A vacuum tube preamplifier is typically used to amplify the signal from a microphone, musical instrument, or other audio source before it is sent to a power amplifier or recording device. Compared to solid-state preamps, vacuum tube preamps are known for their warm, natural sound and ability to add harmonics and distortion that can enhance the perceived "musicality" of the audio signal. They also have a unique "saturation" effect that can add a pleasing "tube warmth" to the sound.
This time I will present you how to make a simple single tube preamplifier with tone control that includes options for adjusting the bass, midrange, and treble frequencies. It also has gain, volume, and input sensitivity controls.
The heart of the device is ECH83 which is a low voltage vacuum tube works with only 12 V anode supply, used especially for car radios. Instead of ECH83, we can use ECH81 tube without any modifications.
The circuit has been presented on several audio forums, and I specifically followed the instructions on the engel-sound blog(http://meatexz.com/engel-sound/ECH83preamp.html), where you can also find other tube projects, and everything is neatly drawn and described.
The preamplifier is relatively simple to build, and in addition to the vacuum tube it contains several resistors and capacitors as well as five potentiometers.
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The elements can be soldered directly to the potentiometers and the tube, but I think that a more elegant solution is with a printed circuit board on which the external elements are connected with flat cables and connectors.
At the input of the preamplifier I added a trimmer potentiometer of 1 Megaohm, which does not change the high input resistance, and at the same time the input sensitivity can be adjusted, so even though the device is primarily intended as a guitar preamplifier, with this modification it can be used for example with signal from a PC sound card, radio tuner, or CD player.
The next stage was testing the device in real conditions:
I do not have a guitar or other electric instrument, so I will test this device with an input signal from the sound card of the PC. Also for testing, I will connect the preamplifier to a 1 Watt tube amplifier with a PCL82 tube that I made previously, and whose construction is described in one of my previous videos .
Unfortunately, you can't really appreciate the full features of this device through a video presentation, but you can at least roughly hear the changes caused by the tone controls. The gain control works pretty good and it provides very dramatic changes. These changes in a certain position of the potentiometer can give a distorted, grainy and fuzzy sound that is especially useful when playing the electric guitar, but in the case of listening to music it can be very unpleasant. As for the tone controls, this is an extremely simple so-called Tone Stacks way of control that was very often used in Fender and Marshall amplifiers, but the controls are usually highly interactive and there is almost always a significant midrange 'scoop'. The ECH83 is only getting a little warm, so the cooling is not issue here.
And now a short conclusion. First of all, let me emphasize that it is about my personal perception of the sound (what is negative for me, can be positive for someone), and if you want to hear its characteristics in detail, you will have to build it yourself. In fact, my main goal was also to investigate whether this preamp works at all at such a low voltage. At least for my taste, primarily considering the simple construction the results obtained are relatively good. At least for my taste, primarily considering the simple construction the results obtained are relatively good. In short, this preamp is not good enough for listening to music. However, as an instrument preamplifier it would probably be good, bearing in mind that the Gain potentiometer has a large effect on the sound, and distortions are desirable in this case. Also, for playing the guitar, the tone controls do not have such a big role, in fact both Fender and Marshall used this type of control in their beginnings.
Finally, the device is installed in a suitable housing made of PVC board with a thickness of 3 and 5 mm and covered with self-adhesive colored wallpaper.
Schematic.rar 
