Reducing ac voltage from 19,2VAC to 18VAC
Let it be if that 7 percent overvoltage is not harmful in your application.  If it is then put 2 double diodes in series. Double diode=2 in parallel, directions = opposite. You will dissipate that 7% so be sure to have 2.5A types and proper cooling. As already proposed, the voltage drop may do the job without any intervention.