January 2015 - The Rise of the "Moondogs"

My love for SET amps, and directly-heated triodes (DHTs) in the output stage in particular, started years ago, when I decided to build my first amplifier, the Welborne Labs Moondog 2A3 monos in kit version. See my article from 2005 on this website for a description of how and why this happened. The Moondogs were quite sensitive/high gain SET amps as they used two indirectly-heated 6SN7 double triodes (just one half of each per mono bloc) in a cascade implementation as drivers. The design of the amps was nothing really special, but the parts quality was excellent and the looks of the amps/the finish of chassis and wooden-cabinet frame were superb. I was immediately in love with how the amps looked and also liked the name Ron Welborne chose for the mono-bloc amps. ;-) That Ron Welborne offered these beauties also as kit version for a really fair price was another bonus for me.

It's worth mentioning that Ron Welborne was at that time one of the few audio dealers in the US who promoted single-ended amplifier designs and was one of the "fathers" of bringing back this SET amplifier magic to the broad audio public in the US and also Europe. Only in Japan, SET amplifiers were then quite popular and widely used within Japanese audio circles. 

The original Moondogs had a capacitor input power supply with choke and tube rectification. Driver tubes and the 2A3 output tube were implemented in cathode-bias mode (i.e. with cathode bias resistor and by-pass cap) plus Jensen coupling caps between driver and output stage. The output transformers (OPTs) used were a pair of nice 3k primary Electraprint designs (made by famous audio veteran Jack Elliano) with 4, 8 and 16 Ohm secondary taps to choose from. I used them wired to the 8 Ohm tap.

That's how my finished Moondogs looked with Mullard GZ37, NOS Sylvania 6SN7s and
superb VAIC Meshplate 2A3 DHT tubes

At that time, I had the right speakers (Altec Lansing Model 15) from my beloved, late father at hand, which were fine for the Moondogs to let me learn about the DHTs sound flavor. And this sound flavor completely got me hooked. :-) DHT SE amps, have in my opinion, what I always had been looking for in reproducing music at home: They deliver the intimacy, tone and emotional impact of the recorded performance. Of course, I also liked the idea of being able to build my own amps - who wouldn't like that! ;-) Due to the accurate manual Mr. Welborne offered with his amplifier kits, also a novice in soldering and know-how of amplifier designs like me, could do it. Although it took me way longer than stated in the manual to finish my 2A3 monos, they worked fine from the first time switching them on! And, to me, they were a great match with the Altec Lansing Model 15 speakers :-). I thoroughly enjoyed listening to music with my Moondog 2A3 amps.

Then, as is often the case when you make the "right" decision for yourself, my personal journey and development of knowledge on my path to musical joy with my audio system went quite well. I met and became friends with DIY tube experts, such as Joe Aigner, who introduced me to his style of tube amp building - with a lot of iron, he is a friend of Thomas Mayer ;-) - which was a lot more refined and advanced due to his many years of experience of designing his own tube amps. I learned about the various flavors of how DHT tubes can sound in different implementations. His excellent-sounding 801A SE amp is a great example. I also heard for the first time the power of a 211 SE amp at Joe's place, which was quite enlightening for me; I heard that his designs were a lot quieter than my Moondog 2A3 monos, mainly due to his more advanced power supply designs and the use of a lot of iron.

Joe also introduced me to the European Triode Festival, where I met Thomas Mayer the first time at the Langenargen event in 2004. Thomas' amps and his wall-filling loudspeaker system at that ETF event are legendary and impressed me (and Joe) a lot. :-)

I later ordered - inspired by this wall of sound-experience in Langenargen - a pair of my beloved Haigner Gamma horns and with these impressive 99dB sensitive horn speakers - a true masterpiece of Austria's most innovative loudspeaker designer, David Haigner, by the way - the ground was laid for my prolonged journey to experiencing and enjoying more low-powered SET amps. :-)

In the meantime, Thomas Mayer decided to start building amps commercially and I fell in love with his designs, especially with his 801A SE Amp, which I finally bought and am still proud to own. This amp outclassed my 2A3 Moondogs as well as my clone 300B WE91 mono-bloc amps (the ones sold by DIY Hifi Supply, Hong Kong) sound-wise by quite a margin, but I still loved the tone of the 2A3 tube. It is such a seductive sounding DHT tube and my Moondogs were, as mono-bloc amps, so neat and light compared to Thomas' "big and heavy" but beautiful-looking and sounding 801A SE amp!

After my hernia surgery four years ago, which went well, my doctor advised me to avoid carrying heavy things in the future. This is bad news when you own big speakers, a heavy TT and Thomas Mayer amps. ;-)

But as you know from my article about Christoph's amps and how we two met, life can take you on different paths and journeys. And that led to the "Rise of my Moondogs". So, here we go...


Christoph doesn't build amps on a commercial basis; he is doing it just for his own pleasure. He is a person who wants to learn and try new things constantly as he loves to gain experience and to make his own
judgment about whether a design works or not. He rarely relies on the opinions of others, as his motto is "Try it yourself and see how it works for you!". I think that is a great quality, especially for someone who
already has a lot of experience in designing tube amplifiers on a DIY-basis and who has an amazing knowledge - through trying and trying and trying over the years - about the effects of parts quality in tube
amp designs. I admire his approach to tube-amp building a lot :-)

So, when Christoph explained to me where, in his opinion, the possibilities of improvement to my already fine sounding 2A3 mono bloc amps would lie (for example the high sensitivity of the amps due to the two driver gain stages) and that with the existing iron (OPT, Choke and power transformer) and modifications in the design of the amps to a more "old school" approach plus some changes in parts, which he believes make a difference in sound, really good 2A3 monos could be the result. I was immediately hooked. As an unexpected bonus, he offered to make these modifications for me, if time weren't too important. Wow, that was too good to be true! And it became reality. ;-) About a year ago, he finished his rework of the 2A3 "Moondog" amp design.

Christoph checking his 2A3 "Moondog" rework (left). His huge GM70 SE stereo amp can be seen in the middle, David and the other 2A3 mono amp in the background

Inside view of 2A3 "Moondog" rework and impressive GM70 in the back

It was now a 2-stage amp design and, therefore, parts count had been reduced. The input/driver tube of choice was the beautiful 6C8G, an indirectly-heated double triode Christoph loves for its tone (this is a predecessor of the well-known 6SL7 double triode). He used one half of the 6C8G and a grid choke (instead of the grid resistor). Modifications to the power supply led to a lower value in the input capacitor and the change from Mullard GZ37 to a Philips 5V4G tube rectifier as the 5V4G provides a lower voltage drop. The ps was further modified by replacing the original ps caps (Elna Cerafine plus some Solen caps) with one 600uf Fischer and Tausche electrolyt cap. In addition, I invested in a pair of 0,22uf Duelund Cast Copper capacitors for coupling of the driver to the output tube.

Soundwise, the amps were impressive on my Haigner Gamma horns. Extremely natural and colorful with more resolution and refinement than the original version. The midrange was truly superb and the highs silky. Voices were a dream with these amps. :-) Bass was on the warm (slower) side but well defined and given that each mono-bloc delivers only 3W output power, surprisingly tight and authoritative. As gain was reduced due to the driver stage modification, my new 2A3 monos worked best together with my beloved LFD Anniversary pre-amp. But the results were also impressive with Martin Küng's new transistor and tube buffer preamps - still in prototype version, a review will follow when they are ready for production. Superb resolution, tonal colors and rhythm. These buffer preamps will be a steal for everyone who is looking for a gainless device in front of his amplifier of choice. :-)

Finished 2A3 rework in Moondog cabinets with GZ32 rectifier, Ken Rad double plate 2A3 in the middle and 6C8G triode as driver installed (all tubes vintage NOS versions)


When Christoph left me his 6B4G monos for comparison with my 2A3 monos, I didn't know what to expect. The 6B4G is a 2A3 DHT with a 6,3V heater and due to not being as well known as its famous sister, still available in NOS qualities for a very good price. Christoph runs the 12SJ7 pentode as input/driver tube in the 6B4G amps with stabilized G2 voltage and the 6B4G in fixed bias mode (the negative grid voltage is now provided by batteries omitting the cathode resistor and bypass cap) with a specially selected copper foil coupling cap between driver and output stage. And although he substituted the rectifier tube in the ps to a solid state rectification with multiple RC filters - I can hear all the die-hard tube fans now scream ;-) -  the music reproduced with the 6B4G amps had more flow and speed than through my already wonderful 2A3 monos. This was especially apparent in the bass reproduction. I haven't heard a faster and more well defined bass performance from a low powered SET amp so far. Thomas Mayer's excellent sounding 801 DHT SE amp - although with more weight in the bass - was slower and delivered bass a bit too "boomy" in comparison.

Discussing these results with Christoph confirmed his preference to run driver tubes or, in his opinion, even better, output tubes with the fixed bias method. Apparently, leaving the cathode resistor and bias cap out in the output stage of an amplifier and providing the necessary negative DC grid voltage by using the means of batteries or another appropriate DC voltage source seems to have advantages.
(Side note: Fixed bias was, in earlier years of tube amp history, and especially in guitar tube amps the more common choice of biasing as this biasing method - due to the implementation of a bias pot for the output tube of choice - gave you the flexibility to bias the idle current of the output tubes used to the same values recommended for the type of tube. Cathode bias - also called "self bias" or "auto bias" - in my opinion, became the choice of the audio industry only because it is more "user friendly". No manual biasing possibility, but the usage of matched output tubes is important!)

Back to the advantages of fixed bias according to my understanding:
First of all, with fixed bias, you reduce the total resistance of your amp design, as you omit one cathode resistor. For the widely-used DHTs like 45, 2A3 or 300B, this is a resistor value in the range of 750 Ohms (2A3) to 1500 Ohms resistance (45; for a 300B tube somewhat around 900 Ohms) in your amp design
depending on the DHT used. Secondly, you get rid of one by-pass cap, which normally is an electrolytic type. Hence, two passive components less in your signal path by using fixed bias is not bad, is it?  So, again proof for Christoph that his principle "less is more" works just fine. :-) Great for me to be his test person of choice. ;-)

Due to the reduced parts count, part quality and part selection becomes more important than ever for the musical qualities of an amplifier. And in the art of choosing the best matching parts, audio design "Maître de Cuisine" Christoph has over the years developed fantastic skills. :-) This he proves with every new amp design!

But there is another factor Christoph highly values regarding developing a truly musical tube amp. It is the right pair of speakers, which are able to reproduce all of the dynamics as well as nuances and shadings of recorded music; this lets him hear if his tube-amp design implementation and  parts selection delivers the aimed for results. In David Haigner's ETF horn loudspeaker, he found the perfect mates for this job! And since then, Christoph's excitement in building tube amps and coming up with further enhanced tube amp designs has grown vastly.


As the results of the 6B4G were so promising, Christoph suggested that I live with these amps for a while and he would take my 2A3 "Moonies" and will try his "fixed-bias" magic with them, too. After some months and some experiments with selecting the best fitting driver tube for my 2A3 monos, he called one evening very excited to say that he had found the perfect driver tube in another DHT tube, the vintage 112A (a directly-heated triode from the 1920!). He really loved the musical qualities of this DHT pairing when he tested the amps in his system. According to him, these were not only extremely musical amps; these 2A3 amps now also had speed, dynamic qualities and bass power. In addition, although the gain of the amps had been reduced on paper to a 1/4 due to the switch from the cathode-biased 6C8G driver to the cathode-biased 112A, one didn't feel this change in the same magnitude as the gain loss on paper would indicate. Christoph found that an active preamp with 6 to 9db of gain was now the optimum partner for the reworked 2A3 amps. My friend apparently had turned my amps into fully DHT driven musical machines. This was too good to be true and I was longing to hear my new amps in my system!

Finally, in early December Christoph visited me for an audio session together with two of our Bavarian friends and audio experts, Wolfgang Braun and Gerald Gessner. We had a blast over that weekend as David, Alexander and Alex also joined listening at my place. We also had a great time listening to music at Alexander's and David's homes. Thank you, friends!

Part of Alexander's wonderful LP collection plus Gamma horn

Relaxed Gerald plus Alex at David's place

My reworked 2A3 Moondogs sound fabulous, by the way! :-) They are fast, clean, dynamic and tonally extremely colorful. Voices are magical! And the bass is incredible tuneful on my Gamma horns. I can now also rock the house with these 3 watt mono-bloc amps when they are driven by an active preamp like my LFD Anniversary preamp. Incredible! The reworked 2A3 amps look great too, in the beautiful Moondog chassis with a pair of globe UX112A DHT tubes as drivers. They show their musical class astonishingly also with Martin Küng's unity-gain Fet-buffer preamp. Ok, you can't rock the house then as there is not enough gain, but you can read the lips of your favorite singer, wow! My new amps in my system are the right partners for experiencing the wildest musical adventures! I have to stop raving about my reborn 2A3 amps... I need to listen and unlock the secrets of my music collection. :-)

Full DHT 2A3 amp in all its beauty :-)

... and here are both

Christoph did it again and DHT tubes definitely rule!

There is one question remaining: Would a dedicated active tube preamp - maybe one using a pair of directly-heated triodes - further enhance the pleasure of listening to music over my system? This is another story, so stay tuned...

Musically yours,


PS: I have to repeat myself: Great to have such a friend :-)


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