TubeNexus

Hope the title didn’t send you fleeing!

I was first introduced to Pentode Press and Richard Kuehnel when I was searching for some detailed design information on the Fender 5F6-A. This was the circuit I wanted to build as one of my first ‘formal’ tube amp projects (as opposed to the informal, point-to-point, parts from the junk bucket ones I had done ’til then). Somehow I stumbled onto his book, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, Second Edition in a web search. It sounded perfect from the blurb:

This book examines this famous amplifier by studying its circuit design in great detail. It starts by breaking the amplifier into its major components: the 12AY7 preamp, the 12AX7 voltage amp, the 12AX7 cathode follower, the tone stack, the long-tailed-pair phase splitter, the push-pull power amplifier, and the power supply. Each component is analyzed determine how it works and to derive the design formulas needed to predict its performance. The results are then compared to bench tests of the actual circuit. Finally all of the components are put together to analyze total system behavior and to discover how and where the amp transitions into distortion.

And I wasn’t disappointed… but the book wasn’t completely what I expected either. It actually contains a true circuit analysis (remember Kirchoff’s Voltage Law and Current Law? … you will after you’re finished with this book!) of the entire amplifier in addition to the standard methods of approximating tube circuits, etc. This book was quite a pleasant surprise to me because the rigourous approach really helped me understand the operation of the amplifier and it gave me an enjoyable satisfaction of understanding a real practical use of circuit analysis… I had been very disappointed with Circuit Analysis courses in University 32 some years ago (even though I really enjoyed algebra and was looking forward to those courses).

Following Richard as he derives a model of each stage of the amplifier may not be everyone’s cup of tea but the book neatly summarizes the results along the way and you can easily fall back on using the algebraic equations that come out of the analysis… Miles O’Neal has even put the gain equation from the book into a convenient web-based calculator.

Shortly after reading the book the first time I happened to be in Seattle on business when I learned Richard was publishing another, Vacuum Tube Circuit Design: Guitar Amplifier Preamps. I tried to pick up a copy for the flight home but was one day early!

Written for experienced builders and aspiring circuit designers, Guitar Amplifier Preamps moves beyond the basics to present a complete guide to the theory and operation of triode and pentode voltage amplification. From the guitar pickup to the second stage grid, every aspect of circuit design is rigorously explained and thoroughly explored using real-world examples from Ampeg, Fender, Gibson, Laney, Marshall, Matchless, Orange, and Vox.

The preamp book covers a lot of older preamp configurations in amplifiers… working out the input impedances and frequency characteristics for the various combinations of bright/normal jack inputs, etc. but also explores the issue of matching input circuits to guitar pickups, interaction with volume controls, paralleling input tubes, and pentode stage design among other things. I find this book good reference, but was a little bit disappointed that it didn’t cover tone shaping at each stage and tone controls in general, but I’ve come to believe that requires a whole book onto itself.

So there I was the other day, diligently trying to add links to the link directory, and I decided to add Pentode Press… surprise… another new book: “Vacuum Tube Circuit Design: Guitar Amplifier Power Amps”

Written for experienced builders and aspiring circuit designers, Guitar Amplifier Power Amps moves beyond the basics to present a complete guide to the theory and operation of single-ended and push-pull power amplification. From the phase inverter input to the loudspeaker output, every aspect of circuit design is rigorously explained and thoroughly explored. Find out what happens when Class A₁ and Class AB₁ circuits are overdriven to distortion levels never imagined by Frederick Terman or the Radiotron Designer’s Handbook. Discover step-by-step how to design phase inverters and power amps to achieve your specific design goals.

This book weighs in at 392 pages and was published January 22, 2008. Perhaps the best way to get Richard’s books is through the PayPal shopping cart on his site… my order went in this morning! Speaking of the Pentode Press site, there is a lot of supplemental information posted there… updates to earlier books, as well as background material like “Tutorial - Tube Amp Algebra” and “Vacuum Tube Archeology”. I’m looking forward to seeing what I’ll learn about the back-end of amplifier design from his latest effort.

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