adenosine(37) in tRNA + dimethylallyl diphosphate = diphosphate + N(6)-dimethylallyladenosine(37) in tRNA Monomer. Belongs to the IPP transferase family.