Guitar Amplifier

10W Old-Style ultra-compact Combo

Two inputs - Overdrive - Treble-enhancement

Circuit diagram:

10W Guitar Amplifier


P1______________4K7  Linear Potentiometer
P2_____________10K   Log. Potentiometer

R1,R2__________68K   1/4W Resistors
R3____________220K   1/4W Resistor
R4,R6,R11_______4K7  1/4W Resistors
R5_____________27K   1/4W Resistor
R7______________1K   1/4W Resistor
R8______________3K3  1/2W Resistor
R9______________2K   1/2W Trimmer Cermet
R10___________470R   1/4W Resistor
R12_____________1K5  1/4W Resistor
R13___________470K   1/4W Resistor
R14____________33K   1/4W Resistor

C1____________100pF   63V Ceramic Capacitor
C2____________100nF   63V Polyester Capacitor
C3____________470µF   35V Electrolytic Capacitor
C4____________220nF   63V Polyester Capacitor (Optional, see Notes)
C5_____________47µF   25V Electrolytic Capacitor (Optional, see Notes)
C6______________1µF   63V Polyester Capacitor
C7,C8,C9,C10___47µF   25V Electrolytic Capacitors
C11____________47pF   63V Ceramic Capacitor
C12__________1000µF   35V Electrolytic Capacitor
C13__________2200µF   35V Electrolytic Capacitor

D1_____________5mm. Red LED
D2,D3________1N4004  400V 1A Diodes

Q1,Q2________2N3819   General-purpose N-Channel FETs
Q3____________BC182   50V 200mA NPN Transistor
Q4____________BD135   45V 1.5A NPN Transistor (See Notes)
Q5____________BDX53A  60V 8A NPN Darlington Transistor
Q6____________BDX54A  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).