Low-cut and Bass controls
Output power: 40W on 8 Ohm and 60W on 4 Ohm loads
Amplifier circuit diagram:
R1 6K8 1W Resistor
R2,R4 470R 1/4W Resistors
R3 2K 1/2W Trimmer Cermet
R5,R6 4K7 1/2W Resistors
R7 220R 1/2W Resistor
R8 2K2 1/2W Resistor
R9 50K 1/2W Trimmer Cermet
R10 68K 1/4W Resistor
R11,R12 R47 4W Wirewound Resistors
C1,C2,C4,C5 47µF 63V Electrolytic Capacitors
C3 100µF 25V Electrolytic Capacitor
C6 33pF 63V Ceramic Capacitor
C7 1000µF 50V Electrolytic Capacitor
C8 2200µF 63V Electrolytic Capacitor (See Notes)
D1 LED Any type and color
D2 Diode bridge 200V 6A
Q1,Q2 BD139 80V 1.5A NPN Transistors
Q3 MJ11016 120V 30A NPN Darlington Transistor (See Notes)
Q4 MJ11015 120V 30A PNP Darlington Transistor (See Notes)
SW1 SPST Mains switch
F1 4A Fuse with socket
T1 220V Primary, 48-50V Secondary 75 to 150VA Mains transformer
PL1 Male Mains plug
SPKR One or more speakers wired in series or in parallel. Total resulting impedance: 8 or 4 Ohm. Minimum power handling: 75W
Preamplifier circuit diagram:
P1 10K Linear Potentiometer
P2 10K Log. Potentiometer
R1,R2 68K 1/4W Resistors
R3 680K 1/4W Resistor
R4 220K 1/4W Resistor
R5 33K 1/4W Resistor
R6 2K2 1/4W Resistor
R7 5K6 1/4W Resistor
R8,R18 330R 1/4W Resistors
R9 47K 1/4W Resistor
R10 18K 1/4W Resistor
R11 4K7 1/4W Resistor
R12 1K 1/4W Resistor
R13 1K5 1/4W Resistor
R14,R15,R16 100K 1/4W Resistors
R17 10K 1/4W Resistor
C1,C4,C8,C9,C10 10µF 63V Electrolytic Capacitors
C2 47µF 63V Electrolytic Capacitor
C3 47pF 63V Ceramic Capacitor
C5 220nF 63V Polyester Capacitor
C6 470nF 63V Polyester Capacitor
C7 100nF 63V Polyester Capacitor
C11 220µF 63V Electrolytic Capacitor
Q1,Q3 BC546 65V 100mA NPN Transistors
Q2 BC556 65V 100mA PNP Transistor
J1,J2 6.3mm. Mono Jack sockets
SW1 SPST Switch
This design adopts a well established circuit topology for the power amplifier, using a single-rail supply of about 60V and capacitor-coupling for the speaker(s). The advantages for a guitar amplifier are the very simple circuitry, even for comparatively high power outputs, and a certain built-in degree of loudspeaker protection, due to capacitor C8, preventing the voltage supply to be conveyed into loudspeakers in case of output transistors' failure.
The preamp is powered by the same 60V rails as the power amplifier, allowing to implement a two-transistors gain-block capable of delivering about 20V RMS output. This provides a very high input overload capability.
70mV input for 40W 8 Ohm output
63mV input for 60W 4 Ohm output
50Hz to 20KHz -0.5dB; -1.5dB @ 40Hz; -3.5dB @ 30Hz
Total harmonic distortion @ 1KHz and 8 Ohm load:
Below 0.1% up to 10W; 0.2% @ 30W
Total harmonic distortion @ 10KHz and 8 Ohm load:
Below 0.15% up to 10W; 0.3% @ 30W
Total harmonic distortion @ 1KHz and 4 Ohm load:
Below 0.18% up to 10W; 0.4% @ 60W
Total harmonic distortion @ 10KHz and 4 Ohm load:
Below 0.3% up to 10W; 0.6% @ 60W
Fully clockwise = +13.7dB @ 100Hz; -23dB @ 10KHz
Center position = -4.5dB @ 100Hz
Fully counterclockwise = -12.5dB @ 100Hz; +0.7dB @ 1KHz and 10KHz
-1.5dB @ 300Hz; -2.5dB @ 200Hz; -4.4dB @ 100Hz; -10dB @ 50Hz
The value listed for C8 is the minimum suggested value. A 3300µF capacitor or two 2200µF capacitors wired in parallel would be a better choice.
The Darlington transistor types listed could be too oversized for such a design. You can substitute them with MJ11014 (Q3) and MJ11013 (Q4) or TIP142 (Q3) and TIP147 (Q4).
T1 transformer can be also a 24 + 24V or 25 + 25V type (i.e. 48V or 50V center tapped). Obviously, the center-tap must be left unconnected.
SW1 switch inserts the Low-cut feature when open.
In all cases where Darlington transistors are used as the output devices it is essential that the sensing transistor (Q2) should be in as close thermal contact with the output transistors as possible. Therefore a TO126-case transistor type was chosen for easy bolting on the heatsink, very close to the output pair.
R9 must be trimmed in order to measure about half the voltage supply from the positive lead of C7 and ground. A better setting can be done using an oscilloscope, in order to obtain a symmetrical clipping of the output waveform at maximum output power.
To set quiescent current, remove temporarily the Fuse F1 and insert the probes of an Avo-meter in the two leads of the fuse holder.
Set the volume control to the minimum and Trimmer R3 to its minimum resistance.
Power-on the circuit and adjust R3 to read a current drawing of about 30 to 35mA.
Wait about 15 minutes, watch if the current is varying and readjust if necessary.