Top critical review
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Has good points but not for everyone wanting to know about guitar preamp design
on 26 April 2011
As someone with a sideline interest in restoring, designing, building and using guitar amplifiers I looked forward to reading "Designing Tube Preamps for Guitar and Bass" by Merlin Blencowe. I have formal training in electronics at the highest level and have made a career in the area but I am well aware that there are always new things to learn and new insights to be had, especially in an area such as guitar amplifier design.
Overall, I was disappointed - primarily because the book is mistitled. It is mistitled in two ways. Its scope is in some ways wider than just preamps - the three chapters on phase inverters and on feedback are more relevant to power amplifiers than to preamps. On the other hand, it does not cover preamp design comprehensively enough to justify its title.
I think it has both good points and deficiencies. For some potential readers, the good points will greatly outweigh the deficiencies. For other potential readers, the opposite will apply.
I think that many enthusiasts will appreciate being able to read in detail the descriptions of valve circuit operation and will get a very good physical understanding of what is actually happening and thus be able to make genuine design decisions, rather than blindly following cookbook recipes with no real understanding. Such enthusiasts will probably find the book very helpful.
The author, apparently self-taught, is enthusiastically willing to share the knowledge that clearly required hard work over a long period to gather. Generally he explains things very clearly. Just occasionally, the explanations get bogged down in a swamp of words. His enthusiasm shines through and I think that many readers will find this inspiring.
The book contains many oscilloscope images, showing actual waveforms and many readers will find this very helpful. A scope image will often show what is really happening far more clearly than any number of words, as well as being more convincing.
I was delighted by the author's comments on writings that talk about the relative tonal superiority of (for example) ECC83 valves from a particular manufacturer. Someone without formal training in circuit analysis is liable to absorb such nonsense as if it were fact. After all, it is widely put about on the Internet - especially by so-called audiophiles. Merlin Blencowe writes:
"These subjective differences are not a consideration for the circuit designer and will not be mentioned here again. (...) Real tonal control comes from the choice of topology, frequency shaping and manipulation of overdrive characteristics, and from a complete understanding of the circuit's functionality, not from the particular manufacturer or vintage of the actual components used."
How very well expressed.
>>Does not address the overall design problem
Although the book gives lots of information on (for example) deciding the component values for a circuit, it hardly touches on the major choices that a designer needs to make before ever getting to detailed circuit design. Such choices might include:
- the sensitivity required from each input
- frequency shaping characteristics to be provided built in to the preamp.
- what controls are to be provided in addition to the tone controls (gain? drive? sustain?)
- what effects (such as tremolo or reverb) are to be incorporated into the preamp
- how many stages are needed, the signal voltages to exist at each stage and which controls should be placed between which stage.
>> Slags off other books and makes unjustified claims
The preface states: "Of the handful of books which are available, they are at best only descriptions of pre-existing or 'classic' amps, and at worst badly written, badly presented, rife with errors and heavily overpriced". It is unbecoming for the author of a book to speak badly of the work of their fellow-authors like that.
The dust jacket states: "Designing Tube Preamps for Guitar and Bass is the world's first comprehensive guide to the design of tube-based preamplifiers, specifically for musical instrument use, in a single volume". This statement of uniqueness is unjustified and comes across as hollow boasting. Two predating books which cover the topic of preamp design for guitar are:
"The Ultimate Tone" by Kevin O'Connor (1995)
"Guitar Amplifier Preamps" by Richard Kuehnel (2007)
>> Editing deficiencies
The book seems not to have benefited from professional editing. One sign of this, which gives it a somewhat amateurish air, is the excessive use of quotation marks and exclamation marks. The book frequently puts words in quotation marks for no apparent purpose except that the word is a bit vague. Professional editors usually eliminate all such so-called scare quotes.
Exclamation marks are grossly overused in the book - there is probably not a single instance where their used is justified. Some readers find repeated use of exclamation marks irritating.
The level of typos is quite high - most seem to be where a typo has apparently resulted in a different word than what was intended, so that it would not be been found by a spelling checker.
Editing deficiencies such as these do not invalidate the information in the book but they will, for some people, greatly mar the enjoyment of reading it.
Another sign of the apparent non-involvement of a professional editor is the structure of the book. An editor would probably have insisted on an introductory chapter. As it is, chapter 1 dives straight in to dealing with analysis of the common-cathode triode, without covering things that you might have expected to find in an introductory chapter, such as:
- Motivation for why you might want to use vacuum tubes in a preamp
- Explaining the scope of the book - what it covers and what it does not cover.
- Outlining the steps involved in designing a preamp and the trade-offs that need to be made.
It would have been good to have had a brief biography of the author, such as many books include on the inside of the dust jacket. The title page carries the letters MSc after his name - but the subject of his master's degree is not revealed.
If I come across a fundamental misconception in a technical book, this greatly reduces my confidence in the book's treatment of other topics.
Chapter 7 is titled "The Cathodyne Phase Inverter". This circuit is more widely known as a phase-splitter - a triode with equal valued load resistors in the cathode and the anode circuits. The author is confused about the output impedances of this circuit and yet he states that "many authors" have been puzzled by the matter.
He states (p. 161):
"The output impedance of the cathodyne is a troublesome subject, as it varies hugely depending on whether or not it is equally loaded. (...) The fact that not only is the output impedance equal at both outputs, but also very low in value (...) has puzzled many authors, yet it is true. The theory behind this is beyond the scope of this book; interested readers are referred to Jones and Preisman."
But if you look at the quoted papers by Jones and by Preisman, you will find they make no such statement. In fact, they explicitly state that the output impedance from the anode circuit is high and the output impedance from the cathode circuit is low.
That is how it is - from a stage that, from its cathode output, is essentially a cathode follower with a cathode follower's low output impedance and, from its anode output, is essentially a triode whose unbypassed cathode resistor ensures a high output impedance. Anyone doubting the facts can look them up in Langford-Smith (formulas 33 and 34, section 7.2) or they can make an actual measurement of the output impedances of a phase-splitter stage - you'll find one in a Selmer Zodiac 50 power amplifier.
The chapter on feedback gets the author skating on thin ice here and there, when he mentions poles and zeros and output transformer high frequency characteristics.
I think that an enthusiast who could already build a guitar amplifier and understands a bit about electronic circuits and who is willing to make the effort to read this book in detail could get a lot of benefit from it and I would recommend it to them - subject to the reservations I have made.
Someone who is not already familiar with circuit design would probably find it over their head and get no benefit from it - notwithstanding the rear dust cover statement "An essential reference text for any amp enthusiast!" .
Finally, an experienced electronics designer who wanted to get clued up on valve guitar preamplifier design would, in my opinion, find that it does not give them the information they need. I'd suggest potential readers in that category should give it a miss.