A self-built transistor guitar amplifier

I completely constructed this guitar amp myself and designed the entire circuit myself. The goal was to build an amplifier that has its own sound. After building several tube amplifiers, I intended to build a transistor-equipped amplifier. It should not bring such a dead sound, as is the case with many transistor amplifiers. That's why it was deliberately carried out differently, like a hi-fi amp. Without pretending to uncompromisingly imitate the sound of a tube amplifier, lots of there usual circuit details was taken up. Negative feedback were therefore largely dispensed with. That is why the use of ICs and in particular operational amplifiers was out of the question. The result is a device that produces very lively clean sounds. Slightly distorted sounds via the boost channel especially are convincing at high volume levels, because then also slight power amp distortion already occurs. The tonal musicality comes close overall to tube amp.

In order to achieve a high input impedance at the lowest possible expenditure, the input stages are constructed with field-effect transistors. So it comes to a low damping of the guitar pickups and the sound character of the instrument used can fully unfold, pickup resonances are included in the sound.

The boost channel is followed by a differential amplifier, which provides a very soft limiting when overdriven. Due to the properties of bipolar transistors, the transfer characteristic has no similarity to that of tubes. The signals from the Clean and Boost channels are merged in a mixer stage and fed to a passive three-band tone control borrowed from tube technology. Via a transistor stage to compensate for the level losses in the tone control, the signal reaches the power amplifier. On a three-pin DIN socket, the signal from the pre-stage can be tapped with two different levels. Thus, the amplifier can also be used as a pure guitar preamp or can be connected directly to a PA.

At the input of the output stage is the (Master) volume control. With the exception of current negative feedback in the driver stage, the power amplifier works virtually completely without negative feedback. As a result, the characteristics of the driver transformer contribute not insignificantly to the sound character. With the exception of the unbalanced coupling of the loudspeakers, the push-pull power stage is strictly symmetrical, as is the case with tube amplifiers. In contrast, however, it is circuited as a series and not as a parallel push-pull amplifier. The type 1N3754 diodes have transistor housings and are screwed to the heatsink of the output stage transistors for thermal stabilization of the bias current with cooling clamps.

The power supply has no special features. The pre-stage gets its power supply via a simple transistor stabilization. This improves the Störsignalabstand and prevents tendency to oscillate.

The housing was glued together from chipboard blanks from the hardware store. It was then painted with water-based acrylic paint. Edge protection is provided by aluminum L profiles from the hardware store. The aluminum front panel painted from the spray can is labeled with transfer letters and symbols. Subsequently, fixing varnish was applied.

The speakers are very old Alnico full-range speakers, which give the sound additional an own character.