John Broskie's Guide to Tube Circuit Analysis & Design

05 September 2006


Book Review

The title of Morgan Jones’s 2004 book, Building Valve Amplifiers, reveals it all. The title isn’t Understanding Valve Amplifiers nor is it Designing Valve Amplifiers. The operative word in the title is “Building,” which includes both construction and testing. The author's previous work, Valve Amplifiers, quite satisfactorily covers understanding and designing tube amplifiers, but is thin on building practice; in contrast, Building Valve Amplifiers is a practical guidebook that carefully explains the ins and outs of constructing, modifying, repairing, and testing tube amplifiers. In Building Valve Amplifiers, Jones leaves out most tube-circuit theory (there are only about ten formulas in the book, for example).

In other words, both books complement each other and can be viewed as a two-volume set. (At over 350 pages, this book is too large to have been tacked on as an appendix to Valve Amplifiers. In addition, I can imagine his publishers doing the math: we can add half again as many pages to the first book and raise the $49.95 price by maybe 15% to 25% or we can bring the book out on its own and sell it for $33.95. Do not get me wrong, I am happy to pay for two books and I prefer many smaller books to one fat book; after decades of reading 1,000-page computer reference books, books so heavy that one dropped from a window could lead to murder charges, I long for easy-to-read-in-bed sized books.)

The punch line is that I very much like this book and I lend it my unalloyed recommendation. Having said that I must admit my prejudices. First, I have met Morgan Jones and we occasionally e-mail back and forth. He is jewel of a fellow and I cannot imagine not instantly liking him. Considering how large is the ratio of charlatans, conmen, and imposters in high-end audio, he stands out as a genuine NOS Genalex KT88 in a crate otherwise filled with Chinese 6L6s.

Second, I hate trying to fix other audiophiles’ broken home-built tube gear. And the only thing worse is trying to do so over the phone or through e-mail. Unlike commercially-built gear, these boat-anchors in waiting may never have worked. Given that fact, how does one sort out the crux of the problem–and long distance, at that–bad component, bad wiring, bad layout, under-specified parts…? I have seen it all: 450-volt power supply capacitors burdened with a 550-volt power supply, grid-stopper-free amplifiers that oscilate at full power at 90MHz, ground buses that look they were designed with the aid of a spiral graph (or illicit drugs), backward rectifiers, huge solder joints the size of cherry pits, heavy coupling capacitors tack soldered in place so flimsily that they easily shake lose and fall to the floor, high-voltage-carrying wire protruding from the chassis. Thus, in terms of my workload, I can only benefit from more tube-loving audiophiles reading this book.

Third, he uses the Broskie-Cavalli-Jones headphone amplifier as one of his practical examples, which was a pleasant surprise. (It was Alex Cavalli who introduced me to Morgan Jones.)

So now that you know that I like Building Valve Amplifiers, why should you like it too? To start, it is well written. Jones writes the way I wish all technical books were written: to the point, clearly, and with a human touch. Explaining carefully, but never excessively, Jones is out neither to impress nor to intimidate. In fact, portions of the book have the slight feel of a confessional, as if it were a catalog of how not to make the mistakes he once made. In fact, he states in the preface,

“This book is distilled from years of bludgeoning recalcitrant electronics, thumping metal, and sucking teeth at the price of test equipment...”

Furthermore, I don’t know where else you would find much of what is covered in Building Valve Amplifiers, particularly the material found in the half devoted to testing. I know that “testing” is a naughty word for many audiophiles, who fear getting too technical might undo some of the glory of their single-plate 2A3s. And many solder slingers are boldly lazy; they maintain that all the testing that is needed is to flick on the power switch, as the equipment will either work or not work. If only that were true.

Analog gear is like life itself, where Perfect Health and Death are but thin lines that bracket a huge expanse of gray area that is filled with varying degrees of tiredness, decay, illness, atrophy, enfeeblement, and stupor. No amplifier works perfectly; each bends and twists the input signal in its own way, as none of its parts is perfect; nor is any immutable, as time ages all that is physical. Testing allows us to see into the gray. It reveals problems that no amount of tube rolling or part-brand swapping will solve. Furthermore, testing promotes understanding more effectively than any other means. If you want to know how an amplifier works, test it, with your ears, a scope, an FFT, and whatever other tools you have available.

In addition, audio testing, like racecar testing, is neither as simple nor as obvious as Consumer Reports would have us believe. Did you know that tube amplifiers can suffer from slew-limmited distortion? They can, and do you know how to test for it? Building Valve Amplifiers expalins how. A key point here is that audio electronics has its own subtleties and important details that do not appear on the radar of most other electronic undertakings. For example, as long as a digital clock keeps accurate time, it little matters if its power supply suffers from poor regulation, ground loops, or high noise.

Moreover, Building Valve Amplifiers is gently suffused with Briticisms and English good humor (or should I have written, “humour”?). For example, in listing the many construction faults revealed in an illustration, he writes: “Gaffer tape is for bodging on stages and studio floors, not internal electronic use.”

So, is there anything I do not like about Building Valve Amplifiers? Well, yes. I wish a full-bodied table of contents and a more-extensive index had been included, so better searches could be performed. For example, I looked in the index for “White cathode follower,” “Broskie-Cavalli-Jones” and "headphone amplifier," but none could be found. Something I don’t particularly need, but which others would no doubt like, is a glossary. One addition that no one expects—but which would nonetheless be useful—would be a link to a Jones-hosted website that would include many more photos, the photos that couldn't be included in the book, photos (particularly in color) of actual amplifiers being constructed and tested.

In conclusion, I don't know of a better way to spend $30* and get so much useful information on building tube amplifiers. Morgan Jones has done us all a great favor by writing Building Valve Amplifiers. We should return the favor by buying it and defending it.

Defending? Unfortunately, I expect this book may not get a fair press. Why? One reason that readily comes to mind is that for every tube-loving audiophile there are at least ten tube-loving musicians. Tube-based music amplifiers are to high-end tube amplifiers what country music is to classical music: related, but neither wishes to admit it. In other words, because of Google searches for tube amplifiers, ten times as many music-amplifier folk will come across Jones’s book, and they won’t get it, just as they won’t get his Valve Amplifiers; it’s more of a culture-based blindness, rather than a mental deficiency (of course years of drug use doesn’t help). The book’s example circuits will look strange and unsettling to them. “Constant-current source? What the hell is a constant-current source," they will wonder. On the other hand, solid-state-loving audiophiles and musicians will not even vex themselves by looking at Building Valve Amplifiers, but then at least they will not bother leaving a disagreeable review at Amazon. So far there are three reviews at Amazon; one gave only two stars becuase the book wasn't more like Jones' Valve Amplifiers and another gave it only three stars because it did not cover his music amplifier interests (only the last reviewer understood the book's value and gave it the five stars it deserved). Prove these jokers wrong, and buy and support Building Valve Amplifiers.


Next time
We will return to more tube circuits.



* I found copies of Building Valve Amplifiers selling for as little as $30 on the web, but since Amazon offers free shipping, its $33.95 price might ultimately prove the cheapest. In fact, I toyed with the idea of buying a case of Building Valve Amplifiers directly from the publisher (Newnes, an imprint of Elsevier) and having it shipped to Morgan's house, where he could sign each copy, as I would love to sell his signed books at my Yahoo store. I know that I would gladly pay $10 more for a signed copy, but that wouldn't cover the shipping costs from England, so I dropped the idea. (Aikido PCB buyers have asked me to sign thier boards! Can you believe it?)





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High-quality, double-sided, extra thick, 2-oz traces, plated-through holes, dual sets of resistor pads and pads for two coupling capacitors. Stereo and mono, octal and 9-pin printed circuit boards available.

Designed by John Broskie & Made in USA

Aikido PCBs for as little as $24

The Aikido tube topology, created by John Broskie, delivers the sonic goods. Since the Aikido circuit came out in the Tube CAD Journal, people have been building it and marveling at its sound. A prediction: just as the 1980s were the cascode decade and the 1990s, the SRPP decade, this decade will be known as the Aikido decade. Why?

This amplifier circuit offers voltage gain, low distortion, low output impedance, and a high PSRR figure—all without a global feedback loop.

Better than SRPP
or Grounded-Cathode Amplifier
The Aikido circuit symmetrically balances imperfect triodes against equally imperfect triodes, which greatly reduces its distortion output; it also sidesteps power supply noise by incorporating the noise into its normal operation. As a result, in terms of distortion, output impedance and PSRR, the Aikido circuit works at least a magnitude better than the equivalent SRPP or grounded-cathode amplifier. The improved PSRR advantage is significant, greatly unburdening the power-supply.

Yet noise reduction is only half of the Aikido’s virtues—this circuit’s flexibility really makes it shine. Different input and different output tubes can be easily interchanged, as long as they have the same socket pin-out and heater voltages. Because the circuit uses symmetrical triodes in place of plate and cathode resistors, a 6AQ8, 6BC8, 6BK7, 6BS8, 6CG7, 6GU7, 6FQ7, and 6N1P could be interchangeably used in an Aikido amplifier ostensibly designed around the 6DJ8 in a 9-pin board.

Always Biases Up Correctly
A fresh triode is not the same as that same triode after 2,000 hours of use. Remember, tubes are not stainless steel yardsticks, being more like car tires—they wear out. Just as a tire’s weight and diameter decrease over time, so does a tube’s conductance. But as long as the two internal triodes age in the same way—which they are inclined to do, as they do the same amount of work and share the same materials and environment—the Aikido amplifier will always bias up correctly, splitting the B+ voltage between the triodes.

Functions Flawlessly
Despite Wall-Voltage Swings
Additionally, unless regulated power supplies are used for the B+ and heater, these voltages will vary at the whim of the power company and your house’s and neighbors’ house’s use, usually throwing the once fixed voltage relationships askew. Nevertheless, the Aikido amplifier will still function flawlessly, as it tracks these voltage changes symmetrically.

No Popping Sounds at Startup,
No Squirrelliness
Moreover, the Aikido amplifier does not make huge popping swings at startup, as the output does not start at the B+ and then swing down a hundred or so volts when the tube heats up, as it does in a ground-cathode amplifier. Furthermore, the Aikido amplifier seems to bypass much of the power supply squirrelliness, making the circuit sound as if it were attached to batteries or a well-regulated power supply.

Less Distortion
Most importantly, the circuit produces far less distortion than comparable circuits. The triode is not nearly as linear as a resistor, so ideally, it should not see a linear load, but a corresponding non-linear load. An analogy is found in someone needing glasses; if the eyes were perfect, then perfectly flat (perfectly linear) lenses would be needed, whereas imperfect eyes need counterbalancing lenses (non-linear) to see straight. In other words, loading a triode with the same type of triode works to flatten the transfer curve of the amplifier.

A Sound that’s Hard to Improve Upon
So, in sum, we get quite a bargain in the Aikido amplifier: low noise, low distortion, low output impedance, and no global feedback loop. The result of all these features is an effortless, clean, and natural sound that is hard to improve upon.

Some have simply hardwired their own, while a few have bought PCBs from others that are in no way affiliated with the the Tube CAD Journal. Many of you requested a genuine GlassWare Aikido PCB, and this is it. This one was designed by John Broskie himself for you, the Tube CAD Journal reader.

What makes them superior? First of all, they look fabulous and feel solid in the hand: extra thick (inserting and pulling tubes from their sockets won’t bend or break this board), double-sided, with 2oz copper traces, clean silkscreen and solder mask. (The board holds two sets of differently spaced solder pads for each resistor, so that radial and axial resistors can be easily used. Each capacitor finds several solder pads, so wildly differing sized coupling capacitors can neatly be placed on the board.) Each board holds two Aikido linestage amplifiers; so, one board for stereo unbalanced or one board for one channel of balanced amplification.

The boards hold two coupling capacitors, each finding its own 1M resistor to ground. The idea here is that you can select (via a rotary switch) between C1 or C2 or both capacitors in parallel. Why? One coupling capacitor can be Teflon and the other oil, or polypropylene or wax and paper…. As they used to sing in a candy bar commercial: “Sometimes you feel like a nut; sometimes you don't.” each type of capacitor has its virtues and failings. So, use one type of coupling capacitors for old Frank Sinatra recordings and the other for string quartets. Or the same flavor capacitor can fill both spots: one capacitor would set a low-frequency cutoff of 80Hz for background or late night listening; the other capacitor, 5Hz for full range listening. Or if you have found the perfect type of coupling capacitor, the two capacitors could be hardwired together on the PCB, one acting as a bypass capacitor for the other.

The board assumes that a DC 12V power supply will be used for the heaters, so that 6.3V heater tubes (like the 6SL7 and 6SN7) or 12.6V tubes (like the 12SL7 or 12SN7) can be used, for example on an octal board. Both types can be used exclusively, or simultaneously; for example 12SX7 for the input tube and a 6BX7 for the output tube. (A 6V heater power supply can be used, as long as all the tubes used have 6.3V heaters; and an AC heater power supply can be used, if the heater shunting capacitors are left off the board.)

The boards are four inches by ten inches, with eight mounting holes. Boards are made in the USA and come with instructions that include schematic and recommended part values.

A real E-mail from a new Aikido PCB owner:

Subject: OH MY GOD!

Thanks, John.

My Aikido boards arrived today, as promised, in good condition, five days after placing the order.

Knowing you through your writings I expected good things. Knowing the high end audio community where cables sell for thousands, capacitors sell for $50, ring clamps sell for $600 and stainless steel record weights sell for $100, I didn’t expect much for $39.

I am tickled to death by the incredible quality of the boards and the thorough documentation enclosed with them. Now I know why there haven’t been any new posts on your webzine since December 10th. You’ve been a busy boy!

I can’t wait to get soldering. But now I’m afraid my parts quality won’t live up to the boards!


One problem, though, these boards are so pretty, I can’t bear to enclose them in a heartless metal enclosure. I have to figure out how to safely let them see the light of day while in use, so that all can appreciate their beauty, even when Aikido is asleep.

Stuart (Happy Camper) P.

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