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
