by Victor Kampf
Tube sounding almost initially became a kind of standard guitar sound, and it is so to this day. The most frequently used in guitar amplifiers, low power dual triodes such as EСС83 (12AX7). Despite the undoubted musical merits of this triode, it does have some drawbacks common to all vacuum tubes (fragility, low efficiency, limited resources, deterioration of characteristics over time, etc.), therefore has long led development engineers to think about creating devices which will replace the tubes, but devoid of the above-mentioned drawbacks.
Back in the 70 years of the twentieth century in the music industry have been carried out first attempts to use low-power semiconductor tube replacements. In some models of company Mesa-Boogie, for example, in the first stage instead of the usual triode 12AX7 was used the components of the company Fetron. It should be noted that the Fetron’s components was intended primarily for telephone exchanges, where they had been used since the late 60’s in order to save electricity. Substitutes of tubes from FETRON were built in a cascоde schematic, which provided the ability to operate at high voltages, but they have a pentode output characteristics, which is clearly visible in the graphs the output current-voltage characteristics [1, 2], therefore, a direct replacement for triode devices with pentode’s CVC led to significant differences in the sound. This circumstance, of course, played a considerable role in that how cold enough the musical community met such innovation in guitar amplifiers and, as time has shown, substitutes of tubes from FETRON not taken root in the world of music.
Some time ago the other companies have also implemented a significant try to the “tube heights”. Unfortunately, the manufacturers have not accompanied its products with sufficient technical documentation, so we have no right to speak about conformity of triode current-voltage characteristics (CVC), or other aspects of the triode vacuum behavior emulation, especially when dealing with high signal levels. We’re just stating a fact of issue some semiconductor tubes, leaving to the music community itself the right to form an opinion about the sound properties of these devices.
AMT Electronics has its own tradition of building tube replacements. In 2007, the company’s engineers used their version of the cascode switching-on JFET and NPN bipolar transistor in the SS-20 preamp, allowing to accelerate the signal without limitation to several hundred volts with a minimum of noise floor. Then, were introduced circuits into the cascades that simulate the grid limitation, and the shape of the signal became very close to that of the signal on the 12AX7. In 2007, on the basis of article  the company started the development of low-voltage cascades that emulate the behavior of triode at the restrictions. Despite the use of field-effect transistors with a pentode characteristics, engineers managed to get the waveforms of signals which are similar to restrictions of a vacuum triode and in 2008, the world saw the first series of preamps LegendAmps (LA). In 2011, as a result of hard work on the modernization of low-voltage cascades, a series LA2 were born, which continues the tradition of using field-effect transistors in guitar preamps.
By 2012, the AMT engineers had accumulated a lot of experience in the development of semiconductor stages, which are in one way or another substituting the vacuum triodes, and in early 2012 the company AMT began intensive work on creating a complete functional semiconductor analogs of popular tubes used in guitar amplifiers.
In this article, we introduce the reader to the main points that characterize the sound of the vacuum triode in guitar amplifiers and their implementation in the solid-state triode AMT WARM STONE. Description of high-power solid-state penthodes and tetrodes expected in the near future in a separate article.