How To Make A 300B Amp

Hollowstate Devices - Duet
The making of a 300B amplifier!
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My name is Matthew Santa Maria (Theaterboy ID on eBay) and I have the
pleasure introducing the new Hollowstate Devices - Duet Amplifier. !
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Before I start the story on the Duet, I will describe how I first became aware of
this amazing amplifiers designer. During a visit to my local Hi-Fi store a
discussion started about testing tubes. After I mentioned I owned a TV-7 tube
tester, the guys at the store said they knew an expert technician - Daniel Nelson
- who was considered the guru of TV-7’s and he could calibrate my tester. They
had always steered me in the right direction before so a couple of days later I
dropped it off and said have at it. !
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As technicians capable of doing this kind of work are becoming fewer in number
than the old bluesman that we have all grown so fond of listening to on our audio
systems - I was not really sure what to expect from this service work. I was just
happy there was someone even willing to do the work at all!
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I received my tester back and was so pleased with the work I would have gladly
payed twice what I was charged. My tester had been throughly gone through
and things I never expected were done. Worn tube sockets replaced, gasket
seal replaced, and the missing testing accessories were included - along with
the calibration. It had a tag on it with the date of service and was signed Dan
Nelson. !
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A couple of years passed and one day I was testing a batch of old tubes and I
was getting a short indication on every EL-84 tube. I decide to call Dan and ask if
the TV7 had issues with this tube. He confirmed it was just a glitch in the tester
design and not to worry if the tubes showed shorts. !
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At this point in our conversation Dan mentioned he had built a 300B amp he
would like me to hear. Over the years I have worked on the retail and
manufacturing side of high end audio. As Dan and I spoke more about his amp, I
was recalling the dozens of times someone had told be about a home brew audio
product they had built. Some of them did sound good, but they all ended up
being something I wouldn’t be happy owning let alone recommending to anyone
else. Then I recalled how impressed I was with Dan’s service work on my TV7
and said “Let’s Do It”.!
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I gathered my best single ended amplifiers - AudioNote Ongaku, Cary 805
mono’s & CAD300SEI - and readied myself for Dan’s visit... !
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Nothing could have prepared me for the experience I was about to have. I am
happy to say the Duet is one of the few amplifiers I will always have on hand
when I want to listen to music. It is simply superb!!
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I will not bore you with the details of the whole listening session here, but I will
say I came away from it wondering how this was possible. I was looking at an
amp hand built by Dan, an individual designer on his own. No team of folks
helping with all the details that go into producing a piece of electronics; taking it
from a concept to a finished product. The Duet handily stacked up sonically
against some of the very best single ended amp designs and had unique
qualities and features the other amps on hand did not even begin to tackle. !
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While the Cary 300SEI is available with remote volume control none of the other
amps in the session had this benefit. Input switching was manual on the other
integrated amps (Ongaku & CAD300SEI). The Duet possesses both remote
input switching and volume control. I could switch from my Record Player, CD
player and then my music server and adjust volume with a remote - I was in
heaven!!
How does the Duet sound? This is where I would normally describe the relatively
small differences the best amplifiers have. Terms like resolution, imaging,
soundstage, tonal balance and any of the other audiophile performance language
terms we have all used to describe our listening experiences.!
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While I have fallen into that camp most of my life. Now that I have reached
middle age and listened to hundreds of audio products; I find these things to be
more of a trap. If they are used as a single parameter to judge the sonic
performance of a component in a given moment in time, one misses the big
picture. These days I strive to listen to a component’s whole presentation all at
once and get a read on how a piece moves me emotionally. While the Duet can
hold its own with the best in all the sonic performance areas mentioned, nothing I
have heard close to its price can best it in any of these parameters. The most
magical thing the Duet does supremely well is get the rhythm & pace of the
music right. It is impossible to not tap your foot or have the spontaneous urge to
get up and dance when listening to music through the Duet. I can pay no higher
complement to any component. !
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One of the other things that's always disappointing to me about modern high-end
gear is the lack of visual appeal. Most minimalist gear eliminates all the very cool
meters and scopes of the most iconic vintage audio components. These features
added valuable input to the owners and had the added benefit of looking very
cool as well. When today’s audio designers do add some visual elements they
either overpower the units visual look or end up seeming very toy like. Dan’s
approach was elegant, adding visual delights that also provides useful
information.!
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On either side of the volume control are Nixie tube meters that move from the
center left and center right with the music signal. The meter is calibrated so 1
watt is about where the power switch and input indicator selector are placed and
as you get towards the end of the meter you are at 8 watts. There is also an LED
just above the volume knob that will light when the amp approaches max power. !
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Dan wasn’t satisfied with the tactile feel or cosmetics of the available off-the-shelf
knobs for the volume control - so he turned his own brass knobs in his shop from
solid bar brass stock. !
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After our listening
session, Dan
removed the
bottom cover and
started describing
some of the
choices he had
made and some of
the design
Over the next several weeks as I got to know Dan better, I realized that none of
this was an accident. He had spent numerous years as the Engineering Manager
for a company that manufactured Wet Process Machinery for the printed circuit
board industry. Dan and his group designed this equipment for companies like
Western Electric, GTE, GM, HP, & IBM which could run as high as $500,000.00
in the 1980s through the 1990s. !
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In his role, Dan was charged with pulling together all the different engineering
teams and delivering prints to the manufacturing side were he would guide the
products to completion. !
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Today besides building amplifiers, Dan repairs & restores high precision test gear
used by companies that have to have accurate results. This testing equipment is
highly complex and run as much as $70,000.00 new. He takes part in a myriad of
other projects as well. When I was at his shop last, he was working on a project
for a national optometry foundation. It was an optical device to help people with
lazy eyes, train the eye in question, and help it return to normal. He explained he
took on the project because the majority of devices on the market were out of the
reach of many folks budgets and he had come up with a more cost effective
version.!
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Then Dan turned his attention to the first batch of the production Duet’s in the
works. He invited me inside his workshop to check things out. This is when I
finally understood the level of skill and meticulous attention to detail Dan brings to
his projects. !
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In front of me were five Duet chassis in different stages of production. Dan also
took the time to show me the four binders he had produced with Bill of Materials,
Schematics, Purchase History, and Prototype information.!
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At this point I was still fascinated with the level of refinement the Duet possessed so
I asked Dan to take me through his journey in more detail. !
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Dan explained that after years of listening to highly regarded Solid State
amplification, he decided to build his first tube amp to see what his valve amp
enthusiasts friends were all abuzz about. He took some iron from an old Fisher
product and built up a Push Pull tube amp on a chassis he already had on hand.
Much to his surprise this relatively low power tube amp sounded much better then
his well reviewed, 100 watt, solid state amp. Now you tube lovers are grinning, as
you already knew the outcome. And you folks with power hungry speakers are
thinking, all these low powered tube guys are crazy. The result of this first tube amp
build was a fire had been lit in Dan’s psyche that fueled a long journey to arrive at
the Duet First Push Pull Amp Build
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The Duet’s circuit had been in the works since 2007 with three prototypes created
before this final design was arrived at. His first single ended 300B effort was pivotal
as during this time he came up with the drive circuit that eventually became the
front end of the Duet. This time he used purpose built mains & SE output iron. Now
Dan had really upset his preconceived ideas on amplification. This 8 watt per
channel amp had him mesmerized with its sonic virtues. Now, you low power,
single ended, triode guys are grinning again as you had already discovered one of
the best keep secrets in audio. These super simple amp designs, which are mostly
modern incarnations of the vintage Western Electric 91 amps theater amps from
the 1930s have a special rightness to their sonic presentations. Once you have
heard it, you can never go back! Oh and you guys with power hungry speakers are
thinking what the hell are they talking about - only 8 watts!!!!
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First 300B Prototype Amp Build
At this point Dan turned his attention to achieving and surpassing the results
he obtained with the first 300B project and designed a set of Monoblock
amplifiers around the Russian GM70 tube. The Monoblock configuration was
certainly the way to go as it opened up a huge soundstage sonically.
Ultimately Dan’s desire to accomplish this sonic result while retaining the real
world benefits of a single chassis pointed him back to what would become the
current Duet!
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First MonoBlock Amp Build
Through this 4-year prototype process, Dan made some refinements to the
input circuit and tried many different parts in the signal path and power supply
to hear what effects they had. For example, most designers of tube
amplification will agree to avoid electrolytic capacitors in the signal path. He
discovered they had negative effects on the power supply as well. Even using
the best versions of available electrolytic’s in the power supply, the sonic
performance drop was significant. A huge gain in sonic performance was
achieved by eliminating them altogether.!
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The third prototype and first version of the Duet was created much as you see it now
except that Dan originally used an acrylic front panel, wood side panels, and the
Dual mains transformers were US voltage only.!
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This was the amp I first got to listen to and I was blown away with what I was
hearing. Dan and I talked about the final production amp a bit and it was decided the
front panel would end up being powder coated aluminum with slots cut to allow the
nixie tube power meters to be more visible. The side panels were also changed to
power coated aluminum and the mains transformers were sourced with both 120V &
240V operation capabilities. !
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He built the Duet because of the previously mentioned fire that was lit after he built
his first humble but superb sounding tube amp and then it was impossible for him to
stop. He wanted to push what was possible in tube amp design to the sonic
performance limit - which for a select group of tube amp lovers can only mean a
Single Ended, Triode, Class A design.!
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This all adds up to about 1250 man hours spent getting to the final Duet. !
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Dan and I invite you to join our Triode Tube Tribe and know it will be a fun
journey. !
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If you are prepared to carve out a space in your place that is dedicated to
making music happen and willing to match up the proper components, cables
and speakers. We can promise you an experience that will put a smile on your
face every time. !
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Not everyone in your circle of family or friends will get it, but that’s half the fun.
You can just tell them that the Duet was born from the original designs from
the beginnings of audio. And that 300B tube based amplifiers were used to
power the loudspeakers when the first talking theaters were transitioning from
silent films. They may still have a dumbfounded look on their face but it will
reaffirm you as the audio gunslinger you know you are down deep. No sense
hiding from your true nature anymore!
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Regards!
Hollowstate Devices - Team!
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