Tuesday, March 1, 2022

D04: vintage 18W guitar amplifier

The D04 is a guitar amplifier built using an old turntable by Perpetuum Ebner (https://de.wikipedia.org/wiki/Perpetuum-Ebner).

The model I've used it's a vintage 70's PE 3015 turntable. When I was a boy friends give away this turntable to me, cause it's not working. I've used this turntable years ago as a standard amplifier, taking advantages of it's aux input, then I've decided to turn this in a guitar amplifier.

Like the D03 amplifier that you can find here http://davidegironi.blogspot.com/2020/03/d03-lesa-boogie-guitar-amplifier.html, this one also is built using a dismissed and not working turntable.

Building technique of the case was almost the same. The speaker were glued together and an hole was drilled in order to makes cables run one side to the other.
I've worked then on the turntable panel to make it fit the top of the speakers.
As on the D03, I've used all the original potentiometer knobs as the panel holes.
The power switch is on the top of the panel.
Then a channel selector is installed.
This amplifier has 2 channels
  • clean
  • overdrive/distortion
The selector switch from the clean to the overdrive/distortion channel. Also the switch act as a voltage divider selector for the overdrive/distortion section. The selected transformer, this time is a 24V single rail, think it's a 40VA. Single rail are cheaper and easier to find than dual rail, so I would like to build something with a single rail.

The preamplifier it's a JFET preamplifier based on the J. Donald Tillman design found in the article "A Discrete FET Guitar Preamphttp://www.till.com/articles/GuitarPreamp/. It use a single J201 JFET and just a few components. It really sounds good. This kind of preamplifier is the same you can find in many other amplifier.

Then the tone stack is a dual tone, as i tell you I would like to keep the original panel untouched, it means to use the two treble and bass selector.
The tone is based on the Fender 6G5 amplifier, it's simple to build and sounds good.

Least but not last, the speakers. Original turntable speakers are used, but they need to be wired in such a mono way.

Then another JFET is used as a Booster to restore the signal due to the tone loss. It's just like the ElectroSmash 1Wamp Electric Guitar Amplifier found here https://www.electrosmash.com/1wamp.

Here comes the overdrive/distortion stage. That's based on Clock of Tone 50 and The Ursa Minor pedal with a few mods. It's basically a single transistor, the 2N5088, based distortion. The selector gives this transistor even more distortion adding a diode clipping stage.
At the end of this stage we need to add a voltage divider in order to makes the distortion volume almost equals to the clean stage.

The signal then goes to the reverb stage. The reverb it's based on the Valve Wizard SmallTime pedal with a few changes. The main IC used in this reverb is the PT2399, a TL072 op amp is then used to restore the signal before and after the reverb.
Two trimpot can be used to trick the reverb sounds.

The amplifier itself is build around a TDA2030 IC. This chip is a class-AB amplifier monolithic integrated circuit. The design used is a based on the main datasheet single rail voltage test circuit.

The power stage is as simple as you can imagine, it's just a rectified AC voltage, with a 9V and 5V output from two separate linear voltage regulators.

Now, how does it sounds?
Really good to me, even if the amplifier is supplied in a single rail way, as the only opamp of this circuit, the sounds of the JFET preamp and the overdrive/distortion stage, makes this amplifiers sounds warm, but when the distortion is pumped up, the sound can be very "sharp".

Find the schematics below.

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  • excuse my bad english