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.
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/
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.
R10,R17,R22 WELWYN 1W or2W 5%
R10 - 27K
R17 - 3K
R22 - 33K
R4,R9 - 10K
R6,R13,R20 - 68.1R
R11,R18,R24 - 95.3K
R8 - 200K
R15 - 800K
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
Attenuator resistor set - Vishay MRS25 0.6W Metal Film 1% 24 pcs, series, log law
C4, C16, C9, C13 LCR Polystyrene
C4 - 100pF
C13,C16 - 220pF (*C16 - should be soldered on Rotary Switch)
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