This Guitar Amplifier electronic architecture adopts a able-bodied accustomed ambit cartography for the ability amplifier, application a single-rail accumulation of about 60V and capacitor-coupling for the speaker(s). The advantages for a guitar amplifier are the actual simple circuitry, alike for analogously aerial ability outputs, and a assertive congenital amount of loudspeaker protection, due to capacitor C8, preventing the voltage accumulation to be conveyed into loudspeakers in case of achievement transistors' failure. The preamp is powered by the aforementioned 60V balustrade as the ability amplifier, acceptance to apparatus a two-transistors gain-block able of carrying about 20V RMS output. This provides a actual aerial ascribe afflict capability.

Technical data:

Sensitivity:
35mV input for 40W 8 Ohm output
42mV input for 60W 4 Ohm output
Frequency response:
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
Treble control:
+9 / -16dB @ 1KHz; +12 / -24dB @ 10KHz
Brightness control:
+6.5dB @ 500Hz; +7dB @ 1KHz; +8.5dB @ 10KHz
Bass control:
-17.5dB @ 100Hz; -26dB @ 50Hz; -28dB @ 40Hz

Notes:

* The value listed for C8 is the minimum suggested value. A 3300΅F capacitor or two 2200΅F 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.
* D1 and D2 can be any Schottky-barrier diode types. With these devices, the harmonic modifier operation will be hard. Using for D1 and D2 two common 1N4148 silicon diodes, the harmonic modifier operation will be softer.
* 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.

source: free-circuits.com