Step 1: What you will need
1. A cassette player with a speaker (Preferably also with an equalizer so you can change the sound settings just like on a real amp).
2. A regular instrument cable.
3. A wire cutter/stripper ( I used a fruit cutter knife :P)
4. A Solderer.
5. Rosin-Core Solder.
Step 2: Instructions
-Open up the cassette player. What you have to do now is to take the two wires from the stripped instrument cable and try to locate the wires in the circuit that gives the sound to the speaker. For me, these two wires looked identical to the wires in the stripped instrument cable, but smaller. To know which the right ones are, simply plug the remaining plug into the guitar and take the instrument wires and find it using trial and error.
-Take the solderer and heat it up. Now take the solder and place it at the spot where the wires of both cables touch (which is 2 spots). Carefully press the solderer onto the solder and it should instantly melt the solder which glues the wires together.
-Make sure everything is dry and find a spot where the instrument cable can leave the inside of the cassette player (I removed one of the buttons and just placed the cable there). Now it's time to put back the cover of the player and reassemble it.
Step 3: Conclusion
To turn it on, simply press the PLAY button off the cassette player. The STOP button will turn it off.
One disappointment was the fact that you could not use headphones. When you plug them in all you get is this really high pitch beep sound. This was maybe a result of something I did wrong (i.e. I might have accidentaly broken something in the circuit when soldering), so try anyways and you might have better luck than I did.
I am actually pretty happy with the sound. It is quite crunchy and distorted. When you play high notes, you get a really funky sound, especially if you are playing on the rhythm setting of the guitar.
Here is a sample (Treble). The background noise is just feedback created from the microphone. When I remove the mic, the only unwanted noise left is the sound of the playing cassette head, which can probably be removed somehow. I know my playing isn't very good... :P but that is not significant right now! Decide for yourself what you think of the sound.
Enjoy your new amp!