Crucial to the function and quality of an amplifier, the output transformer is almost always the most expensive part of the system. The output transformer, which modifies the resistance of the signal to match the resistance of your speakers, has a significant effect on the sound. Yet for a few dollars and a few hours of work, you can build your own transformer to the exact specifications you need.
Attach the paper bobbin to something that will spin it at a regular speed. A simple set-up is a drill with a locking switch and a dowel the width of the bobbin.
Wind the primary winding to the number you need to create the desired resistance, which depends on the diameter of the wire as well as the number of times it is wound.
Attach a length of coloured wire to the end of the primary winding. This will be used to connect the transformer to the circuit.
Cover the primary winding in wax or butcher paper, taping it at the ends. This insulates the primary winding from the secondary winding.
Wrap the secondary winding. The same rules apply here -- number of winds and thickness of the wire depends on the specifications your system requires.
Attach a coloured wire to the end of the secondary winding in the same way you did for the primary winding.
Insert the middle part of the "E" core through the bobbin and attach the "I" part to the bottom, making a box around the completed transformer.