well as they said the ratio is correnct but not the number of turns. 18 turns is like short circuit, you need to determine the number of turns acording to your current trough the transformer, the type of core you use, the value of the volteges you will have trough the transformer. something like you said 18/6 turns, i dont think that you can use that as a audio power transformer. you got to consider that the wires need to hold the current you calculate considering then Win=Wout, and add to it the current that passes trough the primary due to its inductance. if you REALLY can't find all you need to design a transformer, ill do a search trough my old books.