Want tips on designing electron tube amplifiers? Fundamental Amplifier Techniques with Electron Tubes might be the book for you. The author, Rudolf Moers carefully details the science of hollow-state design as applied to amplifiers and power supplies.
The book is an Elektor group publication. So, I asked tube amp aficionado Richard Honeycutt to provide an unbiased review the book. (I asked him to do this prior to taking him on as a columnist for audioXpress magazine.) He agreed, and here’s the review, which is also available in audioXpress April 2012:
Back in the 1950s and 1960s, if you wanted to learn about vacuum tube amplifiers, you could read the Radiotron Designer’s Handbook, a 1,500-page behemoth that covered all kinds of vacuum tube circuits that were known at the time, and also included abundant information on passive components as well. Or you could use the introductory material and example schematics in the RCA Receiving Tube Manual—much shorter and less expensive, and also far less comprehensive. Of course, it did include data on most tubes then being manufactured by RCA. If you just wanted to build your own amplifiers, but were not interested in designing, there was the Mullard Circuits for Audio Amplifiers. For a more scholarly approach, you could check out an electrical engineering textbook such as Analysis and Design of Electronic Circuits by Paul Chirlian.
Now, however, things are different. Although some of these references can be found on the Internet, they are no longer up-to-date. Happily, however, Elektor recently published Fundamental Amplifier Techniques with Electron Tubes by Rudolf Moers, which presents a 21st-century perspective on the science of hollow-state design as applied to amplifiers and power supplies. Beginning with the principles of electron emission, the book progresses through standard vacuum tube varieties: diodes, triodes, tetrodes, and pentodes, after which it covers such general principles as frequency dependent behavior, non-linear distortion, noise, and negative feedback. The book concludes with a chapter on the construction of electron tube amplifiers. Unlike many of the earlier authors of books on electron tubes, Moers is not constrained by a need to cover such specialized tubes as pentagrid converters, or circuits specifically used in radio and TV receivers. Instead, he uses his 800 pages to discuss the physics underlying electron tube operation far more comprehensively than did any of his predecessors. He does this in a way that maximizes presentation of principles while minimizing unnecessary mathematics. In many cases, the physical explanations can be skipped over by those whose only interest is design methods. For the reader who does take advantage of the physical explanations, Moers’s inclusion of an eight-page listing/definition of mathematical symbols makes the explanations easy to follow.
The focus is by no means primarily on physics, however. None of the classic texts provides anything like so comprehensive coverage of the design and operation of half- and full-wave rectifier/filter circuits, or vacuum tube phase shifters, to mention a couple of examples.
Moers’s book assumes that the reader is familiar with basic DC and AC circuit theory, and therefore does not undertake the task of educating those who lack this understanding. The book is written from a scientific perspective in that, while mentioning the disconnect between measured and perceptual performance of an amplifier, the author makes no dogmatic claims about the relationship between the two, other than to opine that most of the “tube sound” results from harmonic distortion components that some people find pleasing to the ear. (Having followed this discussion for about four decades, your reviewer partially concurs, but believes that there are other elements involved as well.) The author lightheartedly introduces the quantity “cm2 of gooseskin/watt” as an example of a measurement of perceptual phenomena.
A consequence of Moers’ scientific approach is that specific catch phrases found in many amateur-oriented publications on tube technology are conspicuously absent. For example, it is difficult to read much about tube power amplifiers without noticing mention of the “Williamson amplifier.” This circuit was developed by D. T. N. Williamson and described in articles in Wireless World in April and May, 1947. It was unique in that it applied negative feedback around the entire amplifier, including the output transformer, thus reducing nonlinear distortion. Doing this required very careful design to ensure stability, including the elimination of interstage transformers such as the phase splitter transformer used in many prior designs.
Moers does not mention the Williamson amplifier by name, but the vacuum tube phase splitter design Williamson used is discussed in detail in the book, as is the method of designing a negative feedback loop encompassing the entire amplifier. Moers also gives a unique explanation of another pivotal power amplifier circuit: the ultralinear circuit invented in 1951 by Hafler and Keroes. It’s a case of content versus jargon.
In his otherwise excellent discussion of damping factor, Moers unfortunately makes the all-too-common error of ignoring the effects of voice coil and lead wire resistance. He gives the common equation for damping factor: DF = (loudspeaker impedance)/(amplifier output impedance). Since the amplifier (modeled as an AC generator or Thevenin source), voice coil resistance, lead wire resistance, voice coil inductance, and reflected mechanical impedance form a series circuit whose actual damping is influenced by all elements, the lead wire resistance and voice coil resistance cannot be ignored. In fact, they can easily swamp the effects of the amplifier output impedance, at least for a pentode stage using negative feedback. However, Moers does not make the further error of insisting that the damping factor be a minimum of 100 as have some earlier authors. Using an 8-Ω speaker having about 6-Ω DC resistance, the effect of a combined output impedance and lead wire resistance less than 0.5 Ω is negligible.
Two shortcomings of Fundamental Amplifier Techniques with Electron Tubes are more or less linguistic. English may well be the only Germanic language in which the verb in a sentence is not at the end of the sentence required to come. Thus syntactical intrusions from the author’s native language sometimes make the text difficult for native English speakers. Also, Moers has chosen to use terminology that is probably not standard in English (at least American English) books on electronics. For example, he uses the term “ anode static steepness” to denote “transconductance” (also commonly called “mutual conductance.”) A common-cathode (or “grounded-cathode”) amplifier stage is called a “basic cathode” stage in Moers’ book.
These three small complaints pale in the face of the outstanding job the author has done in bringing together the theory, design, and practice of vacuum tube amplifiers in a single volume. Anyone who wants to go beyond the Heathkit level of tube amplifier understanding owes it to him/herself to buy and study this excellent volume.
If you’re interested purchasing the book or learning more about it, click here to visit the book’s webpage in the CC Webshop.
Fundamental Amplifier Techniques (by Rudolf Moers), audioXpress, and CircuitCellar.com are Elektor group publications.
© 2012, C. J. Abate. All rights reserved.