Guitar Preamp

1. Basic considerations

This circuit could be considered as medium-gain preamp. Despite that the schematic seems simple, it allows very wide spectrum of arrangements.

 

The circuit is designed to cover the spectrum from clean, to blues, to classic rock levels of distortion, both rhythm an lead. The bass also is welcome, but some components should have different values from rhythm/lead version.

 

Adjustment on test would  be essential for fine tuning the tone, additionally, the user should be familiar to work with HT schematics. 

 

We could provide completely assembled PCB with all necessary components, adjusted accordingly to our understanding, also detailed schematic explanation, description "how to" and other kind of support, etc via email, will be available.   

2. Schematic

The primary task seems very simple - developing quite simple to DIY, reliable guitar preamp for home or/and small recording studio. After much experimentation with different topologies, much common works and discussions with guitarists and recordists, first came the evidence, that the devil is in the details. There is no purpose to show why one or other schematic solution was made. I will try only to give a brief explanation on how it was done and why.

The input gain stage should work only as an amplifier providing plenty of gain. To avoid unwanted distortion the input signal is taken to the input attenuator formed by components R1, R2, R3, J2, J3, J4 for reducing the signal to level preserving the tube from overdrive. 

In practice, instead of fixed resistors, we use a switched attenuator assembled on a 12-position rotary switch. More positions are better, because guitar pickups, depending on construction and other things, generate output amplitude from 25 mVrms up to 1 V or higher. Wider attenuation also allows using this preamp for sound colorification of various electronic musical instruments, etc. synthesizers or drum machines.

Also, one should have in mind, that guitarist playing technique sometimes has a crucial role on pickups output level (experience with a punk band convinced me that the input level limits should be much wider, compared to sweet classical players).

 

The input gain stage (V1A) is standard circuit, with Constant Current Source (Q1, R5) as anode load. CSS greatly increases stage load resistance, flattening the load line and reducing distortion in the valve. The chain C6, R8, JP5, R9 - interstage divider, or in other words - gain control circuit. JP5 - connector, dedicated to connect 1 Meg Log potentiometer. We instead use a 24 pos rotary switch with a chain of resistors from which an appropriate tap could be chosen. Values of resistors should fit logarithmic law.

The second stage (V1B) is identical to the first stage, but cathode biasing differs from the first stage and tonally is most important for sound.
Second interstage divider (C8, R15, R16) reduces output from the second stage to an acceptable level for the next stage grid (V2AB), avoiding heavy overdrive in the third stage.
Cathode biasing of the third stage (V2AB) adds more colours to sound. In practice the second stage is biased to obtain symmetrical clipping, the third stage - asymmetrical clipping. The third stage could be omitted in case if only clean or mild overdrive tones are acceptable - ideal for blues.

 

At the very end of the circuit, the tone stack is placed (R24, C13, C14, C15, JP11, JP12, JP13). Connectors JP11, JP12, JP13 are dedicated to connect the tone stack potentiometers. Tone stack schematic was get from "Tone Stack Calculator" (Fender or Marshall), which is free available at http://www.duncanamps.com/tsc/

3. Components

 

NOTE: Values of resistors, capacitors and other components could be changed to meet personal needs. Ask for advice by email (see CONTACT section).

Front panel is developed and was used for real device assembly. Some design inaccuracies were corrected, the front panel also is available.

Tubes

ECC83 or 12AX7 or 7025

Resistors

R10,R17,R22 WELWYN 1W or2W 5%

R10 - 27K
R17 - 3K
R22 - 33K

 

Other Vishay Dale CMF55 Metal Film Non-Magnetic 1/2W 1% or PRP Audio Metal Film Resistors PRP9372 1/2W 1%

R4,R9 - 10K
R6,R13,R20 - 68.1R
R11,R18,R24 - 95.3K
R8 - 200K
R15 - 800K

Trimmers
R7,R14,R21 - 2K
R16 - 200K

Potentiometers (front panel mount)
Precision Electronics Corporation RV4 2W Carbon Log/Linear

con JP11 - 250K Log
con JP12 - 250K Log
con JP13 - 10K Lin
con JP14 - 50K Log

Rotary Switches (Switched attenuator, front panel mount) - con JP5
Elma Rotary switch Type 04, 1 Wafer, 1 Pole x 24 Positions, Shorting, °15, Eyelets

Cheaper choice

Don Audio Rotary Switch 2024, 2 Pole 24 Position

Attenuator resistor set - Vishay MRS25 0.6W Metal Film 1% 24 pcs, series, log law​

Capacitors
C4, C16, C9, C13 LCR Polystyrene

C4 - 100pF
C13,C16 - 220pF (*C16 - should be soldered on Rotary Switch)

C6, C8, C12 Vishay MKT368​ 10.2 mm pitch orange dipped

C6,C8 - 4.7nF x 250V
C12 - 100nF x 250V

C14, C15 Vishay MKP1837​

C14 - 100nF
C15 - 47nF

4. Recommended Further Reading

Morgan Jones Valve Amplifiers

Merlin Blencowe Designing Tube Preamps for Guitar and Bass, 2nd Edition

    This site was designed with the
    .com
    website builder. Create your website today.
    Start Now