@lynn_olson
I’m really pleased about this. In an era of superb all-digital, all-solid-state 4K HDR video, tube amps continue to make new friends because they sound so good, on all types of music.
Agreed!!!
I believe that class D GaN amplifiers will definitely continue to grow market share and popularity. This seems undeniable. Yet I also believe that there will remain a certain number of music listeners who find that nothing satisfies their musical/listening desires as well as high quality tube products. The two will peacefully coexist.
Both can be excellent while simultaneously being distinctly different. I do not share the gloomy “demise of tubes “ forecast . Time will certainly tell.
Charles
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Charles1dad, thanks for the compliment ... much appreciated. Don and I put a lot of work into these seemingly simple amplifiers.
DHT’s had a rather short reign in audio (much longer in transmitting tubes). It was only from the early Twenties ... the dawn of radio ... to the late Thirties. Once the 6L6 and 6V6 came out (they were designed by the same team), that wiped out the 45, 50, 2A3, 300B, 211, and 845. Even Western Electric abandoned the 300B by 1940 when they designed their new generation of amplifiers around push-pull 6L6’s (WE350). Since the 300B first came out in 1935, it wasn’t in favor all that long.
300B’s have now been in production longer than they were in the Thirties and Forties, rather odd when you think about it. It was the vogue in Japan, Europe, and finally the USA in the Nineties that created the continuing demand for the type and DHT’s in general. It’s been thirty years now, so I think it’s safe to say they are here to stay, along with their pentode cousins.
Class D GaN amplifiers will continue to erode Class AB transistor amplifiers, but I think vacuum-tube amps have an enduring appeal that continues to grow. They now dominate high-end audio, which was not true thirty years ago. I remember going to some CES shows with hardly any tube amps at all, never mind DHT’s, and now they are everywhere.
Now you see quality record players, and tube amps, in movies as a marker of good taste. The movie viewer gets a little buzz when the tonearm descends into the groove, making that distinctive vinyl "click" sound, then you see a tube amp quietly glowing in the background, and wonderful music comes out. The camera pulls back, and you see the protagonist, looking contemplative, and out-of-focus city lights in the background. That alone sets a mood.
I’m really pleased about this. In an era of superb all-digital, all-solid-state 4K HDR video, tube amps continue to make new friends because they sound so good, on all types of music.
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Which returned the Blackbird to the original Karna topology, with far superior power supplies, and the luxury of interstage transformers specifically designed for the Blackbird amplifier.
I give Don full credit for doggedly trying every possible form of coupling, optimizing each circuit with the most favorable operating point, and giving it a serious, I’d even say exhaustive, evaluation. While I sat back with original 20-year-old Karna circuit and criticized from afar. I’m sure I annoyed the hell out of Don more than once.
The selection of an IT for the output section is obvious. The driver, which has to swing a lot of volts at very low distortion, gets to transfer all of its power to the DHT grids. If the DHT grid swings into Class A2 and starts drawing current, no big deal. The power is there, and there are no caps to charge or discharge. Recovery time is instantaneous, unlike RC or LC coupling, and there no risk of DC-coupled failure propagating from driver to output, as there is in solid-state equipment. It really is ideal.
The input tube was another question. In principle, at the lower working voltages, there shouldn’t be much difference between any of the methods, with RC coupling as the obvious and cheapest method. Unfortunately, that’s exactly what it sounds like.
The more serious auditioning over the last year was between current-source + cap coupling, inductor loading + cap coupling (LC), and straight transformer coupling, with no coupling caps or grid resistors involved. And that sounded the best.
Also the simplest. Six parts ... two custom inductors, two good-sized and quite expensive caps, and two grid resistors ... are replaced by one reasonably compact, purpose-designed transformer. The folks who own the "shoebox" format amps, as demonstrated at the show, can be upgraded to the new circuit, which actually opens up space under the chassis. All new amps will have the new circuit, of course.
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Typo above, choke loaded is clearly not RC coupled, but rather LC coupled, sorry. At any rate, full interstage transformer coupling was easily the best sounding.
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Yup. Wires, transformers, and 1930's vintage tubes. That's the entire signal path, from preamp input to speakers.
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Which is why, on my highly resolving system, I could clearly hear the differences between CCS loaded RC coupled, choke loaded RC coupled, and interstage coupled versions of essentially the exact same amplifier. IT coupling won hands down in all areas and is the way these amps are built. The preamp too. No RC coupling anywhere in the signal path.... Of course you need really good transformers.....which has taken a year.
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Direct coupling would have no effect on tube loading, which is responsible for the spectra shown. If we were to re-do the article, we’d try a MOSFET cascode current source load, as well as transformer coupling, SRPP, and RC coupling.
We were surprised that cathode degeneration doesn’t work, and creates some nasty high-order terms instead. Separating the data into even and odd-order terms was essential to unscrambling the chaotic results of the spreadsheet ... a legitimate way of looking at the data, since the underlying transfer curves of odd-order (S-shaped) and even-order (C-shaped) distortion are fundamentally different.
Nowadays, we have the computer power to discover the actual shape of the input/output transfer curve, and exaggerate it enough to be visible. The regrettable drawback of FFT spectral information is that phase is usually discarded, so the underlying transfer shape cannot be found (although it can be inferred).
(What I mean by this is the phase of the distortion harmonics is important. For example, a square wave and a triangle wave look exactly the same on an FFT spectral display; the only difference is the phase of the harmonics. The magnitudes are the same. In real circuits, square waves and triangle waves are created by entirely different mechanisms, so this is important data.)
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Nice article! Too bad direct coupling wasn't tried. That would have been fun to see.
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Investigated in some depth in this 1997 Glass Audio article by Matt Kamna (designer of the Whammerdyne 2A3 amplifier) and myself:
Hidden Harmonics
We found that transformer coupling had the most favorable distribution of harmonics ... by that, the smoothest and fastest drop off. Other forms had more harmonics, with more uneven distribution. Test conditions: 6SN7, single section, 50V rms out, with several different circuits, with and without cathode bypass capacitors. Noise floor with this setup was -118 dB, and harmonics out to the 11th were investigated.
To my knowledge, this was the most thorough examination of vacuum tube harmonic generation at the time, using direct measurement instead of reliance on tube models. Standard assumptions about local feedback from cathode degeneration, and SRPP distortion cancellation, were proven wrong. RC coupling, in particular, was shown to have quite high distortion, while transformer coupling was the lowest.
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@lynn_olson
Thanks for the exceptionally educational and interesting posts. ! Very much appreciated. Your summation of the simplicity of the DHT tubes really explains their longevity, continued desirability and enduring superb sound quality. Talk about withstanding the test of time.
Charles
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Alex, the Shishido 811 circuit is basically uncopyable, since it relies on DC flowing through the secondary of the custom interstage transformer that goes into the 811 grid. Unlike nearly all other audio circuits, this circuit operates the 811 power tube ONLY in the positive grid region ... from zero volts to a substantial positive voltage.
When I met Shishido at the CES back in the Nineties, as technical editor of Glass Audio, I pressed him on this point. In Shishido’s "Inverted Interstage Transformer" designs, the grid voltage swings from zero volts to a higher voltage. It never passes through the zero-bias region (according to Shishido).
This requires DC current to steadily flow into the grid, while the grid is an extremely nonlinear load for the driver stage. There’s only two ways to pull this off: a powerful MOSFET driver with a paralleled current source (MOSFET likewise), or a very special interstage transformer that can tolerate a lot of DC going through the secondary, while current goes through in the opposite direction in the primary. If you did it with MOSFETs the chances of a spectacular explosion would be pretty good. You don’t mess around with transmitting tubes.
Brilliant but the weirdest thing I’ve ever seen. A (very) custom interstage with bidirectional DC current flow. Zany doesn’t begin to describe it. My worry would be matching the current flows to the exact values. Tubes love to drift ... they are not well suited to DC circuits. Tektronix scope designers went to insane lengths to DC-stabilize their vacuum tube scopes, and this amplifier would also require a complex DC-stabilized supply.
How did it sound? I preferred its big brother, the monster 833 amplifier, which was the top-of-the-line Wavac IIT amplifier. That used a hand-selected vintage KT88 from WAVAC private stock as the driver. When you bought the WAVAC 833, they set aside several vintage KT88’s (real British Genelex) just for replacement purposes. Shishido told me that, and I believed him.
I also loved the stunning solid aluminum NC-milled chassis and custom safety glass enclosure for the insanely hot (and very dangerous) 833 transmitting tube with the top cap at many kilovolts. That probably doubled the price, but man, it looked really cool and high-tech.
Transmitting tubes are in the "look but don’t touch" category. In real transmitters, they are behind thick safety glass, with interlocked steel cabinet doors. If they blow up, it’s no joke. The steel doors and safety glass are there for a reason.
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To sum up, we have a circuit with big, simple tubes designed no later than 1939, combined with 21st-Century transformers and power supplies. In that sense, it is a hybrid amplifier, spanning 84 years of time and technology.
(If you want the Blackbird to fly even higher, look to the Emission Labs 320B-XLS or the ELROG 300B with thoriated-tungsten filaments. Those are 21st-Century 300B’s.)
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Hi, Alex!
The original Karna, designed by me, and built in four-chassis format by Gary Pimm (of Portland, Oregon) in 2003, used a 5687 class of input tube. (The 6900, 7044, and 7119 all have the same pinout and similar operating points. The current production JJ ECC99 is similar but has a different pinout.)
I selected that tube for the Amity, back in 1997, and also for the Karna because it had a low plate impedance ... around 2K ... and pretty decent linearity, much better than a 12AU7, which is quite poor and not really suited to driver duty. But I was never entirely happy with the 5687 or the other similar types. I tried just about all of them ... I have quite a stash of 5687, 6900, 7044, and 7119 tubes ... but there was always a bit of glassy, hard quality, nowhere as bad as a 6DJ8, but still there.
There’s nothing wrong with them, again, far better than any 6DJ8, but these are commercial tubes never intended for audio use, and never used in any Golden Age amplifiers, tuners, or TV sets. They were designed for analog computers, commercial radio relay use, and aerospace ... high-end commercial and military applications, at high prices, and not sold in consumer retail channels.
These days they come from military surplus stocks, and only produced in consumer format by JJ as the ECC99. So supplies are getting a little dodgy, twenty years on. Not really suitable for consumer use unless you already have a substantial stash of them, in the hundreds, and all tested and matched, of course.
The 6SN7, and its single-triode predecessors, like the 6J5, 6C5, etc. etc. are famous for their linearity, and they were designed for radio applications in the audio sections of the receiver and power amplifier. Millions were made, in varying quality, but all of them were more linear than the 12AU7 successor, or the quite different 6DJ8 (which was an RF tube never intended for audio). So there’s nothing rare or exotic about the 6SN7, unlike the 5687 family.
I mention "designed for audio" as if it is something special. Well no, not really. But if a tube was originally designed as an RF amplifier, it would never be checked in production for linearity, since RF circuits don’t care about linearity. Nowadays, of course, 6DJ8’s are never used for RF circuits, and only for audio, mostly high-end audio, not guitar amps.
This has the practical effect that vintage (NOS) stocks of authentic 6DJ8’s can be all over the place for in terms of linearity, since that’s not a controlled manufacturing parameter and would have no effect on its performance plugged in to a 1965 RCA color TV set or FM tuner, the task for which it was designed.
In practice, using Gary Pimm’s custom-designed spectrum analyzer with 140 dB resolution, we found that upper-harmonic (5th on up) spectral shapes mostly reflected a given manufacturer, and was surprisingly consistent from year to year. Gary Pimm and I have both worked in manufacturing for big and small companies, and we surmised that consistency reflected the special jigs that aligned the grids, and different manufacturers used slightly different techniques to align the inner structure.
Although tube models are intellectually useful in a design phase, they model ideal tubes that are only available as beautiful Platonic Ideals in a store somewhere in Heaven. Sadly, we humans on Earth have no access to that store. No Platonic Ideals for us.
The tubes we can actually buy were, and are, hand-made by skilled human beings, not robots. The grid pitch is not perfectly uniform, the grids are all tilted just a little bit, electrons escape out each end of the structure, the list of imperfections (and departures from ideal models) goes on and on. These tiny imperfections result in high-order harmonics that can be seen in a high-resolution spectrum analyzer, and heard in a good audio system.
Surprisingly, these departures from perfection are consistent with the manufacturer. That’s why Gary and I surmised it came down to small variations in assembly technique, or even the individual assembler. Again, tubes were never assembled by robots, and still aren’t today. The assembly was, and is, semi-automatic at best.
Frame-grid tubes, like the 6DJ8 or more exotic WE417A, are even more difficult to make consistently, and it doesn’t matter in a high-gain RF circuit anyway. Using them in an audio circuit is a roll of the dice, especially if there is no feedback to tidy up the mess. Harmless in a preamp at millivolt levels, not so good in a power amp.
For all these reasons, Don and I decided to move away from the 5687 family. (Neither Don nor I are fans of miniature 9-pin tubes anyway.) True, the 5687 family greatly simplifies the interstage transformer design, since the plate impedance is about three times lower than 6SN7, but that low plate impedance is the result of high transconductance and more difficult assembly procedures. Part of the reason that direct-heated triodes have a much cleaner spectra is they are big and easy to assemble ... as dumb as that. We’re talking late Twenties to late Thirties technology here ... precision assembly was very difficult back then, especially on a production basis.
Effectively, Don and I took the ultra precision out of the tube and put it into the transformer designer and assembler. That’s where the 21st Century tech comes in. These transformers could not have built in 1939, when the 6SN7 first came on the market (replacing single triodes). The 5687 family dates from the mid-Fifties, with transformer design still in the build-and-try phase, like the loudspeakers of the day. Computer modeling was still decades in the future.
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Here is an example of direct coupling:
http://www.single-ended.com/Lagarto/shishido/811A.png
Is 6f6 grid load easy enough for SRPP?
ECC82 can be changed for 6sn7.
@alexberger It needs grid stop resistors. Its usually a good practice to bypass the output of a regulator like the LM317 with some kind of capacitance to improve transient response; 1uf is a good typical value. You might want a bit more capacitance after the 180 Ohm resistor since any voltage variation where the regulator meets the driver transformer can cause intermodulations. The 6F6 grid is no problem for the SRPP and you should be able to sub a 6SN7 since their characteristics are so similar.
Here's something to think about: many tubes perform better when a cathode bypass capacitor is employed. Yet there isn't one in the output stage. To install one you would need two parts, one for each side of the filament and its 30Ohm resistor (plus pot). It would not have to be a high voltage part but it would need to be a fairly high capacitive value- perhaps about 2,200uf on each side. 10Volt parts should work nicely since the voltage across the resistors and pot might only be 2 Volts.
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Hi @atmasphere ,
I meant that tube with less internal impedance with a transformer load with the same inductance has wider bandwidth in bass. I agree unfortunately 5687s OTOH.
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With 5687 you can get even wider bandwidth especially on bass.
@alexberger All tubes have response to DC. If you hear a difference with the ability to play bass, its not the tube type that's causing it. The reason we like to use 6SN7s is there are several that are new manufacture and most of the NOS types are great so there's a lot out there to support the design. 5687s OTOH aren't being made AFAIK. I like the plate curves of the 6SN7 better too- a bit more linear which is helpful in a zero feedback design.
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Hi @donsachs ,
I have two questions about your new amplifier:
1. When you decided to go to an interstage transformer after the input stage, why did you stay with 6sn7 and didn't move to something like 5687? With 5687 you can get even wider bandwidth especially on bass. What is the advantage of 6sn7 over 5687?
2. Did you try direct coupling between input and driver tubes?
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And the colorations from different part selections are not the same as SET. This makes sense when you reflect on it ... the balanced circuit is cancelling most, but not all, colorations, and the residue left over can be unwelcome and surprising.
SETs produce a quadratic non-linearity, which in turn makes for a fairly prodigious 2nd harmonic. If set up properly (if you see what I did there) the succeeding harmonics will fall off on an exponential curve.
A fully balanced amp will have even ordered harmonic cancellation, so the resulting non-linearity is cubic in nature. So the 3rd harmonic will be dominant, but at an amplitude slightly less than the 3rd is when seen in an SET. Succeeding harmonics should also fall off on an exponential curve, but it will be one with a different exponent- the harmonics will decrease in amplitude faster as the order of the harmonic is increased. The reason for this is distortion is compounded less from stage to stage throughout the circuit. Since distortion obscures detail (in addition to altering the tone colors of instruments) this makes for a more detailed presentation, with less harshness and brightness than an SET can manage, which is saying something. In either case of SET or fully differential, the lower ordered harmonics will mask the higher orders.
The ear treats the 2nd and 3rd in much the same way- in that it finds them innocuous. FWIW, a properly functioning tape recorder will produce a 3rd harmonic as its primary distortion component also, so we have a pretty good indication on that alone that the 3rd isn’t a problem.
Its also worth mentioning that a fully balanced circuit, running zero feedback, will produce a greater percentage of usable power- close to or exceeding 95% of full power, while an SET is doing well to make 25% usable power.
Of course setting the correct operating point is critical in either circuit. In a balanced differential circuit, the best operating bias point will usually be just above the maximum gain that the differential circuit can do, with symmetrical clipping. If this rule of thumb is followed, there will be no unwelcome ’left over residue’. To achieve this, a proper Constant Current Source circuit should be used- a simple resistor to B- is inadequate owing to the rather low mu that most tubes have. It will be found that the current sensing resistor that is tied to B- is quite critical. I usually set it slightly high to allow for variations in the tubes themselves.
A good quality CCS cannot be made using a single tube or transistor- you’ll need at least two devices. Semiconductor CCS circuits can work exceptionally well and offer the benefit of no likelihood of tube damage as the tube warms up due to cathode filament arc-over concerns. If you work with differential circuits you find out quickly how important the CCS really is. In most solid state amps I’ve seen the CCS leaves performance on the table. If it is not optimized, the differential circuit will not achieve its best gain, distortion or Common Mode Rejection. So its important to get this bit right.
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I have explored all sorts of variants of this basic circuit for about two years now, and have learned what I like and understood why. This is a deceptively simple circuit, except that there is subtle complexity. Often, we get caught up in the conventional way of doing things, and then spend all our time refining them to try and hide all of the problems and colorations. Sometimes we need to think outside the box. I left traditional power supply design for tube amps probably 5 or 6 years ago. There are huge threads on the sonics of various capacitors in speaker crossovers and amplifiers. Trust me, I have heard many of the top caps. The best cap is no cap. It took time to arrive at that notion. Time that was wasted auditioning top capacitors instead of thinking outside that box. As we discussed earlier in the thread, your alternatives are to either directly couple tubes, or use transformer coupling. Both have advantages in different sorts of circuits. In this circuit, transformer coupling is best because of the balance at all stages, which is the key to this amp. Think of it as constantly canceling distortion and balancing itself. But you cannot just buy off the shelf transformers from company L or H or E or whomever. For this circuit you need interstage transformers that are optimized for the loads they see and can give flat response from below 20 Hz to above 20 KHz with little or no phase shift. That is no easy task, and you have to work with a very experienced winder and it takes some prototyping and testing. Well, over a year of prototyping and testing. It is not an accident that the best of the vintage tube amps had killer transformers. So, as Lynn elegantly stated above, this circuit is totally revealing and even though you have relatively few parts in the signal path, they have to be really good parts, and some of them have to be custom designed to get the best performance. The audio circuit looks trivial, except that it isn't trivial to get that hand full of parts to work really well together.
Two years ago I would never have built a cathode biased amp, and I resisted that notion at first. Then I thought outside that box and understood what this circuit was doing and why the cathode biasing was best. I had the notion that all cathode biased amps sound slow and syrupy and deliver far less power than a fixed bias alternative. Well, in a conventional circuit that is correct, but not in this one.
The power supply is also highly optimized and we use some tricks to further isolate it from the audio circuit, and ways to isolate the input tube from the driver circuit power supply. Of course the 300b supply is separate from the driver and input supply. The filament supplies also are regulated and isolated from each other. So there is a fairly complex, yet very conservatively rated and reliable power supply that drives this subtly complex, but fairly simple circuit. If you change one thing you instantly hear it. So, in this amp there was quite a bit of tuning, again, to remove coloration. The result is a transparency I have not heard in any other amp to date, plus the ability to drive quite an array of speakers. Again, don't let the 27 watts fool you.
Is this a perfect amplifier? Of course not. There is no perfect amplifier. Again, if I were to magically create a straight wire with gain and play it for 100 folks, half of them would love it, and half would probably hate it. This amp's hallmark is utter transparency and tonal correctness. The piano is in the room with you and you can easily tell a Steinway from a Bosendorfer. I realize that most everything I have heard to date is quite colored, or if fairly neutral, lacks the resolution of this circuit built this way with these parts. This circuit is uncolored, transparent, and highly resolving, and has a boatload of driving ability. It packs serious punch. Unlike a single ended amp, where the idea is to tune the coloration inherent in the design to suit your taste, this project was about removing the coloration so the circuit could really shine. Trust me, it doesn't sound cold and clinical. It sounds like music with all over the overtones in the instruments, the inflections of the voices, etc... It is not sterile sounding at all, but rather it invites you into the music.
To each their own, but this amp is wonderful to my ear and ready for production and this thread has been an insight into our design choices, and the journey. Others make other choices and that is as it should be. I have no desire to build a 200+ watt amp to drive a very difficult speaker, but I do want to drive most speakers in most rooms.
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Reflecting on a recent phone conversation with Don (he’s in BC Canada and I’m in Colorado, a bit north of Denver), I suggested that SET amps are kind of like a paint-box, and a much more fun way of tuning a system than messing with cables. If you are DIY’ing, there are many ways of changing the tone color ... which coupling caps, what kind of passive power supply, which rectifiers ... the options are endless. And a lot of fun if the amp is on a breadboard and you can solder in new parts in a few minutes.
The characteristic SET sound works in your favor, giving a lot of leeway with parts selection. And the amp is fundamentally simple and easy to understand, a godsend when you are tuning with many variables. Like I said, a paintbox. When you learn painting, you learn color harmony and the art of mixing. Nothing teaches faster what XYZ cap sounds like than heating up the soldering iron and swapping parts.
A balanced amp is a harsher taskmaster. Yes, more transparent, potentially by 20 to 30 dB, but not nearly as forgiving. Colorations can sound pretty ugly if the wrong part is in the wrong place. And there is no feedback to tidy up the mess. Maybe more like working with an airbrush, or transparent watercolors, instead of pigments. There’s still balancing to be done, but the high level of transparency, and lack of feedback, exposes everything. I found this out the hard way with the original Amity amplifier back in the Nineties.
And the colorations from different part selections are not the same as SET. This makes sense when you reflect on it ... the balanced circuit is cancelling most, but not all, colorations, and the residue left over can be unwelcome and surprising. The SET experience can be a very rough guide telling you which parts sound really awful, but it will not tell you which sound the best.
This mirrors working with speakers. As transparency goes up, tolerance for coloration goes down. In the absolute sense, this is wonderful, because now you’re really hearing the music. In a way, I’m not surprised the simplest topology won ... less to go wrong, and with the most efficient plate-to-grid coupling.
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No, the comparison isn't really fair. The little Chinese amp is a nice value for the money, but it is a single ended design made of, shall we say, "cost effective" parts. It probably makes 7 or 8 watts per channel. The mono 300b amps are built with cost no object parts and custom wound transformers, with state of the art power supplies, and they really are about 27 watts/ch. They cost multiple times what the Willsenton does. The Willsenton has a chance on efficient speakers. It doesn't have a prayer on a speaker presenting an inefficient and difficult load.
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I certainly confirmed that the 300b monos will drive the Dyn's to the full measure of their sound in my 13'X26'X9' room. Lynn points out the need for speakers of a certain design, as he listed examples off, to have lots of power (current) to those speakers with amps which have a high damping factor. My main speakers are Cube Audio Jazzon single driver speakers which by design, require an amp with a very low damping factor, such as most tube amps I can imagine and ss amps like the Enleum 23R and most of the Pass Labs First Watt amps.
I am very happy that these 300b monos drive both my Cube speakers as well as the Dynaudio C-1's so I conclude that these monos are much more compatible with a larger variety than probably typical ~8wpc 300b amps would drive.
I started this thread with reference to my Willsenton r300 300b tube amp with 8 wpc. I swapped that into the rack to try to drive the Dyn's and it ran out of clean power pretty darn quickly, while the 300b monos cruised along easily and cleanly at very high SPLs. It is a very unfair comparison, but here it is... a 300b tube amp that will drive a lot of speakers that many others likely would not. This fact dramatically broadens the selection of speakers one could pair with these fantastic amps because having a tube amp that only pairs with high efficiency speakers is awfully limiting as one's choice of speakers today might not be the same speakers you have down the road.
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Hi @lynn_olson ,
According to your comparison between RC coupling and IT coupling, IT coupling between input and driver stage should increase slew rate (dynamics, trancients, speed) similar as it does between driver and output stage. Isn't it?
On the other hand, the input stage doesn't have a big voltage swing, and as result of it a linearity issue. It shouldn't drive the driver stage in class A2. 6sn7 with IT has less wide bandwidth compared to 6v6, 6f6, 45 that can be used in driver stage (in my amplifier 6sn7 18Hz-35KHz vs 6f6 6Hz-95KHz -3db ).
Is It wars to try interstage transformer after the 6sn7 input stage in my 300B SET?
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I suspect any electrostat is fine. The weird capacitive/inductive load will not bother a zero-feedback amplifier.
What will not work is something like Wilson Audio or B&W speakers, with low efficiency, a band split into three or more drivers, complex crossovers, and big woofer arrays. Or MBL. They really do need 200 to 500 watts with a high damping factor (lots of feedback). Transistor amps, in other words.
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The regulated filament supplies for each 300b can deliver 3 amps, and the tubes draw 1.2 amps, so they are coasting along, like the B+ supplies. As Lynn described, the amp is lightning fast. I would love to audition the Elrog 300b in it. I have to admit the Linlai replica WE300b is just superb though. I cannot imagine it needing more slam, but my speakers are 97 dB and an easy load, so the amp is just cruising along and is really not stressed. Whitestix told me he tried a pair of Dynaudio C1 on his shoebox earlier edition amps and they drove them with no trouble at all in his smaller room. That is the third or fourth time someone has put a more difficult speaker on the mere 27 watt amps and they had no trouble. I don't suggest you power some horribly inefficient speaker with these amps, but don't let the 27 watt rating fool you. Any rational speaker load is fine.
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What separates the Euro super tubes is massively higher electron emission. For example, what separates the EML 320B XLS from the EML 300B XLS is 50% greater heater current, with correspondingly higher emission. But even the EML 300B XLS, although consuming the same heater current as a standard 300B, has significantly higher emission.
Same story for the thoriated-tungsten Elrog tubes. Thoriated-tungsten is used in high-power transmitter tubes, not "radio tubes" with the typical coated filaments. The peak emission characteristic will sound more like the Eimac giant transmitter tubes than a 2A3 or 300B.
What’s fun about these is they are plug and circuit compatible with classic 300B’s, but they are not replicas. Not at all. They are modern high-power designs, optimized for both linearity and peak emission. And they sound like it ... powerful and blazing fast.
But ... the amp circuit has to match the peak power and speed, or you never hear what they can really do. They sound like just another 300B with generic RC-coupled drivers, for example, and the potential is mostly wasted. Give them a driver that is a powerful Class A amplifier that is transformer coupled, and whoa, stand back. It’s not a meek little flea-power amp any more; put on Mahler or Mastodon and frighten the neighbors.
One unique feature with transformer coupling is 97~98% of the plate power is presented to the following grid. The plate power doesn’t disappear into a resistor, choke load, or MOSFET transistor current source, or jump through a plastic-film dielectric in a capacitor. It’s right there at the grid, with only tiny losses in the transformer.
This is audible as vivid and tactile tonality, speed, and power. Basically, less electronic cruft in the sound, which is what you’d expect. Don and Whitestix will attest to that.
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Unlike the 845, which relies on voltage to deliver more power, the super tubes provide more current thanks to higher emission, and plates are more
Thank you Lynn for your reply.
The EML tubes are a bit larger and heavier compared to my other 300b tubes. The glass envelope is thicker as well. It does possess a heavy duty aura and appearance.
Charles
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The operating points are conventional, well within standard 300B specs, with the plate at about 75% of max rating (which typically gives maximum life). But if you really want to slam the amplifier, well, you can. There’s nothing stopping you. The amplifier gradually turns into a limiter, with the output current saturating on peaks. If you treat it like a guitar amp, running into heavy distortion for hours at a time, the output tubes will wear out more quickly (the same as a guitar amp). Momentary overload, as in music playback, is benign.
If peak power, and particularly, peak current, is important to you, the enhanced-rating European tubes are the best choice. Instead of 40-watt plates of the classic 300B, they have 65-watt plates, and peak current emission is 50% higher, or even more. Although they are related to 300B’s and bias the same, they are in the KT120 class, not EL34 class. Unlike the 845, which relies on voltage to deliver more power, the super tubes provide more current thanks to higher emission, and plates are more generously sized, keeping them cooler.
(User note: Not recommended or guaranteed for guitar amp use. 300B’s are physically fragile and can be damaged by high vibration, excess heat, or operation with a tilted chassis. If you own an electric guitar, get the right amp for the job.)
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@lynn_olson
400 mA might not sound like much until you realize it’s at 480 volts, and the output transformer multiplies the current
To give context to these stated parameters is the Blackbird easy, average or hard on the 300b tube? How is tube longevity affected by the circuit’s operational points?
Charles
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Considering that each Blackbird B+ regulator can crank out 500 mA peaks, and 400 mA steady-state, there’s plenty of current the super tubes can use. The only thing limiting the Blackbird is peak emission from the 300B filaments ... nothing else.
400 mA might not sound like much until you realize it’s at 480 volts, and the output transformer multiplies the current 28.7 times (on the 8-ohm tap).
(The 4-ohm tap has 40.2 times current multiplication, for those with current-hungry electrostatic speakers.)
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@lynn_olson
The Mark I Karna's thrived on a matched quad of Emission Labs 320B-XLS, but that was also a serious investment. The European super tubes are something else
Agreed.
I had early production Elrog 300b tubes that sounded wonderful but had reliability issues. Fortunately that has all been sorted out with the current production Elrogs under the ownership of Thomas Mayer.
I have used the EML XLS 300b tubes over a 9-10 year period and they are utterly rugged reliable workhorse tubes that sound absolutely splendid. Expensive but worth every dime.
Charles
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Yes, I would not mind a matched quad of Elrog 300B's with their thoriated-tungsten filaments. That would be something quite wonderful. The Mark I Karna's thrived on a matched quad of Emission Labs 320B-XLS, but that was also a serious investment. The European super tubes are something else.
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Charles, I am not sure what Meyer would think of the Blackbird! There are some elements of it that are aligned with his thinking and others that are radically different. I would be interested to hear some amp of similar power that he has built in the same system as the Blackbirds. It would be fun:) As I said above, Thomas Meyer builds SERIOUSLY good amps. The man has his own tube factory! I would love to try a quad of his 300b tubes, but they cost serious money. Maybe one day.... I bet they are really good though.
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@lynn_olson
I met Thomas Meyer at the 2004 European Triode Festival (I was the invited keynote speaker). He’s a lot of fun, and super knowledgeable about tube history. He, too, is a transformer enthusiast, particularly with modern transformers
Spot on! Definitely transformers in addition to tubes. I’ve learned much reading his site over the years. I bet that he’d admire the Blackbird amplifier effort and result.
Charles
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I met Thomas Meyer at the 2004 European Triode Festival (I was the invited keynote speaker). He’s a lot of fun, and super knowledgeable about tube history. He, too, is a transformer enthusiast, particularly with modern transformers.
I’m super happy that he made the transition from hard-core hobbyist to the ultra-high-end commercial world. He’s set an example for all of us. It helps that Europe has a fine tradition of artisan-built audio, with wealthy patrons who appreciate the arts.
When I was Switzerland as the guest of Christian Rintelen (host of the 2004 ETF), I visited the museum in Zurich, and astonished to see wooden clocks that were a thousand years old ... and still in working order. The traditions of technology in Switzerland and Germany are ancient, and a deep part of the culture.
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Thomas Mayer builds seriously good amplifiers:) I have his octal phono circuit saved, and one day I will build myself another phono preamp with my power supply and his signal circuit. I bet it will sound very good. His tube blog is wonderful.
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@charles1dad when Thomas Mayer first started building tube amps for his personal use, he built them for 3 phase power. He also loves vinyl, even though his regular job was digital electronics.
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@invalid
I really like Thomas Mayer's 300B amplifiers
I have long admired Thomas Mayer audio products (Particularly his power amplifiers). Given what I have learned regarding the Blackbird 300b PSET mono block amplifiers it’d be most interesting and informative to hear a comparison. Two very different approaches. I have no doubt that both are genuinely sublime products.
Charles
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@alexberger Spatial Audio Lab will be in charge of presentation at shows. I know we are returning to the Pacific Audio Fest in Seattle again next summer. Beyond that I don't know. We are focusing on getting the preamp and amps into production by year end. They are also finishing work on their Q series speaker. A prototype of that was shown in Seattle and they expect to release that by the end of the year. So lots going on. Spring shows may be a bit soon. Given they are in Salt Lake City, I suspect they will focus on American shows though, so probably not Montreal. You never know though:)
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I really like Thomas Mayer's 300B amplifiers, but I don't have deep enough pockets for that. You two seem to be on the right track with your 300B, and it's more affordable than his.
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Don did a marvelous job here. It’s actually quite functional, with a clean and direct front-to-back signal path, and with all wires in the balanced circuit equal length (yes, he went to that much trouble). You can infer that from the top plate.
The power supply side is equally tidy, with two independent B+ regulators, one in front of the other, and the three low-voltage regulators on the other side of the power supply section, all supplied by the custom Monolith power transformer. The soft-start circuit in the back of the amp keeps incoming AC power away from the front-panel power switch, as well as protecting the tubes and regulators from AC line transients.
The amp is considerably simpler to build, because signal flow is obvious, and color-coded wiring is used to keep track of polarity. It is also the same 18"/457mm width as the matching Raven preamp.
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Hi @donsachs ,
Will you present your new amplifier in the Montreal Audio Show in March?
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Quite attractive with an understated elegant aesthetic.
Charles
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This is very close to the production version. It is simpler and more straightforward to build than the shoebox format shown at the PAF show ... more spacious, more direct layout, and the power supplies are confined to their own section of the amp, on the right side of the chassis. The vent holes for the twin B+ regulators are visible on the right side of the amp, next to the VR tubes.
The audio-only circuit is on the left side, with very short signal paths from 6SN7 -> interstage 1 -> matched balanced 6V6 -> interstage 2 -> matched balanced 300B -> Monolith output transformer -> speaker jacks. The input selector switch bypasses the input transformer when XLR is selected. Compared to the show amps, there are actually fewer parts in the production version, with a very simple signal path from input to output. From input to output, there are only wires, transformers, and triodes, in a fully balanced circuit. No coupling caps, no plate-load resistors, no plate inductors, and no dynamic loads.
One subtle difference is each single grid is driven by a pair of balanced plates, so distortion and noise are minimized in every stage of the amplifier. In the show amps, each 6V6 grid was driven by the corresponding 6SN7 plate. In the production amp shown here, each 6V6 grid is driven by a balanced pair of 6SN7 plates, thanks to interstage 1. I was doubtful a good interstage could be made for the 6SN7, but our transformer designer came through with performance from 18 Hz to 32 kHz. Close collaboration with modern transformer design is what made this possible.
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I managed to update my ancient website with the beginnings of 300b project page. There is a photo of the final prototype of the Blackbird 300b monoblock amp at the bottom of this page. Spatial will change the look a bit, but dimensions and layout will be the same. The amps are optimized for sonics and short signal path.
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This thread, with over 500 posts I think, has allowed interested people to understand the evolution of their 300b amps design compared to other designs. A rare insight into the machinations of amplifier design, which to be honest Roger Modjeski from RAM was stellar at doing as well. Lynn astounded me with an earlier post that some designers, maybe it was with speakers, design to specs and that's it. What Don and Lynn have done is an endless process of swapping not only components, but topologies, a very interactive process, subject to both measurements and equally or more important, endless comparative listening observations. This to me is the optimum process to design gear and the process has to be exhaustive, but both Don and Lynn have done this for decades... this iterative process. And, to wit, they have shared their excursion on this thread in masterful detail to inform all of us.
My pre-pro 300b amps are a glimpse of heaven with my Cube Audio speakers and it is hard to imagine how much better they will sound when Don updates them to the final version, but I am certain his promises will be fulfilled.
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Yes, we are pretty much back to an updated Karna circuit with more modern power supply topology, and custom wound iron everywhere. I listened to every possible variant of the circuit and it is obvious that removing all coupling caps and going with all transformer coupling walks all over any other variant in pretty much every way. We are now using custom wound Monolith Magnetics iron for power and output transformers, and Cinemag interstage transformers. The chassis is much wider than the shoebox amps displayed at the Pacific Audio Fest in Seattle. Spatial will change the look slightly, but the layout and size will be the same. There are reasons for the layout, which makes for very short signal path at all sections and also has complete isolation of the power supply from the signal part of the amp. They are just about ready to build in quantity and I would expect them to be available in November or December if all the vendors meet their production schedules for parts. I will say that I cannot listen to anything else now. I am spoiled..... everything else sounds dull and coloured to me now and I have pretty much cleared my house of all other amps and preamps. We shall see what others think!
I cannot seem to post a photo of the final prototype, but will upload to my old website and see if I can post a link to that photo eventually.
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