Tau DHT Amplifier - first build.

After completion of the breadboard stage the questions were laid to rest. After more than a year exploring the circuit variations which tugged on the curiosity the circuit wound up back at the original design with parts section as anticipated. I should add here that the "natural motion" of an interstage or output transformers spoken of in vintage tube building lexicons that does indeed sound more natural than any of the other topologies such as, Ultra Path, Parafeed, plate chokes, plate resistors, plate CCS etc... The AC activity moving in both directions simultaneously within the transformer might have something to do with it. This speaks directly to the hierarchy of circuit over layout, layout over parts.

It was time to build up the first basic unit using mostly off the shelf parts and M6 cores. 

Initial layout and parts mounting.

Wiring begins with earth rails and cathode supplies... 

Rear panel goes on to allow signal input wiring and power entry.

Basic model uses Chimera Labs interstage and LL2766AM Opts.

Wiring complete. 10% silver clad copper in PTFE tubing is used through out. Air is the primary dialectric inside the PTFE as it is not snug. Silver oxide will develop but the copper is hermetically isolated from the atmosphere.

Completed unit in provisional livery to get it going - final design to be published soon.

Power Supply

Since this is the heart of an amplifier it should probably be explained a little. The centre piece is the AC transformer designed for this amplifier. It has low DCR is internal shielded and drained with grain oriented electrical steel. No resistors are used to adjust voltage as they dissipate musical dynamics into heat. Four heater supplies are provided - one for each tube section. These utilise Schottky rectifiers and CMC choke loaded supplies on both positive and negative rails. B+ supplies share the AC transformer, rectifier and ground leg damper diode. The mercury vapour rectification provides not only regulation with very low resistance but also does not sag under load. The current creates a plasma bridge which increases or decreases with load maintaining the same voltage drop. The plasma's response to load also provides a very natural sounding regulation. The low DCR of the AC transformer is not compromised thus. 200H of low DCR inductance does the filtering with low ESR capacitors of high quality in an LCLC configuration for the power section. Inductance dominates the supply. This is furthered by a current source, nickel chokes and VR tube regulation to provide the driver section with clean dynamics. As good as the new Vishay DC Link caps are the long standing favourite of GE 97 and 27 series oil caps still win the day and are used at the final stage of the power section. No other oiler sounds like them.

Wiring in place

Mercury Vapour Rectifier and damper diode.

Cover in place and a little long exposure.

 

Later versions with 226 first section

 Final Version of the Chassis coming soon... as well as impressions of Finemet, Nickel and silver wound interstage, volume and output transformers...

The Tau - essential SET amplifier development

A fine example of a single ended triode (SET) amplifier was required to demonstrate the abilities of our speakers and so developed elements from decades of amplifier building were put together. The concept was then a distillation of learned essentials which were actually not at all conventional but rather imperially discovered though trial and error. These essentials were not an economisation but a focus toward the purpose of fidelity. To go where anaemic circuits with a 300B stuck on the end simply cannot. Conventional electronic design is motivated by finance and this quest was for music - damn the cost.

Once a tube spelunker pokes around enough, chasing the darkness of convention and reviews, which are essentially paid advertisements, one discovers that power sucks. Power sucks the nuance out of playback, yea and transmitter tube amps are dogs with high miller capacitance, huge transformer losses and unnecessary gain stages to drive them. High efficiency speakers and low power amplifiers can do what nothing else does - portray low level detail and the gestalt* of a musical event, to make music something special to hear and enjoy. Euphony. It takes courage to look away from the shadows of the cave wall, what commercial interests have spent decades and millions convincing us of, to emerge into the sunlit terrain. I am grateful I was brought up with live music - the bullet proof, bullshit detector* - which forced me to transcend electrolytic capacitors, untreated ground paths and useless watts. 

The 4P1L replaced the 45 and 46 tubes which got me into SETs decades ago. I intuitively bypassed the 2A3 and 300B. After competing with trophy collectors for tubes it was fun to find and use a tube that was equivalent of the 45/46 in linearity, low level detail and ambient aura that was overlooked, abundant and inexpensive. It is a direct heated Wehrmacht tube, a Russian copy of the RL2 / 4P6, on which there is no information in the German lexicons - they are listed there but with no information. Top Secret? I was curious how it would do in parallel single ended triode operation having used it for years in a Sakuma stye amp - single ended configuration. The 2P29L was chosen as initial drive tube keeping the eventual, almost century old, 226 globe tubes in reserve until after the amp was stable. This tube is also a direct heated pentode clone, CO-257, direct heated pentode, also in triode mode.

So what does this design do different? All DHT, low power = small transformers, two stages = low losses. An over abundance of inductance, some of which is in the ground leg a la CMC. Attenuators are not at the input but used later to increase current drive and slew rate. No cables between preamp and power section to drive which reduce transformer and cable losses in the critical section between stages. Schottky, choke loaded supplies for Rod Coleman's excellent cathode regulators. These were tried with traditional EI AC transformers before finding toroidal transformers sounded better with filament bias. There is overall a plethora of iron - more than any theorist/engineer would endorse. That said the rise time is measurable and is faster than convention endorses. 

This highlights the key element of testing with ears and trusting what they indicate. Music is a series of dynamics, shock waves, over a broad spectrum of frequencies and cannot be dictated by frequency tones or square waves. There was also no politics or economy in parts selection: if it works - its stays, if it don't - it goes, not matter the expense or theoretical/emotional attachment... Measurements bow to the human ear which is what music is for and from.

Many of these concepts were derived from the trifecta of collaboration over decades between Peter Fowler, Bill Jaensch and myself: we three geezers.

On to the build:

Parts collection initiated.

 

Shuffling parts around for layout design. As one can see here the henries of the PSU, Ale's SiC bias boards and Dave's most excellent attenuators.

The beginning of tie down.

Wiring initiated with ground bus.

Provisional power supply...

Clip leads are fun! Here beginning with Ale's gyrator plate load. 

Then on to amorphous core choke plate loads from NP ... and the next iteration of the PSU.

The beauty of breadboards is that a new device or circuit alteration can be changed in minutes.

I became rather fond of the little filament PSU sculptures almost naming them after movie robots... before going to choke loading for the filament supplies. 

This provisional supply used a high voltage Schottky bridge for the B+ which eventually became a mercury vapour rectifier tube half wave. The damper diode is in the ground leg replaces a SS slow start PCB - not only to provide slow ramp up of the B+ but it does more as per JC Morrison's inspirations.

After almost a year on the breadboard with many, many iterations this was the final configuration of the circuit that became preferred. All six supplies are now fully choke loaded with toroids and R cores and the B+ went to mercury vapour rectifier. Nothing, not even TV damper diodes come close to what a MVR sounds like. Yes, I have heard the nonsense that one cannot hear the rectifier in a properly designed PSU. That tidbit of wisdom is for people who do not have good ears IMHO. Hybrid rectification need not apply as they sound predominately like its weaker links. 

The driver section "evolved" to conventional interstage. The "natural motion" of a transformer plate load - "reactor" - as the old text books describe it - sounds the most natural to my ears all theorising aside. Also plastic PP caps and electrolytic have begun to be been minimised and the favourite GE97F series oil caps beginning to populate filtration.

Dennis Boyle's transformers (Ace Amplifer) and interstage are used up to this point with

M-6 Orthosil@ 29 gauge .014” thick laminations. He tested cores of Orthosil@ Super Orthosil@, Permasil@, 40% and 60% Nickel in .009”, 0.10” .014” and .185” lamination thicknesses in both power and audio signal transformers. His findings were that the M-6 .014” EI cores offered the lowest distortion with a wide bandwidth. This was measured and they sound surprisingly good.

That said, Finemet and Dave Slagle's 49% Nickel come in later and clearly do some things that M6 does not. More to follow as we discover some nice things about nickel that the ear and nervous system can easily and pleasantly discern. 

A few more posts on this amplifier will follow - including pictures with less spaghetti...

 * gestalt -

1. A physical, biological, psychological, or symbolic configuration or pattern of elements so unified as a whole that its properties cannot be derived from a simple summation of its parts.

1. A collection of physical, biological, psychological or symbolicentities that creates a unifiedconcept, configuration or pattern which is greater than the sum of its parts (of a character, personality, or being)

1. a shape, a form

Famous Direct Heated Triodes (DHT's) and the Unknown

Monsieurs Slagle and Jackson

Although the legend of the DHT tubes like the 300B, 2A3 and 45 got me into low powered amplifiers these Holy Grail tubes did not continue to hold my attention after a decade or so. It did however open the door to appreciating other low power, direct heated tubes. After hoarding a stock of NOS tubes while trying new production versions, which were often unreliable, it was time to look further. Before powering up a fleet of collectable tubes was the question: "Is it worth a thermal cycle on the tubes for a quick listen?" This is not the way. The enjoyment of music is the pursuit not materialistic and possessive concerns. After years of foraging in the mire for the same truffles everybody wants it became clear that the mind set of NOS tube acquisition was more about curation, trophy collection and possibly fetish than performance. Fact is tubes do not need to be expensive to sound good. Yay, I blaspheme.

Across the industry DHT landmark tubes like the 300B not only dominate - they rule consumers and often do so poorly. The majority of products are cashing in on the publicized trend leaving the consumer slowly to discover, sometimes over years, that the products they purchased, be it amps or tubes, are not at all what they hoped. The US military abandoned tons of vintage PA gear in the wake of post WWII occupation. Intelligent, sensitive ears salvaged and discovered the treats therein and created the seeds of legend with these discoveries. Yet this does not mean there are not others of equal or greater sublimity, in fact it should excite the curious to know what else is out there. Western Electric can sound good but it can also suck. Yay, I blaspheme! Klangfilm, Seimens, EMT and Russian, etc... artifacts are pretty sweet as well. The Eurodyne is probably the finest small venue vintage speaker there is. Yay, I say even better than the 753c or 757a. GASP! Yay, I blaspheme thrice! May lightning strike me if it is not true!

The WE300B can be an excellent sounding tube but they have no monopoly on special sound and to make amps, and replica tubes of them, poorly undermines the expectations that have been set by their mythology. This unfortunately is the market trend. Marketing, as we know, is all about mythology.

It reminds me of motorcycles in the 80s. The Japanese were not yet making decent handling bikes and so the Europeans were the ones to have. The American BMW ads of the time had the slogan: "The Legendary Motorcycles of Bavaria". After I got tired of grinding cylinder heads on BMWs I got into the relatively unknown Italian bikes. After waiting for a buddy a mile or two past an off camber chicane I returned to find him struggling with his BMW in the mud. Once we got it back on the tarmac he said panting: "That is the last time I try to follow a hot Italian bike on a German Myth. We both smiled. Going fast through corners has little to do with ad copy in magazines.

Mercury vapor baby - its hard to beat.

The iconic labels of 300B, 2A3, 45, etc are just popular labels for what a good DHT can sound like. If this is true what then lies beyond the scope of an audio reviewer's limited vocabulary? If they are indeed closer to the divine glow of tubes and share their wisdom across the alter for us to be blessed with must they not disclose the tenets from which they mount their ideology? If another divine glow is found and they do not recognize it does it then not exist? Must we suffer excommunication in order to explore it? What other glows could their be? Ahem, well this is the question but first: "What is wrong with the average big name tube amp out there?"

The 300B is not an easy tube to get working economically and so many implement it with inadequate voltage swing and/or inadequate current and resulting issues - sucky slew rates. etc... Chop it and dice it how you want, unless there is a pentode in there its a three stage program. (a step up transformer input driven by a low impedance line stage is a stage, its just in a different chassis). It is better to have a well engineered and tastefully voiced amplifier with an unknown tube rather than the average 300B amp out there. Blasphemy #1. The output tubes makes less difference harmonically than the driver tube. Blasphemy #2. The circuit design itself has more to do with the sound than any of the parts and tubes are just parts. Blasphemy #3 Yay, I blaspheme thrice, twice! Clouds darken and high voltages range to find a way to rapture me back to the herd - unless it is true - and then there is nothing in front but freedom and fresh air.

The first crack in the mythical facade began when I heard an amplifier which was not anywhere near the divine glow and did not pay any tribute to it at all. It had none of the elements the lore decreed. It used two 25¢ UF4007 rectifiers in full wave with a π filter. It was indirectly heated, it used a common, high gain but wimpy mini tube driver (12AX7, 12AY7 or 5751) with its two sections paralleled cleverly to give it some grunt yet with nothing but a resistor as a plate load, the output tube was a pentode (6CA7, 6L6, 350B, EL-34, or KY-66) run in ultra linear mode. Worst of all - it intentionally used feedback and even had a selector knob for adding this profane corruption! The real problem with it was that it sounded good. Real, serious good. Thus did the candle's flame waver and the shadows on the cave's wall flutter and fail. Light could be seen from the mouth of the cave and soon after the sky and sunlight for the first time. It was not all about the output tube! The amplifier did have unusual implementations which helped it to be what it was but these can be discussed another time on request.

The creator of this amplifier was Dennis Boyle. He lived in an old movie theater full of vintage gear collected over his lifetime. He had been a successful industrial designer before he discovered one day that one audio device could sound different from another after which he lost everything and fell into habitation with aisles of industrial shelving full of carefully collected vintage audio gear. In this closed stack of a humanities museum he lived a solitary life.

Mercury vapor with a damper diode.

Old movie theaters are spooky. They are built for darkness, temporary illusion and the pleasure this provides. They harbor pigeons who leave it everywhere, coo and come flapping out of places least expected. The smell of their guano is sour and makes for headaches. Harvesting vintage gear from these places was like spelunking. It was dark, dirty work which had a bad feeling about it. I would never visit these places without the promise of cheap or free vintage gear and so it was only after Dennis moved out of his abandoned theater and into a metal warehouse, which could hold all of his plunder, that I began to visit him. There was a little mono system which played FM radio through a Scott tube tuner. It was the only system I ever saw him use and it was always quietly playing. He never showed it off or asked anyone to listen to it. It just sat in a stack, speaker, tuner, amp, on the floor against a wall in the warehouse. Dust covered shelves held gear piled beyond reach row after row. After collecting all of this treasure in junk yards, craig' list, military salvage, and endless internet searches, after decades of study and pursuing the rare and precious flotsam and jetsam in pursuit of magical audio properties this modern day tube shaman, self confined hermit monk of audio listened to nothing else. This amp he called the Ace.

I spent some time there on and off over years becoming fond of Ramble, the Timber Wolf. In greeting he would put his snout between my legs and lift me off the ground, easily. It was just an affectionate nudge on his part which instantly lifted my feet off the ground. The first time he greeted me thus I wound up on my hands and knees. Dennis just commented that Ramble did not take to people like that often. Point being that the sound of that amp went into my comparator database unconsciously. Over time I became less happy with most 300B amps I heard. When I hosted a demonstration of the Axiom 300B amplifier brought out from Florida to Texas. This was Dennis' and Bob Hoekstra's (Nascar Engine designer) all out pursuit of a 300B amp done right. It made abundantly clear the tube itself was never to blame for comprised performance but these amps were huge and stupid heavy. They required two men to carry each mono block. With gobs of custom iron, three tier stacked supplies, ten tubes per channel and metal Deco styling from Dennis they looked like something out of Metropolis. (A modern day version of a 300B done right might be Ale's foray into the fray which became quite an amplifier. Ale's 300B)

The Axiom had 20 tubes to ignite for a stereo listen, the Ace had 4. They had things in common however, they were both alive, muscular, ultra clear and still had the inner lifelike clarity that is was what tubes are all about and few deliver. The similarity in sound demanded attention. One was easily an everyman's amp while the other a statement piece and yet they both broke, without even noticing, many of the decrees, laws and tenets of magazine audio. My conclusion? Tubes are not the problem, it is the commercialism which surrounds them. The 300B and its kin are like Merlot - it can be good but usually it is not. Nothing wrong with the grape but its commercial appeal created commercialized production and its sea of mediocrity. Want something better than Merlot? Learn about other grapes and wines.

It is to this venue I extend an invitation. Let us enjoy a little freedom and fresh air away from the marketplace, the ad men's hypnotic distortion field and amplifiers built into MDF cabinets with vinyl veneer to make them look like wood while costing more than a car. If you have not figured it out by now there is no hot, admiring house wife ready to hang on your every word just because you bought the shiny thing in the advertisement. MDF is dust - toxic dust. Let's make real stuff out of real materials with real design work. This means opening the design criteria well beyond the 300Bs of the world and using our ears to determine the value of music. It is after all our ears we are trying to please is its not? This is the way.

An all DHT amp with tubes not so heavily publicized.