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:

Parts:

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

Comments:

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 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

Notes:

A very interesting discussion about Fender and Marshall Tone Controls,
showing differences, frequency response graphics and much more can be found at
Guitar Amplifier Basics

Please note that all impedances involved in this circuit are very high
(valve like, typically 1M) compared to common transistor circuitry: therefore,
to avoid noise and hum pick-up, all connections must be kept very short and if
input and output jacks and the pots are not directly mounted on a printed
circuit board, all wiring to these parts must be done carefully by means of
screened cable.

In their more recent amplifier types, like '59 Bassman and Blues De
Ville/Deluxe, Fender designers have slightly modified the Tone Controls parts
values, thus obtaining a sort of compromise between their older models and
Marshall Tone Controls. The changes were as follows: C2 and P1 (Treble
Control) were left unchanged; C3 was still a 100nF capacitor but P2 value
(Bass Control) was raised to 1M (log.) like the Marshall preamp. C4 and P3
(Mid Control) were changed to 22nF and 22K (or 25K) respectively: i.e. the
same values and connections of the Marshall preamp modification as shown in
the above schematics (left side).

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

Having Questions?