Solid-state Fender Blackface Preamp

All-FET version of the celebrated valve guitar preamp

Can be easily modified to Marshall Tone Controls specs

Circuit diagram:

Fender Blackface guitar preamp


P1,P2__________220K  Linear Potentiometers (or 250K)
P3______________10K  Linear Potentiometer
P4_______________1M  Log. Potentiometer

R1_______________1M  1/4W Resistor
R2,R3___________68K  1/4W Resistors
R4,R5,R6,R7,R8_100K  1/4W Resistors

C1,C7___________47µF  25V Electrolytic Capacitors
C2_____________220pF  63V Ceramic or Polystyrene Capacitor (or 250pF)
C3_____________100nF  63V Polyester Capacitor
C4______________47nF  63V Polyester Capacitor
C5_____________100µF  35-50V Electrolytic Capacitor
C6_____________120pF  63V Ceramic or Polystyrene Capacitor
C8______________47nF  63V Polyester Capacitor

Q1,Q2,Q3,Q4__2N3819  General-purpose N-Channel FETs

J1,J2_________6.3mm  Mono Jack sockets

SW1____________SPST Toggle or Slide Switch


Valve amplifiers and related preamplifiers including various types of tone controls were always the preferred equipment of most guitarists since the rock and pop music early days.
This trend is still alive and Fender or Marshall valve equipment is much appreciated and highly in demand in spite of the cost and difficulties in finding suitable replacements for aged or damaged parts.
The basic idea of this project was to build an exact replica of the Fender Blackface preamp, using the same values for the passive Tone Controls parts and presenting the same input and output impedance and total ac gain. The main difference was the substitution of the original twin triode 12AX7 valve by two simple FET Cascode gain blocks, each block having the same typical ac gain claimed in the 12AX7 data sheets, i.e. >80.
In this way, a cheap unit can be built and easily embedded into an existing transistor amplifier or used as a separate, battery powered, portable preamp.
The circuit should be powered at a dc voltage comprised in the 24 - 30V range, easily available into an existing audio equipment or obtained from higher voltage rails by means of suitable R/C cells.
To power a portable version, the best solution is to use two 9V "transistor" batteries wired in series, in order to obtain a 18V supply: the results will be worth the increased expense.
Total current drawing at all supply voltages (18 - 30V) is only 8.5mA.

Some performers prefer to use Marshall type Tone Controls, presenting frequency curves differing (though not greatly) from Fender ones.
Fortunately, this can be easily obtained by only changing some part values and the center lead connection of the "Mid" control pot, as shown in the circuit diagram below (left side).
Due to the rather high output impedance of the circuit, there could be severe output signal attenuation if the input impedance of the following amplifier is less than about 150K Ohm. Therefore, to avoid this shortcoming (mostly when transistor power amplifiers are driven) a suitable Source follower circuit should be added to the output, and this is shown in the circuit diagram below (right side).
C8 in the original circuit will be omitted and the Gate of Q5 must be wired directly to the junction where C8 was previously connected.

Circuit modifications:

Marshall modification and Source follower

Marshall Tone Controls Parts:

P2_______________1M  Linear Potentiometer
P3______________22K  Linear Potentiometer (or 25K)

R6______________33K  1/4W Resistor

C2_____________470pF  63V Ceramic or Polystyrene Capacitor (or 500pF)
C3,C4___________22nF  63V Polyester Capacitors

Output buffer Parts:

R9______________10K  1/4W Resistor
R10____________100K  1/4W Resistor

C9_______________4µ7  63V Electrolytic Capacitor

Q5___________2N3819  General-purpose N-Channel FET


Technical data

Input Sensitivity @ 1KHz:
6mV RMS (Tone Controls set to center position, Volume control set to max.)
Maximum undistorted output with 6mV RMS input:
3V RMS @ 18V supply
4V RMS @ 24V supply
5V RMS @ 30V supply

Frequency response: See curves at Guitar Amplifier Basics

Total harmonic distortion @ 1KHz: <0.3% at 1V RMS output

Total current drawing at all supply voltages (18 to 30V): 8.5mA