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.
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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