Guitar Amplifier

Posted in Amplifiers, on 2015-08-12

Circuit diagram:

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).


  • 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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