Flesh and Blood -- The Reichert 300B
Article by Herb Reichert
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The light from the streetlights mixes with the glow of the bright emitters in my listening room. Nyiregyhazi plays Liszt in front of me and my skin seems electric but I am completely relaxed. I let out my breath and my pulse is near
flatline. Audio can really be good to us... if we let it. This is why we must seek to create and explore the frontiers of audio. This is why many of you build your own audio equipment.
I started building and designing my own audio because I am a 'gearhead' by nature, not because I thought I could do better than the pros. However, one day I took the negative feedback out of an amplifier I had built and my whole body went limp. Big symphonies by Mahler began to expand and breathe. A strong sense of voluptuousness replaced a feeling of tension and hardness. Average and poorly recorded program began to become part of my midnight repertoire. I concluded I was on to something important. I realized that there must be more design decisions, big ones, like this one with the feedback, that impact the basic character of my system. This is the point at which I became "high fever". I now had a sense of what was possible. I began to engineer with a purpose.
Over the last decade, I looked carefully at each part of the audio amplifier design process. My first rule was: take nothing for granted and keep an open mind. This attitude steered me down many wrong roads; like the six months I spent chasing A/C balance in push-pull amps or looking at pentode driver stages. Big dead ends!
On the other hand, looking into every detail of the single-ended, directly-heated triode amplifier has netted results beyond my wildest dreams. The whole world of high-end audio is changing and believe me, it is the result of the work you and I and our friends are doing in our homes. We are experiencing a 'new dawn' in audio, but this new dawn is only the beginning and there is much
new work to do. So, I want you to take this design as a beginning and build on it. Apply your own audio ethics and benchmarks.
From the letters and phone calls I receive, I know that many of you believe that if you can get a schematic for a great amp, then go out and buy what you think are some great parts, and put it all together carefully, then you will end up with a great amp. Sorry, it just doesn't work that way. You might get a
good amp by this method, if you are lucky and inspired, but to get a great
amp you must suffer. Whenever I build a BAD amp, and I have built quite a few, J.C. always says, "Welcome to the next level." It is the failures and the mistakes that get you to the other side of the mountain. Each failure makes more clear what you do not want. Each success enlarges your vision of what is possible. The process of building is to wire your mind, to focus your sights on that point on the horizon where greatness lays.
I know the Reichert 300B-SE amp doesn't look like an unusual or original design. But, on the test bench, this amplifier design sets very high standards for bandwidth, rise time, and distortion. My main design goals were vividness, body, color, and dramatic contrast. I want a vibrant, breathy, gripping sort of presentation. I am an artist, a painter in the painterly, romantic tradition. I want my hi-fi to paint with strong, rich strokes.
What I don't want is distant, thin, or mechanical sound. Lots of flesh and blood. Lots of drama and empathy and lots of Technicolor and
Panavision; this is what I was after when I designed this amp. Each design decision was a trial and error attempt to get to this end. Please understand, this is not a 'part of the month' design. Each parts choice has a personal evolution, all yin/yanged to my taste. If you change ANY part, it is your amplifier design, not mine.
Amplifiers are engineered from the output terminals backward. The amp/speaker interface is
everything and one's focus must always be on the 'black & red'. Amplifiers are always designed for a particular speaker or speakers and any amp designer who says differently is trying to sell you something he doesn't own. How the amplifier behaves into the chosen speaker load is the first point to consider when making all major engineering decisions. This amplifier was created to power Altec
VOTs, Edgarhorns, WE-755As, and Altec 601As. I have also discovered that it works very well with LS3/5As, AR-M1s, Lowther PM-6s, and Audio Note Model 2s & 3s. I used a circa 1946 Altec VOT system to do the initial design work. Final touches were done with the
Onken/Edgar system and Audio Note Model 2 & 3 speakers.
Output transformers are the cornerstones of any tube amplifier design. Since 1980, I have tried outputs from
UTC, ACRO, Peerless, Dynaco, Fisher, Partridge, Western Electric, Chicago, Hammond, Magnequest, RCA, Audio Note, and God only knows...? I prefer the Tango to any other. I distributed Tango in the U.S. until 1993. Maybe that's why I began this design with the Tango XE-60-5S single-ended output. I do not care how "romantic" you want to be, you must have speed and bandwidth. An amplifier that is slew limited or unstable outside of its passband will never have good tone character.
I believe the ultrasonic and infrasonic behavior of a SE amp must be carefully examined. Excessive phase shift or ringing in these regions will surely sabotage the amplifier's potential for greatness. If the output transformer rotates phase more than 40 degrees below 100 Hz, the amp will sound slow and the bass will seem to lag behind, lacking "tunefulness". Bass transients will sound dull. Likewise, if there is ringing in the ultrasonic region, the amp will sound hard and 'hollow'.
The proper selection of core material, core size, aspect ratio, and winding technique is far more critical in SE designs. Poor choices lead to soft, lazy, unrefined sounding amplifiers. The Tango line is unique in that it is the product of two decades of continual development. World-wide, there are thousands of SE amps with Tango outputs. The Tango XE-60-5S measured 18-80 kHz, -2dB in this amp. This is at 7 watts! Remember, I do not sell these transformers anymore. I just still love them.
I chose the WE 300B tube after living with amps built around the 6B4G, 2A3, 50, 45, 801A, and the 10Y. 10Ys push/pull are still my personal favorite, but even the VOTs like more than 3 watts. Of the available triodes, the 300B plays the most records with the greatest ease and the most refinement. It is voluptuous and elegant. Also, it is the ONLY tube I am aware of with perfect sample to sample consistency. The Western Electric 300B is always quiet and it will last forever. There is a hypnotic quality to the 300B sound that draws me into the performance like no other type of amplifier.
Operating points are next on the design agenda. My best friend and tube maven, J.C. Morrison, has already written the book on this subject: run your triodes hot! With pentodes, I like low plate voltages and high current. With triodes I like high plate voltages AND high current. I run the WE 300B at 425VDC on the plate and 80mA standing current. I want deep class A1. I want to tickle the center of the B/H curve and swing very little current across the power supply. Symmetrical clipping and fast graceful recovery are a combination of power tube operating point, driver stage design and power supply engineering. These three elements work in concert and must be designed together with a clear sonic goal in mind. The amplifier's ability to drive speaker loads with ease and refinement will be seriously handicapped if we make a bad decision here.
I chose the 6SN7 cascade after examining the distortion spectrum and sonic character of the SRPP, transformer coupling (Tango NC-16 and NC-14), 5687 anode follower, the
mu-follower, and several variations on direct coupling. With the R-C coupled 6SN7 and 500VDC raw supply the amp 'locks' into class A1 with a minimum of A2 voltage. The 12K plate load on the driver stage gives me the rise time and low odd-order distortion product I wanted. Don't laugh, but I only like the GE 6SN7GTB in this position. You must use GTBs to get the required plate dissipation and the GE version sounds best to me. The first stage is R-C coupled to the driver stage to hold the output tube grid swing to max even with poorly matched tube sections or aging tubes. Direct-coupled designs seem to change their sound over time even though their minimum phase character is highly appealing. In this design, the time constants have been very carefully considered, so don't go changing any resistor or capacitor values.
The power supply is everything in these little amps and it also seems to be the area where designers agree the least. All the SE 300B amps I have heard sound very different from each other. Some sound more mechanical than the worst solid state designs. The reason, I expect, is wildly different ideas on power supply design. First rule of triode amp design: Solid state rectifiers = mechanical sound. You don't think so? Then you haven't really compared. I promise you, IF there is only ONE thing I have learned in ten years of amp design, it is this first rule. A lot of time went into selecting the rectifier tube for this design. Normally, I use the WE 274A/B, but this amp sounded best with the RCA-5R4GY.
The pi-filter is the heart of this amplifier. If this amp sounds better than others, it is probably due to the choke/cap selection on the pi-filter. I don't like to waste a lot of energy charging and discharging caps. I want a narrow torque curve - high rev supply. Low impedance and minimum storage gives fast recovery and fine texture to the sound. Remember, most amps under 2000 watts are running at or near overload. Let's make them sound unstressed at overload. Then the music will sound
unmechanical. The power transformer should be rated at least 500mA at 800VCT. An even higher current rating is better (I use 750mA) because I want a small value bleeder resistor at the end of the filter. I like to bleed at least 25%% of the total standing current. This stabilizes and regulates the supply. With the small Black Gate caps, the heavy bleed appears to enhance the clipping characteristic.
We want 480 to 500 volts across the output of the pi filter. The choke should have a DCR of less than 300 ohms. I aim for 10-20 Ohms
DCR! This is a big chunk of iron, but it is a very important part of the design. The number of Henrys is less important than the DCR rating. Two to ten Henrys is fine. We are looking for a B+ that is fast and linear, but loosely and naturally regulated. Remember, the reservoir and decoupling capacitors are part of their respective stage's transfer function. This means that they are just as important as the tubes in determining the linearity and character of the amplifier.
For caps there are a few choices. I have tried all the usual stuff: WE oil and paper, photoflash, polypropylene, etc. The Black Gates (47uf at 500vdc x 2) are now my first choice, but only if you play music every day. These are electron-transfer/electro-static and must be kept charged. They take a full 24 hours to recharge when left to discharge. These are not electrolytics. They work more like an electrostatic speaker. There is no electrolyte or electrochemical delay. These caps are super wideband, linear, non-resonant, and quiet. The Cerefine are almost as good. They use a ceramic powder instead of pure carbon like the Black Gates. This ceramic powder allows for a quicker charge up time. Both caps behave very gracefully under A/C conditions. Do not use polypropylene. If you do, this is your amp design, not mine.
I am not going to get into the parts philosophy thing except to tell you I have tried everything I could lay my hands on. My ideas change daily so here are today's recommendations. No Teflon. No silver plated wire. No MIT multi-caps. No metal film or metal oxide resistors. No polypropylene caps. No Vitamin Qs. No Solen. No REL caps. No
Holcos. No metal oxides. No Vishays. No SCR. No MKP-1845s. No solid state diodes. No solid state current sources. No silicon anything. If you use this stuff, you know who's amp design it is NOT.
Please use Allen-Bradley resistors in the plate circuits. Paralleled resistors, in plate circuits, are quieter and sweeter sounding. Use Audio Note Tantalum resistors in the cathode and grid circuits. On the cathodes of the 300Bs use Caddock 50 watt MP 850s. You can use Caddock MG or MX in the plate circuits if you are a noise freak, but I think the A/Bs sound more relaxed and showcase the wood and brass tones on orchestral music. I have only three recommendations for coupling capacitors and they are all Audio Note. Due to my affiliation with Audio Note you probably won't take my coupling cap recommendation too seriously, but that's OK. You lose! If you want even a chance of catching my Ongaku sonically you must use Audio Note silver foil paper in oil coupling capacitors. If you can't afford these use AN copper foil paper and oil capacitors. There simply are no other choices.
Wire is a separate issue. Wire is a system thing. After twelve years of building amps, I only
know two things for sure:
1) Use tube rectifiers
2) You can never have too much silver in the signal path.
Go without food or clothes, but buy lots of records and wire your hi-fi in silver. Audio Note or Kimber silver wire are my first choices for internal wiring of this amp. If you can't afford this stuff, just use Carol PVC hook up wire. Nothing is worse than silver-plated copper. Stay away from all Teflon coated wire if you are looking for relaxed natural sound. Believe me the Carol PVC stuff is good for everything in your system from the tonearm to the speaker. If you can't afford silver, and you trust me, try it.
In fact, if you want to build this amp on a budget, try this... you can still say it is my design. Use the Tango XE-20S or the Audio Note outputs. Use the Carol PVC wire, the AN Paper and Oil Caps (Regular type), Allen-Bradley resistors, and Sprague or Mallory power supply and bypass caps. You will lose some of the refinement but none of the naturalness.
If you make substitutions with parts try to avoid plastic, especially hard plastic. Think voluptuous and colorful. Oh yeah, even if you are on a budget, try to use a copper chassis,
2% to 4% silver solder, and high quality ceramic tube sockets.
Please, try to wire the circuit just as given on the drawing, observing the ground points of the cathodes and the PS capacitors. Do not buss the power supply caps. The pi-filter and the 300B cathode resistor should be grounded at the same point. Likewise the 6SN7 cathode resistors, the driver stage bypass condenser and the decoupling capacitors should all go to the same point.
All of this design talk may be for naught. You see, I believe most deeply that the real magic ingredient in any amp design is the wu of the designer. This wu flows from the designer's hands during construction and raises the effort above the common and imperfect. Therefore it might be best if you design your own amplifier, for your own speaker, based on what you already know and what you think of my ideas. My philosophy rests on the romantic and the expressive. Drama and contrast with the grace and poise of a bullfighter are my audio system goals.
This circuit and these parts choices were developed inside the world of my
hi-fi to my taste! If you want an exceptional music reproduction system in your home you must first develop your internal reference for natural sound. Then you must outline your aesthetic and make a series of design decisions that reflect that aesthetic. But remember, you won't be happy if you acquire your aesthetic from reviews and audio pundits. You must discover your own. Trial and error is tedious and it takes a long time to become a wizard, but I am sure you will look good in the wizard's cap.