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Step 1
Use an old boom box, transistor radio or small amp for your pre-amp. Virtually any electronic device that projects sound has the basis of a pre-amp. You may need to wire a line-in socket and use an adapter in the earphone jack if it's smaller than the 1/4-inch phono jack standard on a guitar. John Lennon, legend has it, used a small transistor radio to get the distinctive overdrive sound on the Beatles' song "Revolution."
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Step 2
Buy a kit. Pre-amp kits are available that have all of the transistors, capacitors and other audio amplification pieces to construct a lead-in device for your guitar. They also come with instructions, so it is easier than starting from scratch.
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Step 3
Use an old tube radio. Tubes give guitars warmer sounds, and distortion was discovered as a musical device by pushing tube amps beyond their recommended usage. Old tube radios can often be found in antiques stores or garage sales. You will need to reroute the wiring to the speaker to an output jack. You also will need to create an input jack.
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Step 4
Use a small amp. If you are trying to a build a pre-amp for a 100-watt amp, using a small 10-watt, 12-watt or 18-watt amp makes sense. You can strip the innards out of the amp's housing and bolt it inside the 100-watt amp housing, rerouting the out-to-speaker wires to the big amp's input and rerouting the small amp's input to be the big amp's input. This essentially creates a bridge so the guitar signal passes through the pre-amp before it gets to the big amp. You can also leave the small amp in its housing but disconnect the speaker if you hear conflicting sounds.
