Faith, Hope and Salvation – The WE91A SE stereo amplifier without 300b tubes – I did it again

End of 2017 I did publish a post about the single ended amplifier designs and my evaluation of power tubes to be more than an alternative of the Western Electric 300B tube. In these days the idea formed from my earlier sound evaluation experience of different manufactured 300B tubes. The audio business focused on tube amplification made single ended tube amplification designs with 300B power tubes for 40 years to be the "cash cow" of growths. As result there are uncountable amounts of makes and brands in the market which explain each as the most original reproduction derivate of the original Western Electric power triode WE 300A from the 1920ties. The major problem here are the extremely developed market values of original vintage rare WE-tubes from all production decades and as reaction to such rareness the market mechanism highly raised the price tags. So modern production tubes create an own market, such are made in different countries but foremostly in China (to keep the quality level high, lol). As earlier stated, if you are not millionaire, able to light your Cohibas with 100$-notes, the modern replacement tubes in the 100 to 600$ range are all more or less crapfunctional in comparison to vintage tubes from the 1940ties in + 2000$/pc range. So relevant amplifiers of the traditional WE91A design topology use on top the WE310A as driver, the WE274B as rectifier, all together the equivalent for stereo in the 10K$ range and if you really want the exceptional finesse of sound, – alternativeless.

Yet, the single ended amplifier topology shows up to the tube audio connoisseur with several advantages if you incorporate a lightweight, fast and fine detailed alnico magnet driven loudspeaker, like horns and full range paper cone speakers (like Altec 755 or similar) or combinations from both principles. To raise the full potential of such systems the amplifier design is in need of radical control about component quality and about its consequent amplification topology. Today almost all decisive vintage components of quality production have disappeared from the market, so it is almost impossible to realize an amplifier design without any compromising cut or ending up in the cost nirvana (known from several Japanese brands reissuing modern Western Electric equipment at a highest possible level). Such set ups easily might create levels of investment in the million dollar range. If you are not the successor in the Toyota heritage or the son of an Arab prince, it might get a bit difficult to follow up such ways to your hobby within the coming years. For all others it needs some intelligent liberation from hardly traded positions in order to find perfect new ways to comparable sound qualities. Here I am going to describe my second sample to a wishless audio happiness with tube amplified equipment, where the tube line up for stereo matches the amount for a well cared piece of organic breeded beef. Yes, 50 $ and all tubes are vintage NOS highest quality production standards long life types or military grade, – and on sale in good amounts.

I am still in the good position to have several of necessary vintage quality parts as spares stored in my cellar, so I am able to realize rarely a design with almost ideal components. My first sample of such conception from 2017 earned so much interest that I did decide to realize a second version build on an identical design topology but realized with completely different hardware, a interesting step to find out about resulting differences.

Partridge mains transformer beside modern open frame output transformers, all covered by epoxy resin boards, like several British vintage amplifiers show up with

Here I did finalize an early Ebay finding after more than 20 years, a British Goodsell GW12 Williamson PP design from 1949, which had lost its open frame Partridge-output transformer. I did look for years to complete it again or to find a matching partner for stereo use, which never happened by its exclusive rareness. So I decide in 2005 to break off the originality line with a heavy heart, but at least in order to bring it back to a new functional life as a single stereo ended amplifier. In these days I did experiment with another rare find, the DHTs from Elektromekano S6 in order to resemble the classic 300b out put tube in such designs. I used the Mullard EF37A to be the driver tube and tested several output transformers to complete this concept, but nothing could convince my expectations in these days (and the illustre matching preferences of this extreme tube). It ended up that I stored the chassis back in my cellar and followed up other ideas like DHT tubes in preamplification stages and so the Goodsell SE-amplifier got more or less forgotten.

The Danish 1940ties Directly Heated Triodes from Elektromekano, the S6 and M7  (4V/1A) are some of the finest manufactured power triodes made ever. Even the Western Electric types from this period don't match mechanically their standard. Unfortunately these triodes don't match the ordinary range of output transformers but they are still some of the nicest Tube ever.

at the far right the knob for the global feedback loop, realized with single switchable carbon composition resistors

Last autum I did start a collection sale of all my long time collected amplifiers and other audio collectibles in order downsize my life in a "less is more" matter. So I had several people coming to look and listen to different components of my personal estate. Many of such auditions ended into sales of exceptional sounding vintage valve amplifiers and dedicated components like speakers, preamplifiers, tonearms, etc. But the most respect raised after end of a particular sales procedure when the question came up with which amplifier I am going to listen in the future? When I demonstrated the "bastard amp" in my favored chain, the most people didn't need a lot more than a minute of listening to understand my decision. Within the two gone years of time the interest into such a special single ended tube amp has grown. So I thought about another sample, well knowing that the prototype was some sort of recycling of leftovers on a highest level possible. I did exactly know, that I never will get a chance to make a identical copy and so could not predict the following result, in particular when using 70 year old parts like the mains transformer, choke (both Partridge brand) and other sound determining parts like the output transformers. The open frame design of the vintage parts made the selection of output transformers delicate in terms of authenticity of design. Modern potted transformers like Japanese classics from Tango, Tamura or later Taiwan successor   James aestetically do exclude themself for this project. One of my technically ideal choices, the Swedish Lundahls, were no option since their design makes an open frame incorporation impossible. I had a pair of vintage Philips universal single ended transformers in spare (1/2/3/5K:4/8/15Z). I had no idea for which power output they were made for, but size and weight let rate me them to something like 7 watts (similar to Tango U808). The next candidate were some classic Chinese laminated oriented steel core OPTs with 3.5K:4/8/15Z@20W, such have been sold as entry level transformers in HK as Mars brand are built into lots of typical Chinese made chrome monsteramps, which are sold through Ebay since years for small money. With the Philips transformers the amplifier performed with a exceptional open refined soundstage, wide, liquid and natural, but with a little slim sound fundament lacking the typical natural substance of a single ended amplifier. Switching to the 2,5 kg Mars transformers the soundstage was perfectly well balanced. I did only try them since they measured at a scope very linear between 30 and 18000 hz, but these lack all the subtile refinements, which have been already on stage with the smaller Philips iron. So I needed to find something with good sound attitude and matching design preferences.

Clearly visible the eight pieces of amorphous c-core parts forming the final transformer core

Beside these experiments I was a bit curious about all the rumors about different new core materials and designs. The final sow which had been chased through the tube audio village in the last years was the Hitachi made "metglas" core material, an amorphous nanocristalline alloy formed with extremely high pressure under exceptional melting temperatures of several thousand degrees. This high tech material is known for its lowest loss properties addressed with "never before heard" tremendous resulting properties. Something which is equally important addressed to even more important rise of the resulting price tag in the audio world, a factor up to three times to an ordinary cut c-core output transformer. Vintage (early 2000) made Tango or Tamura transformers containing amorphous core instead of c-cores or even laminated iron are on sale at Ebay for tremendous amounts... and even serious brands like Lundahl have incorporated such cores to keep up in the vanishing markets.

Today the amorphous cores are used in different high tech scientific applications or in mobile web technologies and Hitachi produces a whole catalog of differently shaped cores and sizes. It looks that only the audio nuts are paying the elaborated high bill for the next "must have gadget". Hitachi has found paths to produce this high tech metal somewhere in India at reasonable market prices. Like with all other new technology some people will find fast illegal ways to sell fractions of such production lines at the web. Then is it only a little step and a question of short time that you will find other people there who make a serious business with such cores and produce audio output transformers for the audio aficionado and sell these for a fraction of the price of established brands. So I found at Ebay. hk.com different designed tube out put transformers with amorphous core material, one type was matching my primary preferences of 3.5 K/20H/15W, unfortunately with secondary windings interlaced for 4 and 8 ohms. So I asked the seller if a custom version with 15 ohm will be available and the friendly people were happy to make me a customized pair. Visually the transformers are wound in a typical British manner with steel mounting frames, so these OPTs did visually match well with my 70 years old Partridge transformers of the Goodsell. So I ordered a pair with 15 ohm secondary tap in the hope that this transformers will match my expectation set by other brands like standard Lundahls, but will offer a better formal integration with this vintage amplifier. I had no expectation about something special as a result from different core material.

The transformers where delivered by Hermes as a standard parcel, no customs, import procedure – how can this be possible? When I opened the parcel a strong smell of the epoxy resin knocked me out for a second, but this smell evaporated within a day and when I had the pair incorporated into the amplifier, their smell was already neutral. The first listening showed up with a well known sound attitude, so I needed to adjust the feedback loop with the new amplifier. In my first post from 2017 I photographed the amplifier with fixed feedback loop with a 3.2K resistor. Later I integrated two wire wound pots of 5k range to have the opportunity to match different speakers than only my Tannoy Monitor Reds with 15 ohms. Here now I planned from the beginning to have the luxury of a adjustable global feedback loop. After some measurements and finalization by listening I ended up with matching value for this amp, ironically it is again 3.2k!

Underneath oil paper caps ballet and a Partridge choke, all high current wire is original vintage Western Electric wire. Top right corner the switchable feedback resistors connected with blue and yellow wire

The most of yours will or can not believe me, but the new realized single ended amplifier sounds absolutely identical to my first version. Yes of course, the design is to 100% the same, but all the very important HiFi narratives about hardware, the use of parts, in particular output transformers, their quality and branding seemed to be what it is – marketing fairytales. Both amps look completely different, but soundwise I cannot detect the smallest difference, unbelievable but true. I have used the same capacitor layout with the same material preferences, paper-in-oil-capacitors as small values as necessary. For example I use 4µF as loading capacitor and another 4µF as smoothing cap. For 30 years of my life it was impossible for me to get such rail calm, now I know how to do and this amp is dead quiet, like a modern transistor amp, but with a "fresh oiled liquidity" unknown from other tube amplifiers. The only coupling cap between driver and power stage is 0.05µF like it should and not like in 99% of all WE91A copies and derivates wrongly transcripted 0.5µF and so on...

If you are looking for a reasonable priced tube output transformer which shows up with exceptional sound qualities and offers results which are on par with the well established brands for a fraction of the price tag ask the people in HK, they will be happy to serve your needs and HK people need some support anyway, their future doesn't look so bright...

Faith, Hope, Salvation - The Single Ended Tube Amplifier

Hi everybody,

since a long time I didn't post any article about tube based audio with vintage components. To my understanding almost everything substantially was said within the last four years about this topic. But now here I will try to bring right in time a known Christmas fairytale with some new accents.
In particular after my series of articles about the rise and reintroduction of tube amplified audio with a synonymously way to create the Western Electric 300b tube as central element of religous passion over the last 30 years. Yes, a vintage Western Electric 300A or 300B tube is a exceptional unique sounding device with unparalleled attitudes – period. But a lot of other triodes like the 45, the 2A3, the 6BG4, the RE604, the R120, the AD1 and several other directly heated tubes from that period before the 1950ties (i.e. mass production) are as well unique sounding devices, each with a recognizable fingerprint soundwise. Not even to note the big bottles 211, 845 and GM70.

To keep the example of the WE300B, there are more than 10 different vintage original WE types known, which show all the originating imprint Western Electric, made at different production places and in periods from 1928 to 1998. Further are additional license productions, like the well known Cetron or the STC 4300B (Standard Telephone and Cables, GB) versions. Each of this different 300B tubes show a clear and comprehensible sound attitude, different to any other sample of this types. The reasons for such might be today found in aging, storage conditions and of course, even in little production differences and materials. Tubes are made to technical given standards and the most of us know, technical specifications can be evaluated through measurements, but sound cannot. This insight brings us to the question if automatically a more rare and a inevitable more expensive tube is generally the better sounding device?

Three WE300B tubes, one from Cetron

For some, who might fire their barbecue with 100 dollar notes, the permamently rising price tag for vintage WE 300B tubes might be neglible. Dripping every day 10 dollars into the money box is the value of using vintage 1950ties original 300B tubes for some hours every day. The latest Western Electric originated 300B production from the 1980ties generates price tags of 2500 $ for a NOS pair, a used single graved base tube from early production will cost a good 3K$, New Old Stock asks for a open range between 10k and 20K$ for a 70 year old pair. Yes this tubes might do sound exceptional unique, hardly to compare them to any later production line, but do they really sound that much better than later versions,  – or just different? Or does the price tag create a very special cognition, which tricks psychologically our auditive perception? (a common sign of market religion) How long will such investment work, yes 70 years ago such tubes could fulfill the WE specifications of 10000 hours minimum service, but what happened in the meanwhile with such tubes? How do they have been stored for the major past time, nobody can tell you here some reliable truth. Recently I did make a lot of tube testing with NOS tubes from the 1940 to 1950ties production period. Top NOS products from GEC, STC, Mullard/Osram and others have been on test. From every 10 piece batch two tubes failed within shortest running time without recognizable reason. I have never listened to a very early produced WE300B tube, I can remember a shoot out between 1950ties and 1980ties production lines and as a third contender a Cetron type from 1970ties production. This shoot out happened a good decade or longer ago, the oldest tube showed up with the darkest, deepest and widest spread sound attitude (or was it just almost gone?).

Today, if you want to buy a new made tube (new for safety reasons, i.e. religion again, when talking about Chinese tubes, I remember my first bought chinese 300B tube ever pulled out of a box in 1992 failed with a first switching), you can get modern replacements made from China, Russia or from the former East production. This modern tubes do sound substancially different than the most of the vintage original products,  – to say it carefully – the most of them does sound infected with cough or hoarseness, without any elegance and tonal refinement, which is so typical for vintage production tubes. To say it straight, they are absolutely useless, if you you are hunting for a three Michelin star graded audio dinner. In deregulated markets the price tag does not reflect any quality aspect, today its more going to be visa vice. In price ranges between 200$ up to 1000$ for a pair of modern 300B bulbs, you will have today a wide range of choices to get the mechanical value of a ordinary electrical light bulb. It seems to be still a good terminated business, to keep the religion running by serving such tubes to people of the 300b-faith congregation. Captured that only 300B tubes can do the trick, people spend still astonishing amounts into such wearparts, driven by the hope of audio salvation. (typical phenomenia within altered capitalism as religion?).

My long term compaignon power amps, the legendary Leak TL 12.1,
one of the finest push-pull amplifiers in history

Comming back to the essentials of this story, the rise of the 300B tubes was on par with a fresh reconception of the Single Ended Amplifier in the 1980ties. This topology can show some exceptional attitudes and advantages, comparing their merrits with other tube amplifier design principles, like the classic push-pull-designs, known from uncountable variations in the longer past, which are based on D.T. N. Williamsons article in Wireless World 1938, the so called the Williamson Amplifier. All the derivates do use pentodes, tetrodes and their kinkless relatives in order to generate a powerfull output of 12 to 100 watts, with theoretically improved distortion figures. Some of these amps, in particular the very early examples do sound exceptional good. The most of the crowd sounds like what is today known as "tube amplifier sound". Dynamic, a bit cheeky, but mostly impressive. The most readers here know my long time relationship with a better example, the early 1949 Leak TL12.1. These push-pull amplifiers are rediculously overenginered in several aspects and they do sound exceptional transparent and natural, I could not find in 30 years of evaluation something from other manufacturers, which was even close beside. I got once a really early set of Williamson Amplifiers, built in 1950ties from a british radio engineer in Bristol, realized at the best possible level of parts and manufacture. Made with Partridge and Parmeko iron as four chassis design (PSU and amp chassis in double mono), these amps are very close to the Leaks, if not on par. Once I will do a own article just about this incredible realisation as piece of important audio history. Both amps use the KT66 from GEC as power output tube, a tube which was designed around the same time of the Wireless World article. The Kinkless Tetrodes KT66 from GEC are the summit of such pentodes or tetrodes in the 25 watt class. Almost every manufacturer of this time did make a own design, RCA began with the 6L6 as base for all of them in 1936, Mullard/Osram did the EL37 in the late 1940ties, another derivate was the US-designed 807 and 5881. In these days a lot of different applications got as well designed, so that the demand for such tubes was immense. RF-amplifiers, military or civil rf-,  radar-, pa- and tv-stage-amplifiers and finally der growing audio and music business created a huge market for such tubes. In the 1950ties almost every manufacturer in in GB had a luxury power amplifier with KT66 in their portfolio, since US-american makers mostly relieed to 6L6 tubes in these days.

Original bochure from Wireless World about the Williamson Amplifier from 1951

These original tubes are mostly exceptional good sounding tubes when compared with modern reissues, mostly made in Asia. So its is understandable that a new old stock quartet of GEC KT66's today will cost almost 1000$ or even more. Everybody who has compared these tubes with any modern variants, even if they look completely identical, is willing to pay the difference of almost 850$ to get the real addictive drug for his guitar or audio amp. If you own an amp which is able to show all aspects of such refinement in sound quality, like with a magnifying glass, than you know that these tubes can do a unparalleled job for you. This is a alternativeless lessons of truth with push pull amplifiers, but it is even more important with a single ended amplifier topology, where only one single power tube is the decisive key element for its finalized sound. Beside the used iron, which is quite important, but in its role for the realism are transformers highly overated, the power tube is a key component.  If you don't believe about, just have a look to the "legendary" output transformer of the even more legendary Western Electric 91A amplifier, the WE171A transformer, which is a small and lightweight piece of iron. In its days almost any signal was limited to a frequency range of 200 to 5000 hz, in cinemas intelligibility was the major demand, speech frequencies between 500 to 4000 hz was the aim of audio designers. This range is what the "legendary" Western Electric amp shows as its best attitude, such performance is even by todays standards a exceptional result (look at the WE aficionados running wide band horn systems with this sort of amps to match their middle frequency performance).
But today the most of us prefer some high end audio quality, let us point it out, a fast, undisdorted and lean low frequency response from 40 to 100 Hz, or even deeper with powerfull liquid distributed dynamic energy output. The frequency extremes are not the foremost attidude of the legendary vintage amps/i.e. its output  transformer/ i.e. the foremost domain of the 300b tube, it was never intended to be so. The wonderful middle frequency response of such amplification with original 300b tubes, was the point which made this combinations legendary once in the early 1970ties in Japan. They got a synonym for a different realized audio response, which is hardly found in much more technically elaborated products from the second half of the 20th century, not to talk about all the contemporary products, which are just feeding only a remnant  theoretical demand of audio, or let us say it in more straight words – religous belief.        

This article aims to those who have not yet come under wheels in their individual sensual independence and are still able to differnence out on a base of personal auditive ability. Those who want a more natural and less technical sound quality, those who do not want to make some drugdealers fat, those who do not want carry on the misleading message, – for those I have some good news. All others who want to consolidate and follow some tribal paths of gone history, religous correlations or faith congretations with different color codes, from black to grey to green, can stop now reading!

Rebuilt from Cary Audio Push-Pull 300B stereo wrack. The stereo set of NOS tubes from
Cetron, STC and Rogers does cost 50 $, together, not one tube!

In 1994 I did buy a already completely slaughtered 300B push pull amp from the than emerging US-company Cary, which was in these days to my knowledge, the only American manufacturer of single ended amps in the US-market of higher faster further audio. With typical sales arguments of extreme heavyness, glossy shine and solid stainless steel cases this company marketed a range of different 300B amps in the US and as well in Europe, as the one and only salvation for the new tube age in these days. Single Ended mono blocks with one 300B or as Parallel Single Ended versions with two or like this example here, as Push Pull Stereo model. I can remember that these amps were quite expensive, all between 5000 and 15000 $US, which was good money in 1992. So I was a bit astonished to see a eviscerated example for some 50$ at a final outsale of a closing local hifi-shop. This amp was completely emptied, without any wiring or components, but with all original hardware. The shop owner told me that several people tried to convert this amp to a singing bird, but all had given up after a while. The former owners loss was going to be my win. (market religion again) I always had the inherent problems of this amp in my mind and so my idea to bring this thing back to another life, was hold back for years. Alone the investment for four Chinese 300B tubes always suffocated my energy in a glance to start some restauration. It got forgotten in my cellar for more than 20 years til I decided in 2015, maybe to do something with this wrack. First I removed the original output transformers and sold these on Ebay for 400$ back into the US. I stripped it down to the clean chassis and found under the massive casted Cary decal at its front the inscription "6L6-push pull power amplifier". So I realized Cary did manufacture with identical hardware a classic 6L6- and a 300b-push-pull amplifier without change. Now I did understand the amount of tube sockets, the layout was dedicated to a classic design with a half 6SN7 as driver stage, another 6SN7 as phase splitter and two power tubes, all served by a doubled rectifier stage with two 5U4G. With a value of  200uF for the charging capacitor (a lot helps a lot-religion) this amplifier gave me a deep insight into a practice of feeding a new and adolescent market.

In 2016 some friends and I made a final shootout with several 300B tubes (will be reported in another special article once), which showed me again that 300B tubes in audio amplifiers are something for people who don't take care about their money. Maybe the same target group which do barbecue only with the best available hand fed Kobe beef. So nothing for me, I know that the Chianina Beef from Italy is as well a good choice and available for a fraction of the pricetag from Kobe beef. I always trusted in my life into long time established cultural path lines, so I know that in Japan beef got fashionable late in the 1960ties. Before that time beef was almost unknown, because of the lack of pasture, so cows did exist in hand counted amounts mostly in the region around Kobe. (legend building as religion) In the common Japanese kitchen beef didn't play any cultural decisive role. In Tuskany the Chianina Cow has been cultivated for several hundred years with very similar excellent life circumstances for the cows. The right living conditions of the animal and the right treating of the beef is manifested as a several hundred year old tradition which can be read back in the legendary Coquinaria from Martino' Rossi printed in 1465, the first printed cooking book in history. This book was as well a present to the Savoyens, which transcripted that knowledge base for the french cooking revolution 250 years later. I rate Japanese cooking to one of the three best traditions in the world, but to be honest, I never was a friend of the Teriyaki style beef cooking (i.e. cutting beef into bite-sized pieces for the chop stick tradition). The Japenese are better with other things than beef. Have a look at the tv documentation showed once on Arte about the Dashi tradition, unbelievable, than you will know what I mean!

Back to our topic, the single ended amplifier, based on its most famous incarnation, the Western Electric 91A model, is still today a wonderful application as tube amp, but there is no real need to perfom the expensive 300B tubes. A minimalist two step amplifier can show up with a lot of advantages against any other tube amplifier design principle. With other nos tubes such derivate of amp can show all typical attidues on a identical level of audio performance as equipped with 300B's. The most of this attitudes are already common knowledge and have been part of their marketing throughout the last thirty years. Its simplicity is is major win towards refinement, but it doesn't need all the religous writebacks which have been cultivated within this time. Such common fairytales are: 1. direct heated triodes sound better than their indirect heated counterparts, 2. about the exclusive audible excellence of triodes, 3. the second-class rating of multi grid tubes in general as disharmonic element, 4. the over valuation of output transformers for the performance of amplifiers. And finally the witchery as such, 5. the global feedback loop known from conventional designs.

When Maison de L'Audiophile released in 1984 their early interpretation of the original WE 91A amp, their MdL'A300b Legend amplifier with Partridge transformers, showed this reincarnation a very good will to do the best to release a contemporary high graded product of this breed. But it came out with some real faults. Some of these faults relay to time dependend interpretations, like the oversizing of capacitors in the PSU, but some were major faults in the transcription of the original schematic, like a 10 times higher valued coupling capacitor between the two tube stages. Further the far to high valued grid resistor for the 300b tube might be handled by original WE tubes, but several modern chinese copies will be overwhelmed in a glance. About this faults and about the original template Fritz Hunold (frihu.com) has made several excellent articles. If you aftermath such details and if you ever try to built such an amplifier yourself, you will not believe how important such little changes are, they shift the overall perfomance of such an amplifier together with a precisely adjusted feedback loop from mediocre bastard to world class component. Yes, global feedback! (inquisition!)

Rebuilt with James Audio 2.5/3.5/5K output transformer, Chicago Transformer Co. Choke, three General Electric 8uF/600V smoothing paper caps and two stage nos tubes,

Coming back to my Cary wrack, it showed up with a acceptable power transformer with values for the plate voltage of 350-0-350 volts and with respectable capabilty for rectifier and tube filaments and a stainless steel chassis with 8 holes for tube sockets. A good base for the transcription of the WE91A topology into a stereo (uuhh, another sin – stereo, pah) amplifier. I did reject two giant electrolytic filter caps (600uF) which gave me the space for the James output transformers. The former OPT's gave enough space for a solid nos potted smoothing choke (8H/150mA) from Chicago Transformer Co. on the right and a set of three oil caps from GE of 8uF/600V capacity on the left side. Keep the impedances of the PSU as low as possible!
First important decision for such a simple two step amp is a reliable penthode-input-/driver stage. Where once WE did set the WE310A as a working horse, thousands of copyiest had nowdays better knowledge and prefered here a better sounding triode (religion again. wow, what a mistake!). The penthode is a must to get the full possible swing to the grid of the output tube with only one tube, the 300B is very selective character at this point, but output tetrodes can be similiar. Coupling in tube amplifiers is one of the most important things, I was always convinced about the perfect coupling abilities of transformers for the same reason. So it makes sense to get rid of the phase shift tube stage in pp-amps, but here we don't have such and pentodes make transformer coupling impossible. Here a small coupling capacitor of 0.05 or 0.068 uF (original WE design is 0.047uF) is mandatory and helps the pentode to charge this capacitor easier. (I started  with a typical value of 0.22uF,  which is four times higher and the difference is the same like the sliding a boat or a hovercraft. In order to couple both tubes perfectly and to keep the amp dynamic and liquid in its physical sound structure this detail is decisive) Almost any of the competitors or the successors of the WE310A is on par with this rare and expensive tube. A natural partnering substitude from that time will be the British EF37A/36, VR56 or EF6 such show all a metallized glass envelopes, wich is important to get this tubes hum free. The direct competitors of the WE310A are the 6J7G or C6C, but these can only be run humfree in solid metall shields, or otherwise later made smaller metal tubes like RCA 5693, the complete European noval row EF40 to 89 and similar types, like the loktal C3g are possible. The typical penthode as driver is a must, period. A whole range of power output tubes can be set up without electrical change: the kinkless tetrodes from GEC in the first row, the KT66/88, their high frequency derivates TT22 and KT8c, the Mullard/Phillips competitor EL37 with its HF-derivate EL38, the Telefunken F2A11, American 807, 6BH6 and 6L6 tubes, STC 5B254M/A, Russian 5881s and several more tubes will work perfectly auto biased on point with a 3.5 k-ohm output transformer. Exactlthe same point wich is typical for 300B tubes with a different filament and a different socket. All of these named tubes here are soundwise in such topology extremely delicate sounding sources with slightly different attitudes. When connected as triodes they all do sound certain classes superior to any chinese production 300B's, – period. Each of them has a clear sound destinction, so is my long time favourite, the EL37/38 are more at the transparent side with clear destinction to better detail resolution of the higher frequencies and on opposite side are the typical 807's with a more darker tonality spectrum and attention to the bass weight. Decisions should be made to personal taste and with the speaker attidudes in mind. But prices for such nos tubes are still low, so low that it is very easy to test them out yourself with oktal adapters, which are available trough Ebay from a Taiwan seller. The most of the gang are easy available for 10$ for a nos piece, some up to 50$, only the space deers from GEC, the KT66/88's and the German F2A11s are quite expensive for different reasons (demand, superiority and rareness). It just needs a single ended gapped output transformer from solid audio production made with some reserve, let's say a silicon oriented steel lamination wound for 3.5 K, gapped for 70mA to 4/8/16 ohms secondary, with 100mA capacity designed for a output of maximum of 12 to 15 watts. Such are available from different brands like German Reinhöfer for 100 EUR/piece or as top product with c-cores from Lundahl for 250 EUR/piece. There is no need for exorbitant expensive Tango or Tamura iron.

Selection of nos driverstage tubes from Mullard and Rogers. Nos power tubes from Mullard, STC, GEC, Vostok,
with oktal adapters are they all pin compatible, as well have matching working point. (Does somebody remember the tube box?
Yes, the British license STC 4300B was packed identical.  ;-)

If you want to get further into audible perfection, you might profit completely from a diy project against a off the rack production from established brands,  than you should match your finalized amplifier with a so called Boucherot Element at the secondary of the output transformer to your paticular speaker/crossover (again frihu.com). You will not only get a safety element for the output transformer, when a speaker cable fails on power, you will get a superior matched combination. Such a matching is able to shift the sound abilities into a certain level of performance, since the impedance peaks of your speaker get flattenend. You will get rewarded with a extraodinary dynamic, open and naturally refined soundstage, without equal. Such an amplifier will be able to transcript the recorded air between instrumentation in a true natural scale, just like to perform a wide and deep open scaled dynamic soundstage, an aspect I always prefer beside tonality as my major criteria for aural excellence. Such an amplifier acts so sophistcated, that I couldn't imagine the result before I started this project, otherwise I argue with myself now why I would have done it some 20 years earlier, but I would't been able to do so back than. Such impedance matching makes as well complex loads possible, like a typical vintage 15 inch coned voice coil in the 12 ohm dc – 3mH section. You will be rewarded with a well matched partner with 6 watts output power, able to drive your vintage immobilias from Altec, Tannoy, EV or Klipsch.
By far the most of commercially available single ended amplifiers cannot outperform the classic push pull designs in this discipline of speaker control. Such always have been named to be short of power reserve to do that job well, which have given them a general poor reputation. The floods of copy and paste chinaware within the last one and half decades for peanut prices and glossy cases, have ruined the credibility of the single ended amplifier topology to a final stage. But it is more than worth to give it another try.

For some it might look chaotic, others know direct solldering to the tube socket is the best sounding variant, highly improved to any series production board or printed circiuts. Here the decisive coupling capacitor from nos Bosch production, some carbon composition resistor and Rosenthal non inductive ceramic wire wounds.

In Sound Practice Issue 14 Herb Reichert stated in an article about a completely transformer/choke coupled push-pull amplifier, the so called "Feral Eye": "This amp, ... has what I call a lubricated clipping characteristic. It has a sort of James Bond like poise ... The Feral Eye plays music very differently than most of us are likely to have heard before. It sounds more full, whole and complete than any traditional PP amp I have experienced. " Since I have finished my single ended amp I can state a similar experience here, a lubricated fluidibility, which I have never experienced before in such a scale. I am absolutely convinced about the fact that the reason for such attidude, just can be found in the improved coupling of only two tubes. Such a majestic attidude is hardly found in other amplifiers, this rare effect, it is prominently credited to transformer coupling. I can state that this particular bahavior for me justified within all my built preamplfiers, the employment of output transformers and plate chokes as to be higly superior for that reason to any other solution. It is a interesting fact, that now here the same advantages popped up with a precisely optimized conventional topology. 

Low impedance filter section with huge paper capacitors and and smoothing choke – a must. Solid wire as well.

But I have to regret, when I started to finish my Cary wrack after years of convinced ignorance (again religion), I didn't expect anything special.  I had no need to change my always winning team for something different. Recently my mood grow to realize it, it was laying around for so long: "You have bought some output transformers some 15 years ago, now just do it. If it fails your claims, it will be sold in the bay next year.
I never anticipated such an outstanding amplifier, which has already set my long time favourites, the Leak TL12.1 into retirement, and this happened with every respect and so well deserved.
If we weren't already a dying group of white old men with strong established attitudes and a lot too much possessions, I would go as far and try to realize in a small edition a series of amps, based on this experience with Taiwanese copied WE91A chassis/housings/cases, with tetrode tubes and Lundahl iron inside. An amplifier for those who need to find the final thing for a final chillout on an island, – but this idea looks like another Christmas fairytale in the wrong religion group.

For everybody a well shaked coming year 2018 with lots of wishes becoming true


P.S.: "Tamensi movetur!", Galileo Galilei, Pisa, 22.6.1633

The Aa Line Preamplifier IV – The Power Supply Part I

Hello,

to everybody. Today I will continue the series about the Aa-DHT-Preamplifier, I assume some people are waiting for the completion of the series. In late summer I did use the good weather to concentrate on other topics than audio, I did start a new blog with a totally different topic, so the work at the preamplifier did stick a bit. Everybody might understand without real pressure the continuing is low since I do use a almost the same preamp for a longer time already. But we will see if this "final" version is able to preserve new experiences.

The same Hamond enclosure like the line amplifier, painted with the same hammer tone silver painting. Lots of wholes  and cut outs were necessary for the quite complex power supply .

I did use the same steel case from Hamond like with the line stage for this power supply. If it would be just a PSU to feed for the line stage stand alone, the expenditure would be very comprehensible. A classic anode supply with rectifier tube and a charging and filtering chain of capacitors made up with the choke would do the job already perfect, since the design of the line preamp is completely class a. But the filaments of the directly heated triodes need to get a well made supply each, since their sensitivity for hum and other unwanted restrictions is a lot higher, than with any indirect heated tube. As already shown in the preamp descriptions I do use for that reason Rod Colemans constant current supply boards. These need each a own raw supply made up from separate secondary windings of a transformer, able to supply quite solid currents, here with two Aa in parallel we talk about 1 ampere each side. Conservativly spoken the transformer should be able to deliver a good 1.5 amperes for safety, noise and long term operation. To work out the final 3.8 volts at the cathodes of the Aa tubes a 12 volt transformer will be a good choice. It has to be determined exactly, otherwise higher voltages need to be burnt down and will create unwanted heat in the enclosure. In the first stage a typical rectification stage is necessary. I will use four 1N5822 fast recovery diodes for that job and low esr capacitors for the filter chain and  dropping resistors per channel to feed the boards.

But with the additional planned phono amplifier I need a more complex power supply. Since this second preamplifier will be realized with indirectly heated tubes, a higher degree of complexity for the anode supply is necessary, but in reverse with such tubes the filament supply will be quite easy to be realised. So my anode supply will divide behind the first filtering cap for a second rail with additional choke and a tube regulated output stage. Here a the integrated tubes of the ECL82/84/85 line (combined driver and power tube in one glass envelope) together with a 90C1 as stabilizing device are a smallest possible physical design for such a stage. This stage needs to supply less than 20 mA of regulated current output. The tube sockets and a extra capacitors are already installed for this later needed output stage.

In order to keep the grounding issues under control I did made a perspex board where all capacitors are mounted off the chassis and can be separatly grounded to avoid hum loops. I did expect to have a quite complex switching schematic, so I will install two separate switches for the different heater modes and anode voltages for both amplifier parts indicated with lamps.

MoD Parmeko mains transformer with typical sealed oil isolated enclosure and porcelain contacts.

Since the middle of the 1990's I did collect at ebay parts for tube amplifiers, which were almost impossible to find before without the web as source. The perfectly made Parmeko transformers are one example of these rare and obsolete parts for the use within tube  amplified devices. With c-core laminations and oil filled sealed enclosures, porcelain isolated contacts and very conservative ratings, these transformers belong to the best made ones in history. The most of them are made for the British MoD (Ministry of Defense) and were used at professional or military equipment and there is nothing comparable made known from any other country. Even with Ebay it is not very easy to find the right values and specifications for all projects in mind, so it took several years to collect this set which is now used for this power supply. A mains transformer with a good variety of anode voltages (300-270-250-0-250-270-300V), a perfect rectifier filament winding with 4-5-6.3V and separate 6.3V filament windings is a very good choice. The two chokes with 30H and 50H are a perfect match for both anode supplies. Only the filament transformer for the Aa's is a modern torodial design.
With this mains transformer I can use rectifier tubes with 4 volts filament, this means I can use AZ1 or AZ11/12 which are easy and quite cheap to obtain even todays where the most rectifier tubes have skyrocketed their prices. There are some early types which are equipped with mesh plates, a sound wise perfect value for such a project with low current drawing figures.

Read on soon about the next part, Volker

The Aa Line Preamplifier III – The Completion of the Amplifier Part

Hello to everybody,

today I want to show the completion of my amplifier chassis with the constant current filament regulator boards. As already stated I did use the predecessor of this preamp for several years with different battery types, since conventional dc regulation showed a lot of problems and did not sound as good as batteries. So I had to struggle instead with contact problems, broken cables and dying batteries for years, which was a real pain. I had started with lead batteries, later did I use conventional AA Nimh's, a pack of three makes 1.5 hours of listening possible. So did use several packs, the everyday use and charge cycles made life for them short, almost every year I had to exchange the most packs.
In 2010 I did learn about Rod Colemans Constant Current regulator boards, which have been designed for this particular dedication, the filament supply of DHTs and for the use in pre amplification stages as special option. These boards can be configured for almost any triode type, even the big ones like 211, GM70 or 845 with high current draw figures can be used. For each triode one board is needed separately, which means here in this case with two Aa's running in parallel mode as couple will be fed by one regulator. They don't run very hot, so they are quite easy to use in the amplifier enclosures.

The Coleman regulator boards are mounted to both chassis sides to dissipate the heat and to be very close to the filaments of both Aa couples, here shown with one board.

The regulators are dead quiet and don't show any deterioration of the sound structure, comparing them to batteries. They will be available though Rod Colemans web page and are a great help for everybody using directly heated triodes in audio amplifying devices. They need to be implanted with a raw power supply each, which should be designed to match the final preferences set by the tubes itself. Any extra power will be burnt down into heat by the regulators itself. As example for the Aa with 3.8 volts for their filament a transformer with separate windings for each raw supply with minimum 11 volts output is needed. Each Aa draws 0.5 ampere of current, together it is 1.0. The boards allow the exact setting of the current to match the variations of such old vintage tubes.

Since the Daven attenuators  are extremely difficult to find nowdays, which means it is almost impossible to find them as stacked stereo units in values like 100K or 250K, I did use one mono type for each channel. With everyday use of such a preamp, separated attenuators for each chanel would drive me crazy. With any control of volume you need to match the stereo balance again, a night mare if the amp is not positioned exactly in the middle of the speakers. In order to make the use more comfortable, I introduced a coupling system. With a o-ring both shafts are coupled together by wheels from a toy steam engine. The o-ring keeps both attenuators in balanced mode since I need to adjust one of them. If, I can hold one since I turn the other one where need. We will see if that system will do comfortable within everyday use, otherwise I have to design a spring supported clutch system…

O-ring coupled shafts for the Daven attenuators.

The back shows several sockets for different dedication. From left there is the power input socket (6 pole) where both raw voltages for the Coleman regulator boards will be supplied. The other two contacts are for the filtered anode supply. Next to this socket are two output xlr-sockets, the two main output terminals of the amplifier, these can be switches for the two corresponding rca sockets as alternative outputs at the other end of the back panel. All paired rca sockets in-between are four different input contacts, which are connected to the source switch, operated from the front panel. Four inputs should be enough nowadays, since the most sources are fed through one dac, like web radio, streaming clients, computers and similar. So there are three inputs for other sources like phono equalizer or a recording unit and here the second set of output sockets will be helpful.

After mounting the two potted chokes on top of the amplifier case and after connecting the tube sockets with four Aa tubes, the amplifier chassis is complete and is waiting for its complex dedicated power supply. When the power supply is completed and will work with the dedicated smooth anode supply of 200 volts and the two raw supplies for the regulator boards, I need to adjust these with a dummy load instead of the real tubes to prevent an overload of their filaments. But till there is quite a long way… As almost everybody knows, a good power supply is by far more complicated than the amplifier part. I will show it with an option of two different versions, a quite simple tube rectified supply with the three different voltages to keep this preamp a simple stand alone solution.

For the aficionados the series of articles will be continued showing a quite unusual pre-amplification concept, which will have the future in mind. Similar to german broadcast preamplifiers of the 1950ties I will show the build of different stages with different amplification factors. A gain amp (+50 db), a catch-up-amp (+25db) for compensation of losses and the already shown line amp for impedance matching to the power amp.
With these modules and a dedicated power supply almost any use in a modern audio set can be managed. For example a typical dac output stage will need roughly a amplification of +50 db, almost the same than a good phono stage before RIAA equalizing. When equipped with a input transformer with different windings and different ratio, like 1:10 or 1:25 (just to mention the extremes), the same amplification stage can be used for both sorts of applications. Since the former digital signal is now strong enough for the impedance amp (line), the phono signal needs further equalization with a RIAA filter with typical loss of -25db at a following stage. So another amplifier with +25db gain is here needed for compensation of the losses to level up for the line stage.

A multi voltage power supply will be needed for such a amplification chain. Such complexity needs to match highest standards and components, potted oil filled transformers and chokes would be first choice, but it is already difficult to find this stuff nowadays. The anode supply develops with tube rectification at the beginning, ideally made with a classic mesh anode rectifier tube lie RGN1054 or 1064 as perfect partner of the Aa tubes, to be continued with several chokes for HV and LV: 1. a well filtered B+ for the DHTs in the line stage; 2. two well filtered low dc-voltages for the Coleman Regulators; 3. a further filtered and tube regulated anode supply for the gain amps, and finally 4. a well filtered dc-supply for their filaments of all indirect heated triodes. I assume the weight, complexity and size of a well made power amp to get an idea about the case.


Read on soon,
Volker

The Aa Line Preamplifier II – The further Completion of the Amplifier

Hello to everybody

interested into the completion of my Aa-DHT-Preamplifier. As already stated, this line amplifier is a perfect solution for the mainly digital based systems, as it can be a exceptional supplement for any analogue chain with separate phono equalization. Implanted into a system with advanced dynamic attributes and with careful set up to harmonic structures, this preamplifier can open a window into the finesse of advanced audio performance, which I never have been able to obtain from commercial made line preamplifiers. A typical variety of incomparable attributes accompanied with directly heated triodes in small signal amplification stages are the base for such a extreme statement. DHT's for small signal output are very linear in their whole operation spectrum. They have a extraordinary refined character, sometimes show up with unique tonalities and exceptional dynamic abilities. Exactly like their more powerful sisters in power stages, but with a much deeper scaled effect because of the smaller signal ratio. The unbelievable attitudes of the Aa-Triode give a extra portion of refinement, linearity and tremendous wide open transparency, combined with harmonic detail I do not know second. Commercially made tube preamps could and can only be designed around available types of tubes, it was almost impossible to find bigger amounts of exceptional tubes since the renaissance of the tube amplifier in the 1980ties for hifi, nor it is possible today. The exceptional icons of indirect heated tubes for small signal amplification, independent of mu characteristics and impedance values, are mostly tubes made for professional dedication, like the WE 417 and 437, the red RCA's like 5691 and 5692, all the Telefunken special series like EF804s, EF801s and the red tipped types of all ECC types, the D3a's, C3-series, from other makes like Bendix, some former Soviet brands and uncountable lots of rare others tubes not named. All these exceptional quality tubes are not to be found in commercial audio amplifiers, where they could show their incredible qualities. But even these exceptional tubes cannot tame up with the same audible benefits of direct heated tubes, they are able to perform tremendous other benefits but rarely with such finesse of dht's.
The tungsten thoriated filament of directly heated tubes is acting as cathode in the signal part and creates very special peculiarities. On one hand responsible for such incredible aural benefits, but as well well for very delicate conditions in everyday use. Everybody common with DHT's in the power stage might be experienced with the typical sensitivities of such tubes for hum and vibration. In reverse to the amount of signal to ground ratio current, these sensitivities are ten times or more higher. This can make the use of such tubes very uncomfortable or even more, – it can be a real pain. I am using such tubes since almost ten years by now in such implementation for every day, within that time it has come sometimes to a point, where I just switched off my stereo, because I was unable to locate some reason for apparent hiss or hum. Completely exasperated after some time of search I decided to find reasons for the problems another day… In particular in the first years, when I did use flying batteries for the heating of the filaments, contact problems with broken cables, clips, battery containers, – flying batteries create here a permanent conflict zone. With a fixed and permanent installed power supply solution (constant current source) the most of such problems are now history. For those which are already on a shortcut, a  common or even complex regulated voltage supply is unable to support the aural qualities in the signal loop. It needs a complex constant current source with own complex (highly filtered with common mode choke) power supply per tube!It has to be said with impertinence, a direct heated triode is not a plug and go installation. If you want such a ease of use, you better should go for something else than tubes and in particular for directly heated triodes in general. Those respecting the extraordinary outlay will get gifted with the most colorful natural soundstage possible.

For those undaunted here a new life with audio might start soon. The Aa-Line Preamplifier its a fantastic solution for the digital chain. In combination with a good dac all the quite new benefits of digital stored music (i.e. better recorded standards, storage and organization improvements, a wide spread program of sources: world wide radio stations with lossless compression, cloud based streaming, etc), will find a exceptional natural sounding input stage, able to perform with a perfect holographic soundstage, if the following components (i.e. power amplifier and loud speaker) are able to support such a level of resolution. All regular readers of my blog have got already certain ideas how to get close to such a rare and uncommon performance. For me the exceptional resolution and the incomparable micro detailed dynamics are a base to operate a huge cone speaker like the Tannoy with wide medium frequency performance to a higher degree of resolution other designs can not support.

Today I will try to show some more completing of the amplifying enclosure. Now the rubber isolated perspex board has been mounted to the chassis, a distance of 20 to 30 mm between board and case is a quite good value since the Aa-tubes have a solid dimension, more known from typical power tubes of the 1930ties like coke bottled 6L6 types or similar. So the tube will be hidden with the socket inside, wile the balloon shaped glass will show out. The tube sockets are mounted at the perspex board with  some turrets to find place for the very few components and their connecting wires, which are necessary for the design of the line amp. This version of the DHT-lineamp is the "short wire version" of this topology. Since it incorporates plate chokes instead of output transformers (the long wire), the physical signal length from input to output sockets is less than 40 cm wire, with just a tube system (plate-grid-cathode/bias resistor, tubes in parallel mode), one oil-paper cap in-between. It is almost impossible to do any shorter design and this explains the tremendous influence to the final performance of the Aa-tube.

Solid silver wire with cotton hoses customized made near to the unwired socket board

Used components

To bring these 40cm to the perfect resolution I did decide to use solid 4-p silver wire (99.99%) for the signal path. I made up the wires myself from solid silver wire (0.5mm or 24 ga) and did isolate them with natural cotton hoses. The negative loop/earth path is made with thicker blank silver wires (1 mm or 18 ga) per stereo side, which get at one point together. The few resistors are carbon comp types of minimum 2 watts heat dissipation and 3 watts metal oxide types for the cathodes. The attenuators are classic potentiometers from Daven with switched 21-step carbon comp resitors for both sides. All capacitors (4 pc.) are oil-paper types, the chokes are Lundahl LL1667/10mA both coils in series (400 H  for 15 K at 30 hz), potted with wax into aluminum enclosures. The chassis for the preamp and the power supply are from the steel 14xx-series from Hammond. The used tubes are nos Valvo Aa's from war production in 1944.

Amplifying chassis from underneath: at top all input and output sockets; left and right Daven attenuators with input source switch in the middle; next level left and right coupling oil caps and b+-support capacitors; the board with input voltage devider, cathtode resistors and all silver wires; knobs with extension shafts for the front panel.

The used parts and components reflect my final selection for this amplifier type, I will definitely use the built sample for the next coming years. I did decide about the rare and expensive parts, till it might be my final line preamplifier and this is going to be a very important component in my audio chain as a result of increased digital format use. It can be made with similar parts and components (chokes, attenuators, resistors, switches, etc.), which are more easy available at a level of 95% of the same performance at the half of the costs. It might depend to the interest out there, may be I decide to make a kit with this design...
Within my next entry the amplifying part of the design will be finished with the inclusion of the regulator boards for the filaments. After this I will start to show the more complex design of the power supply for both pre amplifying stages, the line and the phono stage. Both stages need very different supplies, till the Aa tubes run in pure class a, so they need a quite simplified anode supply together with exceptional complex heater supplies per tube. A class b operated phono equalizer will benefit enormous from a complex filtering chain with chokes and a integrated tube regulation for the anode supply, combined with a simple dc heater design. If you want to follow its realization 

read on soon, Volker

The Aa Line Preamplifier I – The Mechanical Construction of the Enclosure

Hello to everybody reading my blog,

now after the end of the soccer world championship it might be time to start a new series of entries about the building of a DHT preamplifier as central component of a future orientated system. Several times I have quoted my extremely positive experience from my Aa Line preamplifier, which I do use since 2004. This was my first realized preamplifier incorporating directly heated triode tubes in the small signal section of my chain, namely the line stage of my set up. When I realized this concept almost ten years ago, I did not expect to be so much convinced one day that such a preamp will go to be my final word in the line amplification stage.
At the beginning it formed a extremely well sounding approach towards refinement, but in 2004 my chain was not in that developed stage, where it is today. The first remarkable side effect came when I started to rediscover my digital signal sources again. For more then a decade my Meridian CD player was into storage. With my Platine Verdier Record Player and my time living in the US, created a concentration into vintage vinyl as foremost signal source. But around 2005 the digital storage of music attracted my attention. I did start to record my vinyl for digital storage at harddisks. The combination of "lossless" storage and Apples hard- and software (Airport Express w. digital out, lossless format to be supported), formes a easy access to digital systems within my analogue tube amplified audio set up. Together with my new Aa-line amp it created a extremely refined sounding system, with lots of merits I didn't know from my analogue arm. In these days I did use my audio mainly to listen to my analogue jazz record collection, from time to time I made some recordings from vinyl just for storage issues. I didn't take the digital side to serious, I was a lot more shaped by my early experiences from the 1990ties, when early digital recordings and cheaply made transcriptions from vinyl to CD created my worse experience with digital and formed the decision to leave it for the "consumer".

When I had to move my studio in 2010 I had to adapt new listening conditions. At the beginning I did start just to set up a digital streaming chain as first set, in order to have some music by hand. It came as is always happens, new room acoustics did create new courtesies, I started to like the digital arm of my set up quite well. Together with a new interest into actual and therefore newer recorded music, I realized a unknown conservated audible quality. It was very obvious that in the recording part of audio in the last decade a huge quality step was realized. Completely digitally recorded, mixed and finalized takes showed up with a remarkable close and accurate preserved soundstage, in a way of perfection I rarely didn't know from vintage vinyl. All the typical aural fingerprints, made by every analogue component in the recording chain, the microphone, its input transformer, the preamp, the mixer, the tape recorder, the amplifier and serveral other chained components seem now not any longer exist in a conspicuous way. Instead a much more homogenous and digitally corrected sound image seem to be the new standard of actual audio preservation today. A better focused staging with unknown lots of captured air is the typical side effect of modern well made recordings. All the former unavoidable phase shift effects seem to be gone with digital shaping, instead a wide, dynamic and refined window to lively recordings seem to be now some sort of standard. With the exceptional resolution of my line preamplifier, these qualities were the foremost sensual facts to be experienced, which I rarely did only know from a handful of my best analogue recordings. The DHT tubes combine a extreme musical coherence with absolute linear resolution in a way, indirect heated tubes don't show. The Aa Weitverkehrsröhren are exceptional top performers within the common known DHT's for small signal amplification. I did try other tubes as well like the widely used 26 tube ore the TM2 with similar success, as well with transformer coupling, but I will stay with the German Post Tube, since I like its extremely open, transparent and dynamic sound better than any other. They are made once for 10000 hour service in professional installed telephone relay systems and have some other mechanical advantages the most other DHT's don't show up with, because these typical US-american tubes once were made as consumer products for radio receivers in the 1930ties. The availability of German Post tubes is far better in my culture than any US-DHT and there is the variety of three different brands with different properties to find. The rare and expensive original early (from 1927 on) tube from Siemens (with tip and glass tree inside), the common Valvo type (globe type with mica spacers) and the last version made in the former East Germany by Neuhaus Röhrenwerke (with brass socket and coke bottled glass shape and ceramic spacers).

So I started after almost 20 years to buy again some CD's, giving me a new unknown listening pleasure on the first hand, but as well as streamed data beam digital sets a comfort and usability I never could establish with vintage vinyl recordings. Vintage vinyl is still unique in different fields, it wonderful large size covers, the printed informations and a typical liquidity in the timing of their sound, digital formats do not cary. Today I use both systems side by side with mating success, but the operability of streamed data sets the pace for the years to come. So the quality of the line stage is the key for that kind of use and needs to be of exceptional ability.

First  evaluation stage of my Aa-Line preamp in 2004, equipped with battery clips for the heaters and here showed with rare Siemens Aa tubes, identified by the tip.

Solid mechanical construction with complete relying at oil caps and solid silver wire for the signal path. On the left: the variacs for the lead batteries and a stepped attenuator for volume control.

But I will not conceal that there are some problems with the use of thoriated tungsten filament direct heated triodes, other tubes don't show. With directly heated triodes the cathode acts as heating element for the tube, which means the heater is part of the signal path. The most people know this problem from power tubes, where it creates sometimes hum problems, which can be solved quite easily, because of the higher signal to ground ratio. But at a lower signal levels and following further amplification, any little hum sign will be amplified up to solid levels. Here the problems needs to be solved completely. To get it work, it needs advanced solutions for the heater supply. In the beginning I did start with classic regulators (LM317), these bring a lot of radiated heat to the amp and don't sound very good. After burning my fingers at the chassis of my preamp I did decide to look for other solutions and ended up with a very uncomfortable solution, I did use lead batteries first. Now I had to charge every day my batteries in order to have enough energy for some hours of listening pleasure. I did try several different battery types, since the cheap Chinese lead batteries didn't last very long, after 100 loading cycles they lost  energy to be stored. So I switched to nimh-batteries. I got a quite good sound but only for two hours time with one set of batteries. So I had to buy several sets, which needed to be exchanged after a good year of use by charging them every day. I liked the sound qualities a lot, but hated the charging practice and the all around contact problems in practice. Always some hiss or hum happened needed some helping hand to fix the contact problems for further listening. Not comfortable.

In 2010 I switched to Rod Colemans CCS boards, which make life a lot easier. These constant current supplies are sounding as good as battery power and do not burn tremendous amounts of heat like other regulators do, when well implanted. These can be used with almost any DHT triode, independent of current amount with good success. So they are as well a good solution to be used with power amps.
Now after almost 10 years of use with this preamplifier and after final evaluation comparing it to other options with my new speakers, but as well in other chains, I decided to bing this concept into the next level and final stage. This will be accompanied with several blog entries.

It needs a well made mechanical design to find the long term usability. 

Now, in the final stage the line amp is going to be the switching center for all types of sources and needs several in- and output options. Only the best components are prefered for the job to be a final preamplifier, here with Lundahl iron, NOS Daven stepped attenuators and a Elma switch. Classic WE components are a good guide how to make a well organized amplifier.

For my new approach the Line amp becomes a central position in my chain. Beside the analogue system arm with my vinyl record player, the dac will feed the line stage with digitally streamed music using the comfort offered by wlan and with the iPad as a remote system, since all data are stored at the hard disk of my computer. For the future this is a important and foremost feature to be extended as main source? As well a recording option has to be kept be in mind, which will create other problems to be solved with tube amps.
I did try to listen again to some conventional line amps in the last couple of month. After this exceptional evaluation with very different amplifier concepts of advanced stage I was restless convinced about this preamplifier and therefore I did decide to bring it into a final state. For absolute perfection it needs a much lower output impedance to work flawless universal with every tube amp. With the high impedance of the Aa (30 K) it is almost impossible to match this preference, so some tube amps are not happy with the high load and connecting cables are getting a difficult choice. So, staying with the Aa tube and relying to its extreme unique sound qualities make a fundamental change of the implementation necessary. Cutting the internal resistance into half as a result of the doubled tubes into parallel mode, might be the way to get half of the output impedance. It should sound as good as before with much improved load into the most input stages of tube amplifiers.

Read on soon about the next steps, Volker