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Photos and description - Marcin Sławicz
The beginnings of the project
The idea of building my own tube amplifier has been bothering me for the last two years. I am not a maniacal audiophile and using "ordinary" solid state equipment was enough for me (I always preferred to listen to music than equipment). Now, however, my worn-out amplifier is starting to suffer from the ailments of old age, and although I could regenerate it, there is a great opportunity to implement a tube venture.
At the beginning I was thinking about a design based only on triodes, but rejecting the SE circuits burdened with too many inconveniences. A very interesting description of the push-pull amplifier with direct filament triodes can be found on the Lynn Olson website. It is worth taking a look there because of the extremely interesting solutions used in his projects. The described amplifiers, however, have a major disadvantage - cost (mainly due to the 300B or 2A3 tubes and interstage transformers). So I had to look further.
My attention was drawn to indirectly heated double 6AS7 power triodes, once used mainly in power supply systems, but also great as electron tubes in the output stage of audio amplifiers. The cost of electron tubes would be much lower, but due to the low voltage gain factor, in this case, expensive and difficult to obtain interstage transformers or two or more triodes in parallel connection would have to be used. Mr. Russ Sadd described on his website a push-pull amplifier with 6AS7 triodes.
My project took a few more months, during which I slowly became convinced that a successful power amplifier does not have to have triodes in the output stage. I began to consider the use of beam tetrodes working in the gain stage in an ultra-linear configuration. Such a circuit combines the advantages of triode sound (low distortion) with high efficiency and stability of tetrodes and pentodes. I had a choice of 6L6 / 5881, KT66, KT88 / 6550 tubes, commonly used both in guitar amplifiers and in Hi-Fi designs.
Another period of my project is searching the net in order to select the basic amplifier circuit. The amplifier should not be complicated, because a complex circuit does not guarantee high-quality sound, and with limited measurement possibilities, it can be difficult to start up. Mass-produced devices must ensure the repeatability of production and the relative stability of parameters during subsequent operation. When designing an amplifier for yourself, you can often take shortcuts without worrying about the subsequent service.
My choice fell on a well-known layout that has been tested in thousands of homes around the world. It will be the next version of the D.T. N. Williamson. Almost every company that used to produce tube amplifiers had a product to a greater or lesser extent based on this famous circuit. You can find hundreds of articles on the Internet describing different varieties of Williamson amplifiers. So let's take advantage of these rich experiences today.
Design assumptions
In 1947, Mr. Williamson introduced an amplifier circuit that was a real breakthrough in the pursuit of high-quality sound reproduction. The most characteristic elements of this amplifier are the split load phase splitter and the use of a transformer transmitting the signal in the range of 2Hz ÷ 60,000Hz (a necessary condition for achieving the stability of the amplifier with a closed feedback loop).
All stages of the Williamson amplifier are, in fact, extremely simple, but at the same time they perfectly cooperate with each other, ensuring relatively low signal distortion. Nevertheless, the system has several drawbacks, which efforts were made to improve in the following years. The figure below shows the 1949 version of the amplifier with the component values marked.
Read more: Tube amplifier "Concertino"
Grzegorz "gsmok" Makarewicz
