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 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. Back to the advantages of fixed bias according to my
understanding: 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, Norbert PS: I
have
to repeat myself: Great to have such a friend :-)
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 :-)
(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!)
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.
;-)
©The Audio Eagle
2005-2015 •
All Rights Reserved • Disclaimer