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Audio Distortion Kraut Jar

Last Update: 2022-07-30 Ah, who doesn't love a good jar of sour kraut with some crunchy music at the summer picnic? Actually, I don't know who does, but I assure you that two of those things are relevant to this project. The Krautifier is a distortion box which makes use of a pentode vacuum tube and a 741 op amp, built into the lid of a sour kraut jar. The choice of housing is primarily due to me running out of project boxes and the fact that I had not seen something built into one before.


The front of the Krautifier: not your ordinary sour kraut, is it? The back, you can see the tube in there! And it's upside down! Top view of the lid, all controls and jacks are here. A view of the inside circuitry. Another view of the inside circuitry.


The jar lid is the base for all the construction since it is what all the pots, switches, and jacks mount to. Since it was built like this, the lid can be unscrewed to access the circuit easily. Drilling the holes in the thin sheet steel was a bit of a challenge since it likes to rip once the drill bit gets part way through. To solve this problem, I cut out a circle of MDF that fit within the lid. Then I was able to clamp the lid (facing upside down) into a sandwich of the MDF circle, the lid, and another piece of MDF. This allowed me to drill through with minimal tearing. After a bit of cleanup, all the panel-mount parts were mounted to the lid and a copper wire frame was soldered to the back of some of them to construct the mount for the tube and output transformer. After that, it was just the process of carefully soldering all the point-to-point connections without melting too many components' insulation.


The schematic for the Krautifier is relatively simple, with there only being two stages. Since it was built with that I had laying around at the time, it makes use of a 741 op amp for comparator "wave squarer" circuit and a 12EG6 pentagrid amplifier tube. The 741 acts like (but not exactly) a comparator to convert the incoming waveform into more of a square wave shape, causing a good deal of distortion. The input to the 741 has the option of enabling a low pass filter since the squarer works best on low frequencies. The input to the second stage can be switched between the input to the system and the output of the squarer. The output amplifier (or deamplifier) uses a pentode which drives a small power transformer from an old alarm clock (who needs impedance matching, anyway?). Conveniently, the 12EG6 is designed to operate directly from a 12V automobile supply, so no high voltage is required for the plate. The circuit was designed before I had learned how a common-emitter, ahem, common-cathode amplifier works, hence the lack of proper biasing on the tube. Also, I've just pulled up the tube's datasheet as I write this and it looks like positive voltages on grid 3 are bad ... oops. Either way, the system "works" as long as you turn the volume all the way up on whatever you plug into the input side. Schematic of the two-stage distortion box. Top half: 741 op amp comparator-like circuit. Bottom half: tube amplifier output stage.
That's all!
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