Reviewer: Srajan Ebaen
Source: Zanden Audio Model 5000 MkIII DAC; Accustic Arts Drive-1; Audio Aero Prima [on review]
Preamp/Integrated: Bel Canto PRe2; Wyetech Labs Pearl [on loan]
Amp: AUDIOPAX Model 88
Speakers: Avantgarde Duo; Gallo Acoustics Reference 3
Cables: Stealth Audio Varidig S/PDIF, Stealth Audio Indra (x2), Crystal Cable Reference speaker cable and power cords; ZCable Hurricane power cords on both conditioners
Stands: 2 x Grand Prix Audio Monaco four-tier
Powerline conditioning: BPT BP-3.5 Signature for source components; Walker Audio Velocitor
Sundry accessories: GPA Formula Carbon/Kevlar shelf for transport; GPA Apex footers underneath stand and speakers; Walker Audio SST on all connections; Walker Audio Vivid CD cleaner; Furutech RD-2 CD demagnetizer; WorldPower cryo'd Hubbell wall sockets; Musse Audio resonance dampers on DUO subs; Mapleshade 4" solid maple platform under BPT conditioner
Room size: 30' w x 18' d x 10' h [sloping ceiling] in long-wall setup in one half, with open adjoining living room for a total of ca.1000 squ.ft floor plan and significant 'active' cubic air volume of essentially the entire (small) house
Review component pricing: $3,295

Acronyms are great fun. Just think of all the possible ways to distort 'em. Take TDB: Testosterone Depleted Bore? One look at the amp above quickly disabuses the poet in you of any wimpy notions. Total Dominance Bully? Not bad - 43 pounds of boffo weight and build quality agree with you but the presence of those eerily blue-illuminated valves might send you down the wrong alley despite that brutish 250-watt power rating. Tube Driver Blue? Now there's a sporting moniker that just so happens to be the correct one. Does that transform the BK in designer Butler's initials into Blue King? These questions are all best answered by the butler who would be master. After all, what do you make of the apparently single-ended 300B Class A monoblock code-named A100 below that delivers 100/200w into 8/4 ohms, packs 2,000,000uf of capacitance like a fistful of dynamite and accordingly can drive pretty much any load under your moon?

First spotted in my HE2003 San Francisco coverage driving the full-range 4-way Cabasse Karas towers with their tri-concentric cyclops eyes, I only absorbed at the time that the lone 300B triode was not just a predriver but somehow DC-coupled directly to the output devices to mix transistor muscle and valve velvet in a novel, zero global NFB hybrid architecture. Today's bridgeable-to-800 watts behemoth with its twin 6SL7GC dual triodes and 2KW power toroid is apparently based on a very similar concept. It's one of low-voltage operation that employs its tubes at less than 5% of their rated values and carries a full US patent as though every conceivable scheme of wiring and loading a tube hadn't already been thought of and patented decades ago. In the professional tube audio arena for more than 25 years, BK counts among his clients players and groups like Eric Clapton, ZZTop's Billy Gibbons, Pink Floyd, Elton John, Paul McCartney and 100s of others. His Tube Driver Blue has also found its way into upscale car audio. Unable to wrap my mind around this outrageously radical TDB factor notion, I asked the designer to shed some light on today's review unit and, by extension, on the super amp depicted above. BK sent me the "whitiest of white papers" which he penned in response to my request and which contains information never before made available to the public. To wit:

Short Discussion of the new Butler Thermionic Vacuum Tube Technology - BK Butler
My essential goal was simply stated: Achieve the 'all tube sound' with none of the drawbacks of conventional tube equipment - easily stated but difficult to achieve.

It took me 30 years of first-hand vacuum tube design experience seasoned with all the advice and b.s. gleaned from pro musicians, customers and forums during that time, and then some added good resonance found in Mr. Hamm's classic essay, "Tubes vs. Transistors - Is there an Audible Difference?" presented on September 14th 1972, at the 43rd Convention of the Audio Engineering Society in New York. Now that a new US patent has been issued and thousands of amps world-wide are operating reliably with this technology, I can say with confidence that the first stated goal has been achieved. In fact, my latest tube audio discovery reveals the purest essence of the art: thermionic vacuum tube audio.

The fun begins. Starting in 1999, it took more than a year of non-stop Edison-type trial and error work. I learned more than 100 ways not to get the tube sound. After many unacceptable and aborted complex circuits, a light came on. While I had worked with tubes all my life and designed over 100 audio products for various companies, I had never encountered a practical use for what may be called the weak force that tubes produce. As Physics teaches, while there's a weak force on the macro-atomic level which some attribute to the atom's ability to defy conventional perpetual motion constraints, there's also a weak force at work inside vacuum tubes - thermionic action. This phenomenon does not require any external applied voltages. Almost without exception, vacuum tubes in audio are used in a 'boosted' or an electron-extractive mode to produce power or desired gain. This requires the use of high voltages which eventually degrade the internal electrodes and cause failure. Pure thermionic operation requires only the heating of the cathode. The tube is then used without further boosting of electron emissions.

It's important to note that it was unavoidable to use high voltage in tubes for desired amplification properties in the decades before transistors (incidentally, my great uncle Harvey Fletcher was the immediate supervisor at Bell Labs over Bardeen, Brattain and Shockley who 'invented' the transistor). Until solid state came of age, there was no other known way to achieve quality amplification except to extract as much as possible with high voltage from vacuum tubes.

But - where is it written in stone that the only way to achieve the coveted 'tube sound' is to stress all supporting components to hundreds of volts and then force the output signal through hundreds of feet of distortion-producing output transformer windings? There's a much better approach now. I know, I know, this is painful to some very sacred cows within the religion of tube heads, but hope is on the way for the rest of us.

Amassing all the information I had from the sources mentioned earlier, I started with a clean sheet. Of course, I'd been aware for many years of Futterman's OTL tube amps and later his Moscode design which used Hitachi-type lateral MOSFETs to simulate power tubes. I even produced and patented a car amplifier in 1995 called Tube Driver which used a similar approach - tubes driving MOSFET output devices. The results are sonically pleasing but MOSFETs are not tubes and have a lot of problems with biasing, especially in an open-ended configuration. But more importantly, power MOSFETs do not allow for an important requirement observed by Mr. Hamm. They effectively isolate the driving tube from the mechanical-electrical load (speaker)."

"Quoting Mr. Hamm: "Our extensive checking has indicated only two areas where vacuum-tube circuitry makes a definite audible difference in the sound quality: microphone preamplifiers and power amplifiers driving speakers or disc cutters. Both are applications where there is a mechanical-electrical interface" (my emphasis added). So, it's important for the tube to 'see' or be weighted with the output load in order to sound like we want it to? I'm sure of it. But how can tubes be best coupled to the output load? Up until now, the most common and practical way was through an output transformer. This device not only allows high voltage operation, but also converts low current/high impedance levels of tubes to modern high current/low impedance speakers. But there are many undesirable qualities to transformer coupling in power amplifiers. Some major concerns are: inefficiency, phase shifting, distortion, core saturation and frequency response limitations.

In the Butler Thermionic (also called Tube Driver BLUE) circuitry, high voltages are avoided and the transformer is replaced by current-multiplying output devices, in this case, Sanken multi-emitter types. Others can be used, but Sankens have shown extremely good linearity and consistency in production batches. This consistency becomes crucial with my completely open-ended output design.

Now for the other critical element required for tube sound. Again quoting from Mr. Hamm: "The basic cause of the difference in tube and transistor sound is the weighting of harmonic distortion components in the amplifier's overload region" (again, my emphasis added). What? The basic cause of the difference in tube and transistor sound is the harmonic distortion components in the amplifier's overload region? What the ----? Isn't the goal of modern amplifier technology to avoid distortion - the old "wire with gain" idea? Again, what? We need overloaded tubes to get the tube sound? Well (I can really hear the sacred cows bellowing now) the answer is a conditional yes.

My son (or daughter), let me remind you of a simple fact. In the real world, we almost never hear perfectly reproduced sound. Fact is, just the physical differences in our playback environment vs. the recording location and techniques virtually guarantees this. Then we have an endless variation in recording source quality, cabling, preamps, amplifiers and speaker types. Those of us who have heard 'perfect' sound in an anechoic chamber can attest to its lifeless and dry-as-dust quality. What we usually want to hear is natural, enjoyable reproduction. What is natural sound? It's full of real and psycho-acoustically random, imperfect and fluidic elements that our brain interprets as being correct and pleasing. It's a purely subjective human thing.

No two of us will completely agree very often as to what constitutes 'natural' sound from our reproducing appliances, but audiophiles and musicians have overwhelmingly agreed that tubes generally sound more natural than transistors. But Mr. Hamm says that this pleasing sound is greatly attributable to the harmonic overload characteristics in tubes. Guess what? So do the world's best musicians! Designing for the musical qualities of tube overload or 'overdrive' was a good part of what paid my living expenses over the past 25 years.

So how can we produce a tube amplifier with pleasing non-linear or overload tube characteristics with good THD specs and still allow the tubes to interact with the speaker load while avoiding the old transformer coupling scheme?

The basic concepts used in the Butler Thermionic model TDB-2250 are essentially shown in the following diagram:"

The simplified explanation:
Using a very low distortion, discrete Class A front end and voltage amplifier of my own design, the single-ended output is DC-coupled into a pair of triodes (both enclosed in the 6SL7 bulb). These triodes are set up for maximum thermionic action which in tandem produces both the necessary drive and output biasing for a pair of complementary push-pull output current multipliers. Their output signals are then passively summed for the speaker output. The tubes are continuously operated at or near electron-emissive saturation which produces the pleasing, slightly non-linear tube overload characteristics Mr. Hamm says are required for the tube sound. The other condition he emphasizes is that the tubes must be interfaced directly with the speakers. Hence, the tubes receive a reflected and proportional DC load as they play into the output current multipliers.

This interfaces the speaker output with the near-saturated thermionic tube conditions and creates the undeniable and authentic 'all tube' sound. In fact, this approach surpasses most challengers in accuracy, clarity, imaging and presence etc. Again, an added benefit to the thermionic discovery is that the tubes are carefully balanced with each other to create desirable output auto-biasing for open-loop operation. They naturally produce more bias at the crossover point and less bias at full power, eliminating both common problems in open-ended designs: 1) under-bias at crossover and 2) over-bias at large signal excursion. The fact that the tubes are run only thermionically enables them to be used at a zero bias, saturated condition with no chance of any degradation over time. Tubes used in this manner have an almost unlimited life expectancy, on par with the other active components in the circuitry. An added benefit: Since the tubes are operating at full speaker AC line level, there are virtually no microphonics as compared with conventional tube amps."

"For those who are wondering, the answer is yes, parts of this description apply to my rare 300B-based A100 monoblock amplifiers, but not all. The A100 is not simply a single-ended version of the 2250's output circuit with a 300B inserted. I prefer to keep a few things under wraps -:)

The benefits of the new Butler Thermionic vacuum tube technology are many, but here are a few or the main points:

  • No output transformer
  • Power amplifiers of any practical wattage level can be readily designed with this technology.
  • No excessively high voltages. High quality lower-voltage-rated supporting components may generally be used (resistors, capacitors, diodes etc.)
  • Drastic weight reduction. (Allows for 5 channels x 150w or more in a single chassis as in our 48 lb. TDB-5150 model)
  • No maintenance. No periodic bias adjustments or re-tubing required. The amp may be left on continuously or switched via internal relay as desired by the customer.
  • Low heat production. The thermionic tube auto biasing reduces operating and standby idling temperatures to about the same or lower than solid state amps of similar power ratings.
  • All Tube sound: This proven new vacuum tube technology delivers as promised!"

What started off as simply another amplifier review had, with the appearance of this whitiest of papers, turned into something else - my first taste of what appears to be a truly novel implementation of tubes in consumer audio. With the designer's explanation delivered a priori rather than in a manufacturer's reply, we're now in the enviable position to test the implicit claim that a 250-watt power amplifier with two small-signal tubes can mimic, clone or elicit the instantly recognizable sound of a 'pure' (or better perhaps, traditional) zero-feedback SET or push/pull tube amplifier that employs regular output tubes and output transformers. I've decided that the above explanation was way too interesting to withhold until the review proper, hence this sneak peek not so much at the amplifier under review but the technology that drives it. More soon, with thanks in the meantime to Mr. Butler for giving our readers more than the somewhat opaque explanations thus far published on his website for reasons of intellectual property rights protection. Gracias, BK.