Guitar Amplifier RED Pa .pdf


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Guitar Amplifier
10W Old-Style ultra-compact Combo
Two inputs - Overdrive - Treble-enhancement
Circuit diagram:

Parts:
P1______________4K7
P2_____________10K

Linear Potentiometer
Log. Potentiometer

R1,R2__________68K
R3____________220K
R4,R6,R11_______4K7
R5_____________27K
R7______________1K
R8______________3K3
R9______________2K
R10___________470R
R12_____________1K5
R13___________470K
R14____________33K

1/4W
1/4W
1/4W
1/4W
1/4W
1/2W
1/2W
1/4W
1/4W
1/4W
1/4W

C1____________100pF
C2____________100nF
C3____________470µF
C4____________220nF
C5_____________47µF
C6______________1µF
C7,C8,C9,C10___47µF
C11____________47pF
C12__________1000µF
C13__________2200µF

63V
63V
35V
63V
25V
63V
25V
63V
35V
35V

Resistors
Resistor
Resistors
Resistor
Resistor
Resistor
Trimmer Cermet
Resistor
Resistor
Resistor
Resistor
Ceramic Capacitor
Polyester Capacitor
Electrolytic Capacitor
Polyester Capacitor (Optional, see Notes)
Electrolytic Capacitor (Optional, see Notes)
Polyester Capacitor
Electrolytic Capacitors
Ceramic Capacitor
Electrolytic Capacitor
Electrolytic Capacitor

D1_____________5mm. Red LED
D2,D3________1N4004 400V 1A Diodes
Q1,Q2________2N3819
Q3____________BC182
Q4____________BD135
Q5____________BDX53A

General-purpose N-Channel FETs
50V 200mA NPN Transistor
45V 1.5A NPN Transistor (See Notes)
60V 8A NPN Darlington Transistor

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Q6____________BDX54A

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60V 8A PNP Darlington Transistor

J1,J2________6.3mm. Mono Jack sockets
SW1____________1 pole 3 ways rotary switch (Optional, see Notes)
SW2____________SPST Mains switch
F1_____________1.6A Fuse with socket
T1_____________220V Primary, 48V Center-tapped Secondary
20 to 30VA 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: 20W

Circuit description:
The aim of this design was to reproduce a Combo amplifier of the type very common in the 'sixties and the
'seventies of the past century. It is well suited as a guitar amplifier but it will do a good job with any kind
of electronic musical instrument or microphone.
5W power output was a common feature of these widespread devices due to the general adoption of a
class A single-tube output stage (see the Vox AC-4 model).
Furthermore, nowadays we can do without the old-fashioned Vib-Trem feature frequently included in
those designs.
The present circuit can deliver 10W of output power when driving an 8 Ohm load, or about 18W @ 4
Ohm.
It also features a two-FET preamplifier, two inputs with different sensitivity, a treble-cut control and an
optional switch allowing overdrive or powerful treble-enhancement.
Technical data are quite impressive for so simple a design:
Sensitivity: 30mV input for 10W output
Frequency response: 40 to 20KHz -1dB
Total harmonic distortion @ 1KHz and 10KHz, 8 Ohm load: below 0.05% @ 1W, 0.08% @ 3.5W, 0.15%
at the onset of clipping (about 10W).
Notes:
SW1 and related capacitors C4 & C5 are optional.
When SW1 slider is connected to C5 the overdrive feature is enabled.
When SW1 slider is connected to C4 the treble-enhancer is enabled.
C4 value can be varied from 100nF to 470nF to suit your treble-enhancement preferences.
In all cases where Darlington transistors are used as the output devices it is essential that the sensing
transistor (Q4) 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.
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 R9 to its minimum resistance.
Power-on the circuit and adjust R9 to read a current drawing of about 25 to 30mA.
Wait about 15 minutes, watch if the current is varying and readjust if necessary.

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