if you can fit a toroid in there, then look at the antek site. They have small filament transformers and may have what you want. AN0205 is 25VA with dual 5V windings so will give you 2.5 A x 2. They are fine for what you want. they also sell transformer covers. If you want to regulate the filament supply for each 300b then you can get a 2 x 7V.
300b lovers
I have been an owner of Don Sachs gear since he began, and he modified all my HK Citation gear before he came out with his own creations. I bought a Willsenton 300b integrated amp and was smitten with the sound of it, inexpensive as it is. Don told me that he was designing a 300b amp with the legendary Lynn Olson and lo and behold, I got one of his early pair of pre-production mono-blocks recently, driving Spatial Audio M5 Triode Masters.
Now with a week on the amp, I am eager to say that these 300b amps are simply sensational, creating a sound that brings the musicians right into my listening room with a palpable presence. They create the most open vidid presentation to the music -- they are neither warm nor cool, just uncannily true to the source of the music. They replace his excellent Kootai KT88 which I was dubious about being bettered by anything, but these amps are just outstanding. Don is nearing production of a successor to his highly regard DS2 preamp, which also will have a unique circuitry to mate with his 300b monos via XLR connections. Don explained the sonic benefits of this design and it went over my head, but clearly these designs are well though out.. my ears confirm it.
I have been an audiophile for nearly 50 years having had a boatload of electronics during that time, but I personally have never heard such a realistic presentation to my music as I am hearing with these 300b monos in my system. 300b tubes lend themselves to realistic music reproduction as my Willsenton 300b integrated amps informed me, but Don's 300b amps are in a entirely different realm. Of course, 300b amps favor efficient speakers so carefully component matching is paramount.
Don is working out a business arrangement to have his electronics built by an American audio firm so they will soon be more widely available to the public. Don will be attending the Seattle Audio Show in June in the Spatial Audio room where the speakers will be driven by his 300b monos and his preamp, with digital conversion with the outstanding Lampizator Pacific tube DAC. I will be there to hear what I expect to be an outstanding sonic presentation.
To allay any questions about the cost of Don's 300b mono, I do not have an answer.
875 responses Add your response
Yes, plenty good enough for the job. Don’s comments above are right on the mark. What you want is isolation. Stage-to-stage, and isolation of the critical filament supply. And if you really want to get hardcore, make sure all the cathode circuitry, of each section, comes down to a single star ground on the main ground bus-bar. So, star ground for 6SN7 cathode components, a few inches away, the star ground for all 6F6/6V6 cathode components, and another few inches away on the main bus-bar, all the cathode components for the 300B. The idea is keep all audio currents local to that stage, and to that stage only, and only have DC return currents on the main ground bus-bar. You would be surprised how few high-end components do this. The presentation below is for advanced students. You know who you are: |
Its been a long time so my comparisons have a set of our M-60 amplifiers in between if that makes sense. The M-60s were overall less colored by distortion with wider bandwidth and a greater sense of palpability. Peter Moncreif had us do a direct comparison at the show. So the M-60 compares to the class d in that they have a similar tonal balance- with no grain or edginess in the mids and highs. The big tell that we hear and that customers report is that the class D is more focused in that its easier to hear what’s going on in the rear of the sound stage, pick out details and that sort of thing.
If I can add to this, make sure that the grid circuit and cathode circuit of each tube employ a single wire that goes to ground for both of those circuits. If you are using terminal strips, to do this the grid resistor and cathode resistor would tie to the same point and then a single wire to ground is used. Tubes amplify differentially, which is to say the grid and cathode are out of phase with each other. So if a single wire is used for ground and noise is injected into that wire, it will be rejected by the tube. If you use separate wires the tube will be more noise susceptible. |
Just a quick update that Don and I are continuing to refine the Blackbird, with a bit of Raven and Karna Mark I thrown in. The chassis will be 18" wide to give a more spacious layout, a simplified build procedure, and visually match the Raven. The circuit continues to be balanced throughout, with a tube lineup of a 6SN7, a pair of matched and balanced triode-connected 6V6, and a pair of matched and balanced 300B’s. The power supplies (both of them) have a slow-start circuit that protects against hot-start transients if the AC power flickers off for a second or two, as well as controlling tube warm-up. Sonically, Don and I are prioritizing depth and realism of tone color, like 300B SET amplifiers, combined with a clarity and directness usually associated with high-performance Class A and Class D transistor amplifiers. |
Since nobody is reading this thread right now, I’m going to throw out my wish list, my note in a bottle, to the wilds of the Internet: * I’d like to see LinLai or JJ or any of the other tube vendors, try something a little out of the ordinary. A true triode, using an octal KT88 socket, that biases up exactly like a triode-connected KT88. An indirectly heated triode, in a KT88 package, with only three elements ... cathode, control grid, and plate. With no screen or suppressor grid, and the control grid correctly spaced so the whole tube mimics a triode-connected KT88, so it can plugged directly into a KT88 socket in an existing amp and work right away. What is the benefit over a standard KT88? Well, with no useless screen or suppressor grid, the one remaining grid can be optimized for lowest distortion ... in particular, the lowest proportion of high-order harmonics, like a direct-heated triode. It’s not the direct heating of the M-shaped filament that’s responsible for the very low distortion of DHT’s (compared to triode-connected pentodes and beam tetrodes). It’s the clean, uniform grid structure, and the carefully chosen spacing from the cathode (or filament). So there’s no reason a purpose-designed true triode can’t be designed to fit a standard KT88 (or EL34) socket that has the same DC bias characteristics as it’s more complex brother, but also much lower distortion. Literally, a simple plug-in improvement for all the hundreds of thousands of conventional PP-pentode amps out there. No change in bias, no change in cathode circuit, no change in fixed-bias operating point, just lower distortion, ideally approaching DHT performance if the grid is correctly designed. You could call the new tube a TR88 to distinguish it from a KT88, while signifying it is plug-in compatible (thanks to the same DC biasing). Or TR34 if it replaces an EL34. |
@lynn_olson - that’s a cool idea Lynn. There are certainly plenty of audiophiles out there with KT88 amps that are running them in triode mode that could benefit from these, not to mention folks with KT88 amps that don’t currently have a way to run triode connected. i don’t currently have a KT88 amp, but would probably build one if such a tube were available. |
People have gotten the weird idea that somehow the filament of the 45, the 2A3, the 300B, and the 845 are responsible for the ultra-low distortion, and the super-vivid tone color, of the DHT family. Wrong. It isn’t. It’s the grid. DHT’s have a physically large grid, well spaced away from the whirling cloud of electrons called a "space charge". Surprisingly, electrons are not directly emitted, pass through the grid, then strike the plate. Instead, they whirl around in the space charge, find a passage through the venetian-bland repelling field of the grid, and are accelerated to the plate. It’s the grid geometry that sets not only the DC characteristics of the tube, but also its linearity (especially high-order terms). This is the most critical part of the entire circuit. If you need pentode or beam tetrode characteristics, fine, you’ll have very low Miller capacitance, very high output impedance, and easy drive characteristics. This makes an excellent RF modulator, where distortion doesn’t matter. But if low distortion comes first, and you’re not asking for 20 to 50 dB of feedback to linearize the whole amplifier, a true purpose-made triode should have the lowest distortion. Since there are already lots of octal sockets in PP power amps, why not make a special triode tube just for them? There is some design work to optimize the grid structure so DC biasing is the same as a triode-connected pentode, but the absence of all those other grid wires should help. If the design is good enough, it could rival the 300B without the hassle of direct heating and the complex filament circuit. |
@lynn_olson : that's such a good idea! I'd buy a set for my pair of Dynaco Mk3's! An octal-base power triode would sell well! |
nice wish list.... unfortunately, the power race is on to build even more powerful KT88 to get KT120 and now KT150. They generate more watts if the amp can be biased to run them, but they don't sound as good as a good KT88. I think it is the watts race that is driving development though. What Lynn suggests would be really interesting and I would certainly try working with such a tube. |
The sonics would be interesting. The target device would of course be the 300B, but with an octal socket with a 6.3V indirect heater, and KT88 biasing. The 300B is more physically fragile than a KT88 because of filament sag, which can happen if the amp is tipped on its side while the filament is hot. If it sags enough, the filament will touch the grid, and ZAP! the tube turns into a full-power diode, which destroys it and the cathode circuit. Indirect heated tubes have the heater coiled up inside the cathode box, or cylinder, so it won't go anywhere if the amplifier is tipped on its side. This is why guitar amps are so rugged ... it takes enormous abuse to damage a KT88. The customer base for a TR88 would be much larger than a 300B. If the sonics and harmonic structure were like a DHT (not a KT88), a lot of people would be interested, assuming the price would be in the KT88 range. If it had a 300B price, that would greatly diminish the market, since it would be far outside the KT88 class. |
Funny you talk about filament sag. I used to restore tons of vintage tube amps when I collaborated with Jim McShane. So I worked on many amps that used the EL84 type tubes. There is a Russian variant that is quite a good sounding tube and you could get them for peanuts. The only thing was that for a series of years they made that tube, they suffered from filament sag. They were tough as nails and worked for years, but it you tipped an amp on its side to work on something while it was running.... bbzzzzt. Fried output tube and usually the cathode resistor as well. You could run any other EL84 type in any direction, but not those Russian ones of that decade or so! You could run them upside down without an issue, but not on their sides. |
Hi Lynn and Don (and the OP!), it’s been great fun watching this thread. I heard from several PAF attendees that the Spatial room was a highlight of the show. A friend and Audiogon member directed me to this thread (Hi Mark!). In reference to Lynn's suggestion regarding a cathodic triode, several years ago I worked directly with JJ Electronic on an octal based cathodic style "300B". This was originally a joint project with Matt Kamna, who also desired an easily implementable cathodic 300B derivative. We tried modifying all the usual suspects (KT88 included) but the best candidate was their EL509S. With that tube we could simply remove G2 and the beam forming plate, tweak the G1 pitch, and you get a viable power triode, with a hell of a cathode to work with...
Grid Curve 300Vp fixed:
Plate Curve -56Vg1 fixed:
Cheers,
|
I am impressed you took the project that far, but the minimum requirement for 10,000 units on an unknown tube is a steep hill to climb. Even at OEM prices, that’s a half-million dollars on a gamble. The "X" tube would have to be very very good, and very very popular, for that gamble to pay off. In other news, the all-IT, no coupling-cap version of the Blackbird is the best version yet. The LC coupling on the 6SN7 is going away and getting replaced with a custom IT with 18 Hz to 35~40 kHz bandwidth. This gets rid of six parts - a pair of 100 Hy inductors, a pair of copper-foil coupling caps, and two 220K grid resistors. No RC coupling, no LC coupling, and no current sources, either as plate loads or in the cathode circuit. The signal path is copper wire, high-nickel magnetic cores, and vacuum tubes. As Don mentioned on a recent phone call, there is no direct electrical coupling between any of the stages, which filters off any RFI incursion before it gets amplified. Homes are much noisier in the RF spectrum than they used to be, with Bluetooth and WiFi everywhere. Tube lineup remains 6SN7, matched pair of 6V6, and matched pair of 300B. Tube rolling is welcome so long as pairs are matched. |
@eddie138 Thanks so much for chiming in. Really interesting post and the economics are of course as you stated. It is not like getting a custom capacitor made where you only have to have 100 of them for an order. I guess that explains why manufactures stick with the known tube types where there will be a market. Now if someone would start cranking out moderately priced 45 tubes......:) But you would have to sell a lot of them to make it worthwhile. |
@donsachs Yes, new tube types are a daunting investment from the manufacturer’s perspective. My top wish list, without a doubt, would include an ST style 45 (which is the driver of choice in my personal PP 300B build). Followed by a round plate 6SN7, and then maybe an 801A (have not tried Mayers versions yet, but the originals are spectacular). Regarding iron, I saw Monolith Magnetics mentioned here. I have had great success with everything I’ve tried from them (OT’s, interstages, and plate chokes). I can’t recommend them enough. Their IT02 in particular is stunning, and defies what I thought was actually possible with magnetic coupling. Getting back on topic, I’m elated to see a fresh PP 300B amp hitting the market. The PP Triode topology has been a personal favorite of mine for many years, and this was most certainly inspired by Lynn’s work/publication of the original Karna, well over 20 years ago now! I have to say I’m really looking forward to hearing the production Raven :) |
I have to agree with Eddie about the ST-style 45. Aside from driver use, the charm of the 45 is that it is really easy to make a superb, low-power amplifier with it. It has the cleanest distortion spectrum of any tube I’ve measured (closely followed by the 300B), but unlike the 300B or 2A3, it is super easy to drive. This makes two-stage amps simple ... in fact, I’ve yet to hear a bad 45 amp, while I’ve heard plenty of not-great 2A3 and 300B amps. Good 300B amps in particular are quite difficult. 45 amps also sound louder than you’d expect ... they easily keep up with 2A3 amps, despite what the numbers say. If 45’s were more abundant and at different price points, they would quickly find a market. I have a dark suspicion that modern 2A3’s are simply below-spec 300B’s, and not related to original 2A3’s at all. Of course, that’s not really true, since 300B’s have 5V filaments while 2A3’s have 2.5V filaments. Modern 2A3’s are quite different than the bi-plate RCA originals ... I’m not sure where they fit, actually. Returning to the 45 triode, I really think people would be surprised just how good 45’s sound, and how crisp and vivid they are. They are very different sonically than 2A3’s or 300B’s. They actually sound more like 845’s than anything else. Very clean and fast, no murk at all, and nothing like any pentode (no hash or grain). If you can get your hands on a good pair (or quartet) of 45’s, they are quite impressive and worth exploring, |
Umm, hard to describe. More "there" there. More sense of a physical presence, and a better feel of the performer’s musical intentions. More hall sound. Most of all, a feeling of the performer being in the room instead of somewhere "out there". Not the usual audiophile verbiage, but it’s immediately evident when you hear it. By now, the Blackbird is very close to the original Karna, but with greatly improved power supplies and a much more practical monoblock construction, which also improves performance. Full credit to Don for pulling off what I thought was impossible. As you approach the highest levels of audio, the sound kind of takes a right-angle turn from the usual path of improvement. Instead of just getting more and more clear and vivid, suddenly, there’s a physical sense of musicians being right there in the room, instead of sounding like a very good recording. The instruments have physical size and feel like you can reach out and touch them. A piano sounds like it’s five feet away, and it sounds BIG. Even "bad" recordings sound like this. Is something being added? No, I don’t think so. Instead, a mechanical quality is no longer there. This phenomenon is well known amongst triode practitioners. It’s not a secret. Unfortunately, it is almost never heard at hifi shows, so don’t expect it there. I’ve heard it several times at the advanced DIY level. Never heard before from any mainstream or transistor system, which is why I started working with direct-heated triodes in the early Nineties, after hearing and reviewing the Audio Note Ongaku and the Herb Reichert Silver 300B. |
These are the kind of conversations I used to have with Harvey Rosenberg, who definitely "got it". Unless you knew him, you didn’t realize his clowning around, and occasional jaunts into speculative metaphysics, was an act designed to chase away mainstream audiophiles (which it did). Of course, I can speculate about metaphysics for hours on end, as folks who met me at the PAF discovered. But the clown act was pure Harvey ... I’m too sobersided to joke around like that. But we did discuss the subtler aspects of musical perception, and how they interacted with culture and worldview. I am not sure where capacitor coloration comes from, or what’s causing the hours-long "burn-in" process. It can’t be measured, and the plausible mechanisms are pure guesswork. But it’s there. In a positive sense, direct-heated triodes, in the right circuit, have very special qualities. Not everything is revealed by the spectrum analyzer, and the usual audiophile lingo falls short of describing what triodes really sound like. Sanskrit or Japanese might be better, but I don’t speak either, although I am conversant with a few basic Zen, Taoist, or Vedanta concepts. English is not particularly good at discussing subtle aspects of perception and consciousness. Now that Don has built a version with zero coupling caps, he’s having the same experience I had when I first heard the Amity in 1996. The Karna was a development of that, and the Blackbird is a development of the Karna. |
Basically what Lynn just said. Once you hear it you cannot go back. Even the best caps cannot touch it. Now, that said, you will not hear this if the rest of your system is not up to the task. You put a $500 DAC in front and a mediocre preamp that uses small signal tubes, and a mediocre pair of speakers behind and you will mask all of this. You are not going to get this with cheapo ITs either. The Lundahls I played around with last year sounded good, but rang at 15KHz. That doesn't mean Lundahls are bad, just that they were not designed specifically for this circuit. So if you get really good quality ITs that are designed for your circuit, then you will hear what Lynn described. These replaced VCap ODAM, and I also tried top shelf Miflex and Duelund JDM copper foils. All are very good caps. None has the realism of the custom wound Cinemag transformer. I have no doubt the Monolith iron is superb and I am about to try their OPTs whenever they arrive. But you need iron of this quality. You wouldn't judge all coupling caps based on a cheapo Solen, nor would you judge all ITs based on something like a Hammond or Edcor. There are levels and to my ear in our circuit (important caveats), the top quality IT is better than the top quality coupling cap used in the LC with a very good anode choke. My 2 cents and feel free to disagree, but it is what I hear. What you will hear in your circuit .... I don't know, and it doesn't make me right and you wrong or vice versa. |
See page 21 for a construction article on a push-pull, zero-feedback 300A/300B amplifier. This is about the same time as Columbia introduced the LP microgroove record, so it’s very early days for high fidelity. What surprises me about this article is that 300A’s and 300B’s were even for sale to the public; they must have been pulling them out of prewar theater equipment At the time of printing, the only records you could actually buy were 78’s, and FM radio was very new. Note the primitive state of tonearms and phono cartridges ... also, 78’s had no standard equalization, so preamps had to cover several curves, including "acoustical" for pre-electronic records. The reign of the monophonic LP record was surprisingly short; ten years later, almost to the month, stereo LP’s were announced by all the major labels, and stereo cartridges and stereo preamps were also on the market. |
This is entirely up to the transformer designer. They need to know the Zout, or Rp, of the tube driving the primary, and the load on the secondary, which will either be a pure capacitance in the 60~80 pF range, or paralleled with a load resistor, typically 100K or so. The method of extending HF bandwidth is interleaved windings, and this falls into the realm of modern computer modeling. Back in the old days, this was cut-and-try, now, it can be modeled. Interleaved windings extend bandwidth, but the interleave pattern has to be carefully chosen so there is no HF ringing into the intended circuit. (This is why they need to know the Zout of the preceding stage and the load of the following stage.) They will want to know your expected bandwidth, power handling within that bandwidth (particularly below 40 Hz), and how much square wave overshoot you will accept. And the DC parameters ... if SE, how much quiescent current does the tube run at, if the circuit is balanced, how much DC imbalance do you expect from the pair of tubes. This affects core gapping, which in turn dictates core size and transformer size. A small air gap linearizes the transformer, but can also double the required core size, which in turn affects HF bandwidth. The DC parameters are critical for the entire transformer design. As you can see, this isn’t a matter of selecting an off-the-shelf part, but consulting with the transformer designer and telling them what you need (and what they can do). They *might* have an off-the-shelf part, or they might not. If not, what is the minimum order, and how long will that take? I haven’t mentioned sonics yet. Aside from meeting minimum technical specs (which you and the transformer designer both agree on), there’s the matter of subjective sonics, and how it fits with the sound you are aiming for. This might sound trivial, but if the amplifier designer has no subjective sonic goal, you will not get there. "Perfection" is not a goal, it’s a marketing term, like "Perfect Sound Forever" for CD’s back in the Eighties. Are you familiar with the subjective difference between RC coupling, LC coupling, active current-source loads with capacitor coupling, and interstage transformer coupling? (With this amplifier design, Don built and auditioned each one.) This is very useful to know as the amplifier is tuned subjectively. Similarly, the driver stage design has a major effect on amplifier sonics, aside from inter-stage coupling. The driver section affects slew rate, HF distortion, and subjective colorations in the mid and upper frequency range. It’s useful to know the sound of a 6DJ8, 12AU7, 6SN7, and a power-tube (45, 6V6, KT88) driver ... they sound quite different from each other, and can dominate the sound of the entire amplifier. |
I apologize for making this sound confusing, but I wanted to give the readers of this forum a taste of what Don has been through. There are many possible ways of getting a zero-feedback amplifier wrong, and from the standpoint of mainstream audio engineering, all zero-feedback amps are wrong ... not just in practice, but in principle. An all-transformer coupled amplifier is especially wrong. The accepted path is DC coupling throughout, using transistors, with lots of excess gain for plenty of feedback. Modern feedforward techniques (Bruno Putzey, THX, et al) can get distortion into the parts-per-million range, so why look elsewhere? Unless your goals are subjective, and you have a weird hypothesis about linearizing each stage, as much as possible, without using feedback. That’s why Don took a gamble on the Karna topology, a circuit out of the late Twenties and mid-Thirties. |
Again, basically what Lynn said. At each stage of the project whether in the preamp or amps I discussed the exact parameters and even shared the relevant part of the circuit with Dave Geren at Cinemag, who is a very experienced master winder of small transformers. He models everything and makes a prototype and then I build with it and listen. Then I give my impression and measurements of the frequency response to him and we do round two where he tweaks the design and that is that. Dave is also a maestro of core materials and interleaving. The result is that all coupling caps are now gone from the input to the preamp to the output of the amplifiers. Certainly you can accomplish this with direct coupling in places. Every approach has strengths and potential pitfalls. One advantage of transformer coupling is the complete isolation of every stage and the banishment of hum, and also far greater immunity to ambient RF. Also, if a tube goes south, or a customer does something odd like put the wrong tube in the wrong place (yes I have repaired things where people did this), or pulls a hot running tube, any potential damage is limited to a very small portion of the amp. Direct coupling can lead to a daisy chain of failures, cascading through your amp. These are considerations for commercial gear, where numerous units will go to many environments over which you have no control. I can build whatever I want for my living room because I know how it works, and I can fix it. But if a commercial product I want reliability. Transformers are very reliable. I will say that I have had the luxury of semi-retirement to spend a over a year with these preamp and amp circuits. That has allowed me to try pretty much every permutation and combination of power supply and coupling topologies. We have finally settled on everything and the final tweaking is about done. When you are trying to make a living selling gear you generally get something that works and sounds quite good, and is reliable and then you make them. When you are semi-retired you can go down every rabbit hole until you find the exact sound you seek. Good enough isn't good enough..... So my comments above are based on this approach. I cannot tell you whether you will like all transformer coupling vs. RC or LC using your favourite capacitors. I much prefer IT coupling, but I had everything custom wound by a very experienced winder, and we did a prototype and final version in each case. I can do this because I have time and am willing to devote some money to the project. For your one off amp you are forced to buy off the shelf transformers, but there are some very good winders out there and if you communicate what Lynn discussed with them, you may well get a very good solution. Just don't expect to pick a transformer off some web site and have it work perfectly. You will have to communicate with the winder. I wish you all success. If you persist, then you can probably get a great transformer for your circuit. I will also say that if you heard the gear at the Pacific Audio Fest in June, you heard prototypes and the set in my living room is considerably better, both preamp and amps. Now they are about done. The cases will be redesigned by the folks at Spatial Audio and the amps are getting physically larger so we can fit one more mod under the hood, which I expect to bring the sound up yet another notch. Then they will be commercially available, probably late Q4 or early Q1. There are already 4 or 5 folks on the wait list at Spatial and I would expect them to have gear very late this year or early next. So if you are on that list, your patience will be rewarded with gear that is considerably better than what was shown in Seattle. |
Lastly, I will say that internet forums are both a blessing and a curse. There are people who tout one amp topology over another, or whether they prefer SS or tubes, or interstage transformer coupling over LC or RC, etc... When you read these things consider the source. There are lots of people who have strong opinions about what sort of coupling to use between tube amp stages, or even whether they should be direct coupled. We all have our biases. You need to figure out whether that poster has actually tried custom transformers for example or whether the opinion is based on some off the shelf midrange transformer. Or the person who states that coupling cap quality doesn't matter much, or they love this or that cap. The question is what have they heard before, what circuit is it going in, etc.... I have had several cherished notions overturned by this project, simply because I experimented thoroughly with different approaches and let my ear by the final arbiter. I started out with the silicon assisted tube amp approach and ended up with all IT coupling, like the original Karna amp. So my point is that you have to keep an open mind. I am definitely a tube guy and solid state makes me unhappy when I listen for long periods of time, but that said, if a pair of Ralph's latest and greatest Class D amps appeared in my living room I would give them a serious audition and chance to shine. I hope to hear them at a show some day. As I said way up above many pages ago, there are many paths to audio nirvana, and we all may have our own. |
@alexberger And just like that you put your finger on why a direct coupled driver can be so effective. No bandwidth issues (able to go to DC) and plenty of drive for a hungry, highly capacitive grid. If the driver is pulled from the amp while its on, the output tube goes into cutoff. Similarly, the power tube does not conduct until the driver tube warms up. You might think the negative voltage power supply to be too expensive to include in the design, but its a lot easier to get right than the design of a good interstage transformer. If you think a type 45 sounds nice, try them in push-pull! |
Don is being modest. The last year, going through the present, really made Don pursue every obscure byway of amp design, building and listening as he went, every step of the way. Don started with an obscure version that I called the Symmetric Reichert, which was literally a Reichert 300B done twice, with a phase-splitter transformer at the input. All RC-coupled. He built that and called me out of the blue, about a year ago. Don then tried separate B+ supplies for the input+driver and output section, and an interstage transformer between the driver and 300B’s. A few months later, Don used a triode-connected 6V6 instead of the hard-to-find 45 driver. Thom Mackris and Don independently tried this at just about the same time, pretty much on the same day. Don (but not Thom) then used active current-source loads for the input 6SN7, instead of resistor loads. That was the Stereo version Don built and shared with the Spatial team and the first customers. Next, replacing the active current source loads with custom Cinemag inductors designed for the purpose, and using the shoebox-format monoblocks that became the show amps. My Colorado neighbor, Thom Mackris of Galibier Design, has been following along in a parallel project, with a SET architecture, but with passive CLC B+ supplies and damper diodes for rectification. That’s where all of us were a month ago ... Don Sacks, the team at Spatial, and Thom Mackris. The latest from Don is an IT between the 6SN7 and the 6V6, replacing six other parts with a much simpler approach ... provided the IT was up to the task, which it is. The IT has turned out to be superbly designed, exceeding expectation, and also making our lives simpler. Don and I have gone full circle, and re-invented the Karna (after trying every alternative), with far more advanced power supplies that were not available in 2003. Don really has tried every topology, one after another, and carefully measured and auditioned each one. RC coupling, active loads, LC coupling, and now, IT coupling. By lucky coincidence, Thom has been walking a parallel path with his SE topology. All four groups ... Don, Thom, Spatial, and myself, have been exploring this zero-feedback approach for several years now. If other folks want to build transistor Class A, Class AB, or Class D, more power to them. Those designs have an entirely different set of challenges that have nothing to do with triode amplifiers. In triode amplifiers, the devices themselves are exceptionally linear, and the appropriate circuits take advantage of that. |
Hi @atmasphere , McIntosh MC30 has the similar cathode follower driver 12ax7 to drive 6L6 directly. The output tube works in fixed bias in this schematics. Isn't it? Can you explain how the output tube bias is self adjusted? |
I’m no expert on the MC30, but it is very unusual. The cathode feedback (from a special tertiary winding in the output transformer) results in very low gain for the 6L6 power tubes, so the driver has to swing 100 volts, putting extreme demands on linearity. And I think it operates in nearly pure Class B, with a very small Class A region. This requires substantial feedback (which it has) to linearize the output section. The Class B operation requires very close coupling between the tertiary and primary windings, otherwise the circuit will have tube cutoff glitches with every zero crossing. So the whole thing is very much a package ... multiple feedback loops, a unique output transformer, a wide voltage swing from the driver, and cathode feedback for the power tubes. Almost the polar opposite of the Brook 2A3 amplifier, which relied on the linearity of the 2A3 power tubes instead of massive feedback. The high-power (30 watts!) Brook amplifier used sliding bias to keep the output section in quasi-Class A. It should be mentioned there was no awareness of slewing distortion at this time, because signal sources had very limited HF bandwidth (12 kHz) and limited peak energy. Phono cartridges were very primitive and could barely track LP’s at 5 grams. When the first writings about slewing distortion appeared in the late Seventies (25 years later), things were very different: moving-coil cartridges with exotic styli were flat out to 50 kHz, and cutterheads could put down tremendous levels on the disc. Tweeters were much better as well. |
Yes. R21 (120K; refer to schematic) sets the bias point of the 12AX7 (which to me seems a terrible choice for this application- a 12AU7 or 12AT7 would have suited better), which in turn sets the bias point of the power tubes. The operating point it based on the idea that as the tubes weaken, at some point you just replace them, rather than readjusting the bias as the tubes age. The output power is not a function of the bias- its affected by the condition of the tubes! That is why the operating point was chosen to be class AB2 so the actual operational point is not critical and the power tubes will run cool with long life, high power and low distortion (due to the various feedback means). A good driver tube will present the output tubes with a very consistent bias voltage over time- that circuit is quite stable and has some ability to handle some grid current in the output section. The bias rectifier and its power supply play a role in this. The original rectifier was a selenium device which has a larger voltage drop across it so if you renovate the amp and replace the rectifier with a new device (recommended- those old seleniums were terrible) this is a minor thing to pay attention to if you want to set the same operating point, although as I said its not critical.
|
Interesting circuit ... thanks for posting it, Ralph. It’s kind of weird the actual gain/driver tube is a powerful 12BH7, while the cathode follower is limited to 1mA or so from the 12AX7, which is about the worst possible choice for a driver. Very strange. Maybe an intentional current-limit for the power tubes? Also was not aware the global feedback network has its own winding, making four secondaries on the output transformer. Most everybody else samples the 16-ohm tap, which captures the entire secondary that powers the speaker. I kind of wonder if McIntosh did this to make the circuit hard to copy. From this distance, I wonder about circuit stability. I see at least two feedback networks, one nested inside the other, and the outer loop not actually sensing the voltage on the speaker terminals, but a separate winding (which will never be exactly the same, especially at high frequencies). The circuit has a massive amount of forward gain (12AX7 -> 12AU7 -> 12BH7), so the feedback networks are definitely active. |
I rebuilt a number of classic mac tube amps when I was restoring vintage gear. They all sound very nice, but the mac house sound is a bit soft to my ear. Sort of on the warm side. Pleasant sounding, but not the absolute last word in detail. The preamps sounded like that too. Very good gear if you like that sound and very reliable, but warm.... CJ gear was also a warm sound. |
They all had a different "house sound". Marantz was crisper and more resolved, a more hi-fi sound, and Fisher sounded like a really good console. Not sure if the H.H. Scotts had a consistent sound or not ... they were mostly known for their FM tuners. Dynaco was always the "value for money" brand, like a VW Beetle. |
The HK Citation gear was the most razor flat sounding, and the Fisher receivers were actually quite good. The Scott gear was also quite good, and my favorite small integrateds were the Sherwoods. Their little EL84 based integrated was quite good if rebuilt. None of these are as good as the really good modern tube amps, but only the really good gear. The transformers in the much of the old gear were really good, but they didn't have the quality signal path parts we have today. Also, they had all sorts of features that robbed signal quality, like miles of wire in the signal path to support things like stereo/mono and phase reverse switches, and of course tone control circuitry. Very long signal paths in many of the old integrated amps and preamps. The Dynaco ST 70 had good transformers, but the driver section was barely adequate and if you replace it with one of the more modern boards, especially the all octal tube ones, the amp is greatly improved. The small chassis limited the quality of the power supply you could fit in there though. Still, if modded, the ST70 is a great little amp for the money. |
Playlist Here’s what I’ve been listening to for the past year. Probably zero percent overlap with Don Sachs, and only 10% overlap with the folks at Spatial. It’s a mix of dense heavily-produced rock, London Sound, and German techno. Joywave: Traveling At The Speed of Light (7:34 version) Joywave: Smokestacks Big Data: Unglued Big Data: Put Me to Work Disclosure: Help Me Lose My Mind (featuring London Grammar) Soul II Soul: Keep On Movin’ (demo quality) Elderbrook: Inner Light Elderbrook: I’ll Find My Way to You Ulrich Schnauss: Far Away Trains Passing By (whole album) Phosphorescent: Song for Zula Tears for Fears: Elemental Tears for Fears: Woman In Chains (demo quality) Preferred Listening Format: * Shadow Vector, Surround Master, or DTS Surround system * Open-back planar headphones * DHT triode system Probably half these songs would get me thrown out of most hifi show demo rooms ... certainly the first four, for sure. They’re all really emotionally intense, which is why I listen to music. Both "Keep On Movin’ " and "Woman In Chains" have an unworldly beauty unlike anything else, and Ulrich Schnauss’ "Between Us and Them" is quite a sonic excursion. I’ve been lucky enough to have several experiences that changed my life ... when I was ten years old and heard stereo for the first time, and heard a live performance of The Messiah the same year ... fifteen years later, hearing Shadow Vector quadraphonic for the first time ... twenty years later, hearing the Ongaku and Reichert DHT amplifiers on my new Ariel speakers ... and ten years after that, my own Karna amplifier. Profound, deeply moving, out-of-body experiences. That’s why I find audiophile discussions of "accuracy" to miss the point. I don’t listen to music to sip a single-malt whiskey, smoke a Cuban cigar, scratch my chin, mark up a twenty-point checklist, and write long reflective essays about this or that subjective aspect of the sound. That’s silly. What’s the point? Who cares what XYZ critic thinks? It either gets me flying or it doesn’t ... and if it does, how high?
|
@lynn_olson They were quite stable and reliable. But the circuit is carefully designed around some serious issues with phase margin, which is why you really don’t want to try any serious mods other than updated coupling caps unless you do the math and think it thru.
@donsachs The real weakness of the ST-70 is the rectifier. Its not got enough current and so is the most likely tube to fail in the amp. If you replace it with solid state the B+ will be too high. It is possible to use dual rectifiers but you have to replace the power transformer to do so. Triode Electronics of Chicago makes a power transformer that drops into place. You can use the filter can location for the other rectifier; the power supply caps can be installed beneath the chassis. There are adapter sockets on ebay that allow you to run a 6GH8 in lieu of the 7199; that gets you a far more common tube that also runs the amp with a bit lower distortion. |
I’ve said it before and I’ll say it again. This is such a great thread. Oh! And hi @eddie138 ! Nice of you to drop in! |
@atmasphere Actually, I have rebuilt a few ST-70. I agree, the rectifier is inadequate, but so is the driver section, which is at clipping at or just before the output section clips. I have heard a couple of drop in boards and the octal version was by far the best. Much better amp with 6sn7 input tube and 6sn7 drivers. Still, the small case keeps you from installing a really nice C-L-C supply built with only film caps rather than lytics. When modded it is a cute little amp, but hardly worth the trouble compared to other designs in my book. I recall another vintage amp or two that used 6GH8 or similar for driver section, probably Scott, but too long ago. Yes, we re-wired the sockets to take more common variants of the triode/pentode combo tubes. They are all nice little amps, but not in the same universe as the topologies we are discussing in this thread. However, would make nice little systems though with good speakers. I recall a Fisher flagship el34 amp that was really rare that actually sounded quite good when restored as well. So long ago I forget all the model numbers! When I was doing all the vintage work I could probably have rattled off the model... |
@donsachs Unless the power transformer is replaced, I think its a bad idea to drop in a board using 6SN7s as much as I like that tube. 6SN7s have a 600mA filament; running 4 of them is 2.4 Amps! The transformer would burn up in due time. Yes, you can install a supplemental transformer...As much as I like the ST70 (it has decent output transformers) IMO when you get that far into it, I feel like starting from scratch with a chassis that is large enough to really do what you want is a better move- and leave the ST70s for what they are good at: inexpensive and competent tube power. |
@atmasphere Hi.. you are only running 3 6sn7 tubes. Input tube and then a long tailed pair driver on each side using a single 6sn7.... |
@alexberger Mostly because of the voltage swing. RC coupled circuits are a bit inefficient if you also want current to deal with that input capacitance (which admittedly isn't all that much). That is why transformer coupling or better yet, direct coupling, does the job better.
@donsachs So 'only' 0.9 amps more... I think I've seen too many power transformer failures in my life, although that's mostly been due to bad filter caps. I treat old power transformers carefully- to avoid Bad Things happening. |
@atmasphere the ST70 power transformer spec was allegedly 2 x 6.3 V windings @5A each. Pair of el34 is 3 A, even a pair of 6sn7 is only 1.2A, so that should be only 4.2A. Should be safe |
What makes the 45 or 300B "hard to drive" is that it is good practice to have the majority of amplifier distortion in the final power device, not the driver. In other words, the driver should be cleaner than the 45 or 300B ... which are the lowest distortion tubes ever made. In a feedback amplifier, the source of the distortion doesn’t matter much ... the feedback sweeps it all away. Which is why the substitution of the higher-distortion 12AU7 for the lower-distortion 6SN7 in the mid-Fifties didn’t matter much, since feedback was in universal use by then, and it didn’t show on the low-resolution distortion analyzers of the day. To be honest, zero-feedback amp design is kind of a cult audiophile thing. For that matter, any kind of tube amp is a cult audiophile thing. If distortion numbers come first, THX or Class D are the answer, end of story. Don’t mess with tube amps, just buy the solution off the shelf. It’s an esthetic decision to build zero-feedback amps, whether bipolar transistor, MOSFET, or vacuum tube. I think it is good practice to design low-distortion zero-feedback driver sections, but I have seen (but not heard) all-transistor amps, with lots of feedback, used as 300B driver sections. Which begs the question, why use a 300B at all, if it’s just an expensive distortion generator? Returnng a little more seriously to the original question, there are a lot of SET amplifiers with marginal driver sections. I’d go out on a limb and say the majority of 300B amps on the market sound mostly like overstressed driver sections, not like a 300B. That’s why some of this discussion might sound like we are at cross-purposes. If state-of-the-art SINAD numbers are your goal, please look elsewhere. Forget all tube amps, whether pentode, triode, or hybrid amps. If you want a taste of "tube flavor", get a preamp with a 12AU7 in it. I own a charming little Xduoo TA-10R, which is a AKM 4493 DAC, a 12AU7 gain stage, and a simple two-transistor Class A output stage. 2 watts per channel, sounds great, and all for $320 from Apos Audio. Lots of power to drive planar headphones, and a fun alternative to the usual Topping or SMSL. Designers of zero-feedback 45 or 300B amps have different goals, which are also different from designing re-creations of Golden Age PP pentode amps. A lot of it comes down to esthetics and design philosophy. |
