Guitar preamp circuit – over drive using 12AU7

This Guitar preamp circuit is a guitar booster for those with a love for music. act as preamplifier or over drive signal acts soft voice before to the power amplifier or headphones. Highlight is a very clear sound without noise. Because we use 12AU7 vacuum tubes is the main equipment, which is better than any other preamp circuit. It also easy to builds because of the a few parts.

How it works

In Figure 2 is circuit diagram. When We play the guitar, micro phone or contact will get sound signal from the electric guitar to J1. Then coupling through a capacitor-C1 to a grid pin of 12AU7 vacuum tubes-V1/1, by have a resistor-R1 is setting an input impedance of them, and has VR1 act as resistance of cathode to set bias current of vacuum tubes V1. When the bias is changed cause gain also changed so Operating VR1 adjusts the sound as we played guitar.

the circuit diagram of Guitar Preamp - over drive

Figure 2 the circuit diagram of Guitar Preamplifier – over drive.

The first signal section will be coupling through capacitor-C2 to a grid of V1/2, which is the final preamplifier. Then coupling through capacitor -C3 to VR3, Which acts as the volume to adjust the output level of J3. To application to headphone or guitar power amplifier in sequence. The capacitors C4 and VR2 will adjust the tone according to the user needs.

How to build it

After collecting the necessary parts in this project. Then assemble the component on the PCB.
Starting from several points short circuit wire. Then resistors The eyelet for soldering wires. Next capacitor. But because this circuit using less amounts. As a beginner, it can be assembled quickly.

The final step is to install all three volumes and Jack inputs and outputs. Including input jack for DC power. It may test the wires to the outside before. If we have any problems going to be solved easily.


Testing and application

Since this projects have less parts. And have test point a little. If the circuit is correct, nothing wrong. We created this project to be able to run the course. Before apply dc power supply to this projects should disconnect the tube is V1. Then begin to apply voltage to circuit. Next use voltmeter to measure DC voltage at pin 5 of socket on PCB compare with ground of circuit. It must near supply voltage, because it serves the heater, then plug the tubes into the socket packet tightly.

Reductions VR3 low volume, then enter the signal from the guitar to the point J1, And the the output signal from J3 to the amplifier or headphones to test it. Adjustment of VR3 the volume up a few. The experimental electric guitar, listen to the sound that out of the output signal. If have one it indicates that the project can work normally.

If no signal is found, remove power, then check the soldering equipment that the various loose. Assembly and the tubes socket to tighten it, there is heater voltage. The various signals point, wires, must complete. Then check step by step until the circuit worked normally.

The components list
Resistors ½ watts +-1%
potentiometer resistor has a single class, short axis.
VR1—50K (B)
VR2, VR3—100K (A)
Capacitors (polypropylene)
C1, C2—0.047uF 400V
C3—1uF 400V
C4—0.01uF 400V
LED1___ LED3 Red
Other parts
V1—12AU7 vacuum tubes with socket.
J1, J3— Jack Mono.
J2—Jack DC in.
Universal Box, Knob, small wires, shielded mono cables, etc.


This circuit I do not test it. So, I cannot confirm that It works well. You must consider carefully before creating.

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9 thoughts on “Guitar preamp circuit – over drive using 12AU7”

  1. Can u run a 12ax7 with this circuit and use higher voltage. i see you are using
    high voltage capacitors but this circuit is only suppose to run at 9v.

  2. Me again.
    This works and is quite awesome.
    But let me say it, 12au7 heater doesn’t work under 12V in series arrangement. I made this project on 12V.
    And take care NOT to make it on higher voltage, at least don’t this way! You will burn the heater -> ruin the valve. It is a common good practice to separate heater from the GND and B+ and join them at the input filter capacitor. Otherwise there is probably no reason this design wouldn’t work on much higher voltages. If you wanted to use 9V at all means, then paralell the heaters (use joint 4 and 5 pins and 9th pin for 6,3V heater arrangement and regulate voltage down to 6,3V (7806 IC). With higher voltage, use 7812 IC to

    Made it in PTP – messy as hell, but easy and quick.
    Cathode biasing with the pot is not an optimal way to get gain – it would have been better to use used and usual resistor paralleled with a cap, and then maybe use a pot on the cap.

    It would have been interesting to stack two of those in series (will try it with low voltage caps on another – oops, i realized I just did, look at this schematic:
    It has a fixed resistor (of the same value) instead of gain pot (and in + version bridged with few uF cap) – i will probably try . It also omits (in my view) unnecessary input coupling cap (at list for guitar use).
    This design is the first stage of my DIY amp and they sound really crazy in series. You can see that this design lacks tone control and has a input grid resistor (radio frequencies prevention). Doing some additional tweaks could lead to a nice tube overdrive pedal. Due to something muddy sound I am thinking about BJT-powered buffer in front of it. Or even op-amp powered.


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