Revisiting the classic pentode driver
Radiotron's answer to the WE91A
Right before the end of the 20th century, a friend brought over a Japanese built pair of WE91A clones sporting top of the line Tango X-3.5S output transformers, matching chokes and power transformers, input tubes were original WE310As and 50s date code WE 300Bs, WE274B rectifiers, Allen-Bradley carbon and NOS wire wound resistors, Vitamin Qs for coupling and can type paper in oil caps for the power supply. The amp was huge easily dwarfing my JEL SE300Bdx monoblocks and the quality of construction and wiring was impeccable. Although this WE91A clone had great qualities in the midrange my friend was convinced my amps sounded better overall. Does that mean my amp design is absolutely better? Definitely not! Only in the context of my system - Denon DL103R/SME 3012/Garrard 301 feeding Tamura TKS83 MC transformers into a JEL deluxe preamp, JEL SE300Bdx mono blocks driving a pair of Altec 755As in open baffles - since I optimized the signal chain so that it sounds good according to my standards. I told my friend that his decision to purchase those amps should be based on how it sounds in his system not on what he heard here.
Click to zoom
That amp made a lasting impression and reinvigorated my interest in the classic pentode driver topology which has a lot of esteemed followers and practitioners. After my friend left that night I pulled out Joe Roberts WE91A article in Sound Practices Issue No. 1, reviewed my notes on the 6SJ7-->300B I briefly prototyped - slight upper midrange glare through the 755As, the top end was a bit rolled off with a tubby bottom. I was thinking....could it be the pentode 6SJ7 being a later compact version that kills the magic? Remember that even in the 90s WE310As were already unobtanium. So I thumb through the pages of the RC-12 tube manual in search for a close equivalent and found the 6SJ7's grandfather, the 6C6 which has similar electrical specifications and bears a physical resemblance (BIG plate and grid cap) to the WE310A. The 6V filament is a convenient bonus. By that time my all triode input/driver circuit already gave the qualities required to maximize the potential of the 755A/OBs and for the simple crossover network equipped Altec 2-way, a fine tuned 2A3 version. I pursued other interests and kept the 6C6 project in the back burner.......
At the beginning of the year (2007) I was looking for a winter project that will not entail chassis building. I reread Joe Roberts article "I never met a 2A3 amp that I didn't like" SP15 and noticed a familiar looking SE2A3 amp schematic using a 6C6 pentode driver from the Radiotron, 3rd edition. He had good things to say about this circuit and it looked easy to graft this topology on my long-time SE2A3 test mule with Tango U808s. I took inventory of my tube stash and was frustrated to find just a single RCA 6C6, a couple of 6-pin sockets but needed 1/4" medium sized grid caps.
One chilly morning I downloaded an email from an internet buddy Sergey Didkovsky from Ukraine. We have exchanged emails a few years back when he was building the JEL classic 300B circuit. He was telling me about a pentode driven SE amp that he thinks sounds better. This was a fortuitous moment because the clairvoyant Sergey was talking about a Radiotron derivative. In the course of our correspondence he pointed out a quick source for additional 6C6s and grid caps with several links and attachments of Japanese and European DIYers' single pentode driver stage schematics including tips on operating points and parts tweaks.
I started my listening evaluation with this circuit with component values taken arbitrarily from the R-C chart for a 6C6 and sent my impressions to Sergey for comments:
"Try please, to a little increase the cathode resistor (up to 1,2-1,5Kohm) and reduce anode resistor (to 75k-82k). It will allow to increase Ua up to 150V and to receive ratio Ua:Ug2 = 2:1 ratio. The sound should become more "interesting and old". That is especially good for jazz. Still mine 5 cents....use good NOS Oil capacitors in G2 circuit. I use TCC Visconol. I think, oil caps in the grid - adds a bass. Reduction of the anode resistor - improves the hi range a little."
His use of adjectives "interesting and old" prompted me to get as close to the suggested 2:1 plate to screen grid suppressor voltage ratio as it seems to be a key ingredient for his recipe. So far I have achieved a 1.8:1 ratio and was still hearing a slight upper midrange glare that I originally attributed to the 6SJ7 years ago. The BIG plate = FAT midrange I was expecting from the 6C6 was already there but the glare did not disappear until I replaced the plastic 2uf cap with can type 2.8uf paper in oil caps for the screen grid (suppressor) and as Sergey said it did improve the bass. Maybe the paper in oil decoupling cap will give the same results with the 6SJ7....
"Thanks for your patience, not looking at my very bad English language...."
There is a saying that music is a universal language, Sergey apologized about his English but I assured him that his communication skills were fine. Certainly driven by his passion for good sound, I had no problems understanding his tips and it was quite evident that he has done some serious research, listening and is blessed with good ears!
With these thoughts in mind I did not argue and simply looked up to him as a mentor. In this revised circuit I captured the 2:1 ratio, to be exact 1.98:1 with 152V on the plate drawing about 1.8ma. of current. I changed the grid resistor from a 270k to 330k because not only does this make an easier load for the driver stage but from experience the sound becomes warmer as the grid resistor value is increased but only to a certain extent. If you exceed the maximum 500k grid resistor value specified in the tube manual for a 2A3 you might find the plates running away cherry red! So check your tube manual before messing around with this tweak on other triodes. A 220k will give more focus and detail but lower than that value the driver stage will have limited headroom. It will clip prematurely from not having enough voltage swing to drive the 2A3 to a full 3.5W output since the grid resistor is effectively in parallel with the plate load resistor.
I am grateful to Sergey for sharing his experience saving me time and effort to build a classic pentode driven SE2A3 amp.
not so OBJECTIVE....
As a budding audiophile in the 1980s I became aware of the existence of another type of triode aside from the 9-pin miniatures inside a Dyna PAS preamp through an article by Robert Fulton in Fanfare magazine "Prerequisites for Capturing the Musical Experience" where he extolled the virtues of Directly Heated Triodes - 01A, 27, 56, 45, 2A3, and etc. - with adjectives like simple, honest, pristine, speaks with dignity and majesty. Those words haunted me for years but I had no clue where to find DHTs let alone how to use them until Sound Practices and Joe Roberts reprinted the article in issue 1.
The main reason we get into the audio hobby is to enjoy fine music reproduction. In order to discern that what we are hearing on playback is a resemblance of the actual sound we have to understand that every musical instrument or human voice has a unique set of odd and even harmonics that defines its sound due to the harmonic overtone series (further discussed below). This phenomenon became the basis for a worthless but standard audio test procedure - total harmonic distortion - on the premise that if a sine wave is injected to an amp it should not alter the quality of the signal. Sounds perfect in theory but a sine wave is not a representative of any musical instrument or human voice. And I quote Radiotron, 3rd edition published in the 1930s from which pages this circuit was derived, "The total harmonic distortion is not a measure of the degree of distastefulness to the listener and it is recommended that its use should be discontinued". It is now an accepted theory that the reason why people find tube amplifiers more musical is due to its inherent high even order distortion figures. The Single Ended DHT topology is plagued with this deficiency!
Driving the amp with a chosen sine wave signal frequency (typically 1khz) right before clipping (visual distortion on the scope or with a THD meter) determines power rating and square wave testing are methods that address the dynamic nature of music. By definition a square wave is a waveform theoretically comprised of an infinite set of odd harmonics starting from a fundamental tone. It gives a visual representation on a scope of transient response and upper frequency ringing. However a modern IC op-amp chip equipped amp can probably pass an indistinguishable square wave from input to output but may not sound as realistic as any of these ancient SE circuits.
Scope jockeying can be fun and I am trying hard to be objective so please indulge me.
100hz, 1khz, 10khz - both channels
Radiotron derived SE2A3 - 6C6 pentode driver, Tango U808
Power output = 3.8W per channel before clipping
Frequency response = +/- 1dB from 30hz - 15khz @ 1W
( - 3dB @ 20hz & - 5dB @ 20khz)
100hz, 1khz, 10khz - top trace is the signal generator output
JEL SE2A3dx - 76/6SN7 input/driver, Tango U808
Power output = 4.5W per channel before clipping
Frequency response = +/- .5dB from 20hz-20khz @ 1W
( - 3dB @ 50khz)
100hz, 1khz, 10khz - both channels
JEL Simple 45 - 1/2 6SL7 [or 6SF5], Tamura F475
Power output = 2W per channel before clipping
Frequency response = +/- 1dB 20hz-20khz @ 1W
(- 5dB @ 30khz)
Compared to my favorite topologies in driving power triodes, this R-C coupled 6C6 pentode has slightly narrower bandwidth and as shown in the square waves, it has the slowest rise time and softest leading edge at 1khz and 10khz (in theory a more vertical rise time and sharper leading edge = faster transients and better high frequency extension) and pretty much equal at 100hz (less tilt = better bass response) with the Simple 45 having a slight edge but that uses a different OPT. To put things in perspective a 30hz-15khz +/- 1dB bandwidth is not a slouch if you refer to the table below.
If I remember correctly the pentode driven WE91A amp which brought talking films to the public shares a similar bandwidth if the negative feedback loop is disconnected which is how they are used nowadays. With NFB bandwidth is improved but does not necessarily result in better sound. In my experience NFB chokes the sound and I do not use it in my preamp or amp. This is often noticeable in a typical NFB equipped amp 10khz square wave showing an almost vertical rise time but the leading edge will have tiny squiggles before going horizontal which is also known as ringing or unwanted high frequency resonance.
Past and present audio engineers mean well and we have to be grateful for their contributions that help us understand audio science. Unfortunately marketing hype distorts scientific research to exploit the market in order to sell a product. I believe that good engineering aided by good ears is an important factor in building a musical sounding amplifier and cannot forget a post I read several years ago at a forum "....nice people build good sounding amps, bad sounding amps are designed by annoying....." and so it goes.....
Let us take a detour to discuss frequency and harmonics (or overtones). I started playing the violin at 5 and my dad trained me to recognize pitches and conscientiously practice to achieve good intonation. By the time I got to Juilliard we were expected to hear complex chord progressions, tone clusters and be able to write the notes down as ear training exercises. Using the table above I can convert a note or harmonic progression to a frequency or band of frequencies for audio applications. This skill has served me well in tuning and voicing my preamp and amp circuits, adjust the crossover point and sensitivity of the horn with an L-pad to achieve the smoothest sonic transition between the low and high frequency drivers of the Altec 2-way horn and despite the published 70-13,000hz response of the 755A, in an open baffle I can still enjoy listening to a Mahler Symphony with the double bass section playing contrapuntal lines going down to the 40-32hz region. This is due to the harmonic overtone series (please read this link carefully). It is worth noting that some Hi-Fi guide books from the early 1950s managed to discuss this topic to some extent to educate prospective buyers but is hardly mentioned since. For the sake of simplicity I will use absolute harmony and concentrate on the fundamental and second harmonic. I will not go into detail about the third, fourth, fifth and further harmonics since those are farther up the no brainer frequency range of the 755A.
When a very low E:40hz fundamental note is played by the double bass section there is a prominent second harmonic (first overtone) component around 80hz (E2 - same note octave higher) that is within the lower range of the 755A (70-13,000hz). The 755A mounted on a 35.5" wide open baffle according to this table provides a barrier to prevent a 90hz sound wave from canceling out. Since the 80hz second harmonic is only 10hz away (the approximate equivalent of one whole tone or note on a piano) the OB surface area should still provide adequate barrier to prevent the 80hz E2 note from canceling out. So far we have been talking theory in free field. Once I optimize placement of the 755A/OB in the listening room additional low frequency reinforcement will be provided by the back legs, top plate and room boundaries and the rolled off 40hz is no longer such a far fetch reality especially when we factor in the further harmonics up the frequency spectrum that makes the unique double bass sound. Numerically the distance between 40hz and 80 hz seem great but in music that is only a one octave scale simple enough for a beginner to play on a piano. E:40hz is the lowest open string in a double bass that does not have a low C:32hz extension the use of started in Mahler Symphonies. Granted it is a compromise to listen to Mahler through the OB/755A and the Altec 2-way horn system gives a better sense of scale and proportion but it is still not flat down to 40hz. It is even possible that the resonance from an enclosed bass reflex loading is inflicting more damage to further overtones.
While we are on the subject of pitch or frequency recognition, there are some people who claim to have been born with perfect pitch. Scholarly research places the Baroque period concert tuning of the note A (open string on a violin) at 415hz, in the 19th century it started to rise, A:440hz is the present standard and some major orchestras even tune a smidgen higher (441- 442) for the extra brilliance needed for bigger concert halls of the modern era. For the past 300 years the tuning standard has been rising so does absolute or perfect pitch truly exist? As far as I am concerned the ability to recognize pitch is developed through musical training. The main advantage of a musically gifted person is he or she will learn faster or without formal training will notice if someone is singing or playing out of tune. But that is not perfect pitch; it only means the person has good ears!
The human ear is an extremely sophisticated device and many things we associate with good sound are still beyond the capacity of man made audio test equipment to measure. Meanwhile anyone with a basic background in music and has interest in electronics has the capacity to maximize the potential of their audio system by developing their hearing acuity. To those who would like to DIY their way into recognizing pitches and frequencies I suggest getting a $15-$40 tuner as a tutor. I urge every music lover and audiophile to print this table to use it as a guide the next time you turn on your system and listen to your favorite LP or CD. After a while you will realize that most musical activity is centered well within the 755A bandwidth. Get this range right first and then try to extend both extremes without significantly compromising what you already have. This has been my motto towards musical satisfaction. Hopefully these tips can save you from upgrades, help make decisions for future purchase or DIY project and more importantly, lead to happy listening!
!!!!WARNING!!!!
Contains explicit material that may not be suitable for some audiophiles and DIYers. Please consult your audio analyst before proceeding :o)
....and the very SUBJECTIVE
Does it sound better?
Are we supposed to listen to sound or enjoy music? The longer I indulge in this hobby the less I care for better or best. My recent adventures in monaural playback opened a fresh perspective on how not to listen audiophile style. Lately I have been listening mostly to authentic mono software chosen primarily on musical merit in true monaural fashion - single channel all the way. I highly recommend monophonic activity to anyone interested in honing their listening, voicing, tuning and most of all enjoyment skills!
Soundstage....
When you go to Carnegie Hall or attend an orchestral performance elsewhere do you listen to the soundstage? If so you missed the point of paying for a ticket because you should be there to enjoy the concert! Next time give the ticket to a struggling conservatory student. I studied in NYC for 10 years waited in line for student rush tickets to attend concerts and was fortunate enough to have played in Carnegie Hall but never realized it has a soundstage, I just know it has fine acoustics. Ironically I was a subscriber to TAS at that time......fortunately there was Fanfare at the music library to keep me grounded.
Imaging....
Sorry I think you got here by mistake.....try this or maybe here?
Is it accurate or colored?
I lost track of the true meaning of these over used audio adjectives. What is accuracy? The absolute sound of a violin is neither a Stradivarius nor a Guarnerius del Gesu. Each instrument has a unique sonic trademark again due to the harmonic overtone series even if it came from the hands of the same luthier. To add further obfuscation the difference will be magnified by the acoustics of the room as well as the player for better or worse - Perlman will always be Perlman even on a bad day whether or not he is playing his del Gesu, Strad, Bergonzi or a modern fiddle whereas a fine Strad in the hands of an incompetent player will just sound bad. Violinists choose a fine instrument as a tool to synergize with in order to communicate their musical ideas and sentiments!
The New York Philharmonic will project a certain sound in Avery Fisher Hall and another at Carnegie Hall simply due to the acoustic properties endemic to each concert hall. Which one is the accurate sound of the NY Phil? Hmmm....again we forgot, the reason we are there is to witness the artistry of the conductor, musicians and their collective ability to play the repertoire to the inspiration of the moment!
DIYers and audiophiles, stop your search for the holy grail in cartridges, preamp and amp circuits, speakers and etc. The perfect circuit or transducer that will reproduce everything accurately does not exist we all need to compromise!
Colored? Guilty! I have an assortment of vintage flavored MC and MM cartridges with heavy tracking spherical styli that gouge musical life out of the grooves each time I play a record and maintain five SET amps with a variety of output tubes and driver circuits because of the harmonics they add to the color palette (or shades of gray if you are into B&W) I enjoy hearing when I relax and unwind.
OK after killing all the joy and excitement in audiophilia I should be sentenced to describe the sound.....
This Radiotron derivative is a musical sounding amplifier and I will keep it in my amp fleet. I felt transported back to the '99 listening experience stated in my opening paragraph except this time via 2A3s driving the Altec 2-way horns and later cross referenced to Jensen Duettes. I do not contest Sergey's claim that this amp sounded better because he probably voiced it to match the rest of his system. There are two important variables to consider, the Audio Note AN-J/L is a contemporary design while my tweaked Altec and Jensen are vintage; and the 6SN7GTB--->300B circuit he built and evaluated has evolved to a whole new design due to the refined sonic qualities of the type 76 triode at the input stage and revised power supply. I relayed my findings to Sergey and this was his response:
"Though you are right, all depends on all components of system. I use 2-way loudspeakers Audio Note AN-J/L. And alas, never heard Altec, Sergey"
BRAVO Sergey!
My correspondence with Sergey reminded me of the early days of cyberspace when Joel and I exchanged ASCII schematics back and forth on dial-up. Nowadays there is far greater information available to download one just has to learn how to distinguish valuable ideas from flack and hype. I am not dogmatic about DIY sans NFB circuits, SE DHT amps, vintage high-efficiency drivers/speakers, idler drive turntables, cartridges and tone arms. I just found musical satisfaction from this type of set up. However a vital aspect of the audio hobby that is rarely discussed and often overlooked is the synergy between components in the signal chain. I intentionally avoid regular participation in forums because a lot of audiophiles and even DIYers get carried away with out of system context details that lead to unproductive debates - vintage idler in heavy wooden plinth vs. acrylic laden belt drive turntables, high vs. low mass, MC vs. MM cartridges, step-up transformers vs. active MC devices, tube vs. solid state, SE vs. PP, DHT vs. IDHT, paper in oil vs. plastic caps, exotic core material for transformers, designer interconnect cables and wires, alnico vs. ferrite magnets, paper vs. plastic cones, horns, open baffles and etc. - as if this is a competitive sport. Let us not lose sight of the BIG PICTURE which is simply how to blend components to achieve the best compromise towards a musically satisfying system. In general collectors and users of vintage equipment seem to be notable exceptions; they enjoy listening and accept the real world compromise from classics that have been minimally altered or not at all and should be looked upon as role models. You want competition? Try outdoing yourself by setting up a system you can enjoy!
Years ago I warned my SETUP friends that DIY and the ability gained from the experience to voice ones' system towards musical satisfaction will cure the upgrade syndrome and quite naturally force them to withdraw from the recommended components competition....they took up golf and went home to unwind to good music. Shindo components are probably the most expensive and revered of all audio exotica currently available in the market. Based on reviews and press releases I have read, his products are the result of over 30 years of listening and painstaking tinkering. There is a choice for music lovers who demand the finest music reproduction system - buy a six figure ear tuned Shindo audio system or learn to DIY - either way may be the anti-dote to high-end audio affliction but I am not so sure.....
The next challenge I plan to undertake is build a mono block version of this circuit. Hammond has graciously offered a JE Labs commemorative amorphous core 125ESE with pure silver winding, ultra low DCR power transformer and choke. A token of appreciation for all the years my website contributed to their sale of iron. I will use the 200% copper free total conductivity quadruple shielded platinum prototype cable sent for evaluation by Radio Shack to connect their smelted from a vintage Patek-Philippe Tourbillon wristwatch 24 karat gold RCA jack to the Chinese OEM grid cap, pepper the chassis with NOS mil-spec carcinogenic PCB loaded paper in oil caps and then further fine tune the operating points of each tube using a Tektronix 570 curve tracer interfaced to a brand new Sony VAIO laptop with MS Vista OS to do the number crunching so that I can match my epoxy and C37 coated A-B resistors to .001% tolerance for the ultimate monaural playback system. But first I have to track down an obscure transducer with idyllic Qts parameters that is supposed to work best in a slim and tall open baffle fashioned from 2" thick extremely damped (resonance is claimed at RF) Teflon coated acrylic for the penultimate sonic transparency and imaging within its +/- 10 dB 90hz-7khz bandwidth (guaranteed feel of a solid 40hz E from Eddie Gomez double bass). This scarce (so far only 2 are known to exist and are in Japan) handcrafted 1920s vintage field coil phenolic coated 9 7/8" paper cone driver augmented by a whizzer is attributed to Peter Jensen's valet......please do me a favor, stay away from my bid at eBay.....;o)!
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