Building your own guitar preamp is a great way to customise your tone and add versatility to your rig. The job of the preamp is to process the signal of your guitar before it gets to the power stage. The preamp consists of a circuit board, power transformer, tubes and potentiometers. These are connected by various wires, resistors and capacitors. The most simple approach to preamp building is to use a preamp kit. All of the parts are compatible and ready for assembly. Typical preamp kits come with a wiring scheme and assembly instructions.
- Skill level:
Things you need
- Tube preamp kit
- Soldering iron
Search for a tube preamp kit online. These kits come complete with all key components as well as a pre-cut enclosure and schematic.
Read the wiring scheme which has all of the electrical and circuit related information that you need. It illustrates the layout of the printed circuit board and includes all resistor and capacitor values. Familiarise yourself with the circuit layout before beginning construction.
Populate your printed circuit board. This is where the signal processing takes place. The type and value of each surface mounted component influences the tone of your output. For example, higher value resistors attenuate the line level of the preamp signal. Lower value resistors do the opposite. Follow the assembly instructions and load each part in order on to the top of the printed circuit board. Solder them securely to the board from the bass. Solder the tube bases to the circuit board. These go in the pre-cut holes.
Wire the potentiometers. A potentiometer is effectively a valve that governs the flow of current. When you turn a dial on your amplifier, the potentiometer at the rear of the dial increases or decreases the flow of current through a certain part of the circuit accordingly. So when you crank the bass, the potentiometer sends more current to the bass handling part of the board. Each potentiometer has a solder lug. Solder the wire to the lug and then connect the other end to the correct input terminal on the board.
Screw the board into the chassis. Typically the printed circuit board will be drilled. Now solder the input jack to the board.
Load the power transformer. This component transforms the high impedance guitar signal and turns it into an electrical signal. This signal is then sent to the circuit board for processing. Fit the power transformer into the pre-cut hole on the preamp chassis. Make sure that the solder lugs sit neatly at each side of the hole, then solder the transformer in place.
Wire the board to the power supply. Solder a wire from the power supply to the circuit board so that it is powered.
Mount the tubes into their bases. Slot them in vertically.
Screw the enclosure lid on to the base and test the preamp.
Tips and warnings
- Test your circuit with a volt meter before you plug in. The power ratings will be listed alongside the assembly instructions.
- Inspect your parts before assembly. One faulty resistor or capacitor can cause the entire preamp to malfunction.
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