I'm trying to figure out the best way to lower the voltage for a transformer. As it stands the transformer converts from 120 to 14.5 - both VAC and the primary draws about 3.7-4 amp (that would mean ~ 33 amp on the 14.5v side). So the ratio is 8.276:1 I would like to get the voltage to 13.0vac which would mean that I should have a primary voltage of 107.59 VAC for the ratio to give 13v.

Now I'm not sure if the resistors should be used on the primary or the secondary. If everything was ideal I would be able to adjust the voltage from 103.45 - 111.73 which would give 12.5-13.5 on the secondary. I'm concerned about the current if putting the resitor on the secondary but I don't know if that is a valid concern or not.

What is the best way to calculate this and where should the resistor(s) go?

IDK if more info is needed but the output from the secondary will be put through a full bridge rectifier and have a smoothing cap.

 

Very bad idea. First off, it would require about a 50-100 watt resistor - those ain't cheap. Second, your primary voltage (thus, your secondary voltage) would change with the current draw.

Your better bet would be to remove some windings from the secondary. The secondary windings are usually on the outside, so this shouldn't be too difficult.

 

IDK if more info is needed but the output from the secondary will be put through a full bridge rectifier and have a smoothing cap.

13.5 volts on the secondary will give you around 19vdc - 20vdc after rectification and smoothing.
+1 on removing turns, typically 2 to 3 turns/volt.
Max.

 

If you can't or don't want to change the number of turns, you could add a buck transformer on the primary.
A 120 to 12V, 4A transformer with the secondary connected in series with the main transformer primary (polarity out-of phase) will give about 108V on the primary, close to what you want.