Ralph has written in the past that the 6SN7 is a sufficient driver tube “dependent “ on its implementation in a given circuit. I have been told that the 6EM7 provides much more current, power and drive capability compared with the 6SN7.
I will say that I’ve heard an excellent sounding PSET 845 amplifier that utilized the 6SN7. I could certainly be wrong, but isn’t the 845 a more difficult tube to drive than the 300b?
@charles1dad The issue is always how much voltage swing will drive the tube to full output and how much grid capacitance is there. The more grid capacitance, the harder the tube is to drive.
You can get a 6SN7 to drive a single 300b no sweat. But doing it the way is traditionally done in SETs will likely but the tube at a disadvantage.
The 6EM7 does have one section that is considerably more gutsy than a 6SN7 has. But you will always be looking on the collector's market for replacements.
The way to get a 6SN7 to really do the job is to have it wired as a cathode follower direct-coupled to the grid of the power tube. In this way the coupling cap used in the amp stays rather small since its driving the 6SN7 rather than the output tube. Smaller caps are more transparent...
This is the technique we've used in our OTLs for several decades now. Its very reliable. One 6SN7 section is thus able to drive a number of highly capacitive triode grids with the voltage swing needed. I do not understand why this approach hasn't been used in SETs since the design consideration is similar. It might be because a negative power supply would be needed. But from what I've seen, SET users are not particularly worried about cost if the amp gets the job done for them.
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@atmasphere
I do not understand why this approach hasn't been used in SETs since the design consideration is similar. It might be because a negative power supply would be needed.
Ahh, I see. The usage of the 6SN7 with your preferred utilization mandates a negative power supply. Does this add noticeable complexity and cost to accomplish successfully? This could be the explanation for its lack of popular use.
Charles
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Gavin,
Don is licensing the production of his Valhalla integrated amp that owners on this forum have praised. I recall that the Valhalla amp is one of several that Clayton Shaw uses to demonstrate the best aspects of his speakers.
I am the least knowledgeable guy on this forum about electronics. I clearly admit it. I remember listening to Roger Modjeski explaining the design of one of his preamps at Burning Amp a few years ago where an attendee asked about worthy driver tubes. RM's minced no words in responding that 6SN7 tubes were a very poor choice of tubes -- "better employed in old TV's", because of their inherent high noise levels. Well, I had had Don's first preamp with 6SN7 tubes (derivative of the Roy Mottram's SP14 design and with Roy's full approval), and his Kootenai amp with the same driver tubes, and wondered how to square away RM's comment with the dead quiet sound from my system. When I shared RM's comment with Don after the show, oh boy, it was an interesting conversation. It is true that few audio designers use this tube in their designs (Modwright and Supratek come to mind), but Don does and is doing so with his new 300b monos and my ears tell me that Don is correct in his use of the octal tubes in all of his designs. Hearing in believing.
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@whitestix
I believe that the 6SN7 is actually a pretty popular tube for input/drivers of power amplifiers and In preamplifiers. I think that the degree of success is overwhelmingly dependent upon the talent, knowledge and skill of the particular designer/builder.For example, Atma-Sphere and VAC utilize them quite effectively.
Charles
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Charles,
Evidently more use this splendid octal tube than I was aware of. There are lots of ways to get to sonic splendor and Don advocates one particular approach that is clearly favorable to my ears. Owners of Ralph or Rogers' gear might say the very same thing and they would be right. SQ is such a a subjective notion that there are plenty of paths to pure listening pleasure for all of us music lovers, because the gear is just that which yields the SQ with which our ears most happy.
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@atmasphere
The approach you described is used in DIY world for DHT SET, particularly 300B SET. There is a group who prefers this approach and there is a group who prefers the more simple way - using half or the 6SN7 in parallel as one triode and interstage coupled with the next stage, or alternatively using a more capable driver, including a DHT such as 300B itself, 4P1L, 10y, 801, EML’s 20 or 30, etc.
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@ffzz
Based on your listening experiences have you found one approach to provide better sound quality or is it simply an implementation issue?
Charles
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@gavin1977
No, I am not going to do kits... I am retiring and letting a partner build the 300b project and matching preamp. That should be going before the end of the year. If you make kits you have no control over how they are built and you get LOTs of questions so customer service is a big issue. Just because most folks are competent and can build a kit, even if only 5-10% of people are in over their heads you get a support problem. It is far easier to build a product, test it, and then warrant it. Plus, these amps use multiple regulated supplies per amp, and it took a while to work it out and get it right. They are very stable when built correctly, but not trivial to do the build. They are designed to be easy to service as well, but kits... no. The amps need about 1.5 volts for full output so any preamp can do that. They also feature XLR inputs, which are very easy to drive.
I a quite smitten with this design and think they sound great, and of course I have not heard all the other great amps discussed in this thread. Hopefully someone who comes to Seattle has and can tell me their opinion!
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@charles1dad
I belong to the camp that prefers simple triode circuit with interstage coupling in DHT SET. It sounds more dynamic and transparent to my ears.
I don’t think it’s just an implementation issue though. DHT output tubes such as 300B are not that easy to drive and will not sound at its full potential if driver circuit is inadequate. The coupling method affects driver tube’s ability to swing voltage/drive the output tube, hence has direct impact to the final sound.
Interstage coupling allows more voltage swing to be squeezed out from the driver tube in theory and practice. In addition, the output tube can recover from overload very quickly with interstage coupling as the grid current can easily go through secondary winding of the interstage transformer to ground - an unique benefit of interstage transformer. These benefits are not insignificant in DHT SET.
I suspect that interstage coupling is less important for IDHT output tubes since IDHT output tubes are often easier to drive compared with DHT output tubes.
Also, it’s easy for me to advocate for interstage coupling in DHT SET as that’s not difficult to do in DIY - where cost, weight, and space are often less important (in fact, interstage transformers are often cheaper than high-end coupling caps). But if a manufacturer needs to fit everything in a single box, then the choice is very limited.
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@charles1dad
I used the 6SN7 to drive both KT66/6L6 and KT88 in two different amps and they are great for that. They are marginal to drive a 300b. As @atmasphere said, there are many approaches and schools of thought about drivers. Many paths to audio nirvana as it were. There are also many power supply approaches. As for SE vs PP, well, traditional PP has issues with phase splitter circuits right in the middle of them, and SE amps do have certain types of distortion. These amps do PP a different way and limit the phase split issue and avoid some SE distortion. Again, there are many fine examples of all sorts of amp topologies, but these don't sound at all like other PP amps you have heard. They sound like a great SE amp with the drive of a PP amp. That is the design goal....
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@ffzz @donsachs
Thanks for your very informed feedback and perspectives. The builder of my 300b (Israel Blume) Coincident SET mono blocks decided to go with an interstage transformer and 6EM7 driver tube. As has been openly acknowledged, numerous paths to success. In my case I can happily say that the sonic outcome is superb.
Charles
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RM's minced no words in responding that 6SN7 tubes were a very poor choice of tubes -- "better employed in old TV's", because of their inherent high noise levels.
@whitestix I encountered that with him as well- although by that time he was beefing about the 6SN7 linearity family of curves. Turned out his source was an early Tung Sol tube date manual that didn't reflect 99% of most production tubes. RCA and Sylvania of course figured things out with the 6SN7 early on and in another year or so, so did Tung Sol. RM had the bad luck to have a Tung Sol manual that was outdated.
There is a group who prefers this approach and there is a group who prefers the more simple way - using half or the 6SN7 in parallel as one triode and interstage coupled with the next stage, or alternatively using a more capable driver, including a DHT such as 300B itself, 4P1L, 10y, 801, EML’s 20 or 30, etc.
@ffzz I've found that implementation plays an enormous role in zero feedback amplifiers: grounding, component quality, power supply design and of course getting the operating points right in the circuit design. I challenge the idea that using an interstage transformer is actually a simpler approach- it is once you have a suitable transformer, but a good transformer design is the tricky bit; on that account direct-coupling is IMO easier. It has the same advantage of being able to support class A2 operation as well as instantaneous overload recovery, plus wider bandwidth and lower distortion.
The lower distortion may well be why it sounds less 'dynamic'! The use of that word when associated with SETs is always tricky, since most of the 'dynamic' nature of SETs has to do with how they make distortion. IOW its normal for a lower distortion circuit to sound less 'dynamic'.
These amps do PP a different way and limit the phase split issue and avoid some SE distortion. Again, there are many fine examples of all sorts of amp topologies, but these don't sound at all like other PP amps you have heard. They sound like a great SE amp with the drive of a PP amp. That is the design goal....
@donsachs +1
I've been harping on this very issue for years. Our amps do not have a dedicated 'phase splitter', relying instead on good CMRR in the differential voltage amplifier.
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@atmasphere great post. My new amp does use an interstage transformer and a larger driver tube. The 6SN7/300B amp I'm listening to in the mean time is darn good. Not great, but darn good. So lets not walk away with the understanding that all 6SN7 implementations in 300b amps are junk. but I agree a bigger driver is a good thing.
Jerry
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@carlsbad2 Yes the Reichert amp used 6SN7 to drive 300b and though I have not heard one, those who have said it had a sonic purity that was hard to beat. So it was a good starting point and 6SN7 driving a 300b can sound good. But it can be improved if you are looking for all out performance.
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Thanks for the kind thoughts, whitestix. For those who are curious, this amplifier is a more advanced, second-generation Karna amplifier, first designed around 2005, and the result of a years-long collaboration with Don Sachs. More practical, two chassis instead of four, and benefitting from Don’s decades of experience in what works and what doesn’t, especially in a production environment.
My projects are usually proof-of-concept and kind of out there. Don’s experience makes all the difference for this collaboration, getting it off the ground and into the real world. He has a lot of great ideas, taking it even further, and these will be in the pre-production amplifiers at the Seattle show.
Whitestix, don’t feel left out. The amps you are listening to now are essentially identical to the show amps, just a little earlier in the production cycle.
It’s a fully balanced circuit, with 6SN7 input, 6V6 driver, and 300B output. It’s a high current, high speed design. I tested an early 2003 prototype at 500 kHz at full power, with clean-looking sine waves on the scope, with no visible flat-topping or triangle shapes. No, I won’t repeat that test again, and I don't encourage anyone else to do it, either.
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Gavin,
I had a stock SP14 for a while in my bedroom system and it was a wonderful glimpse what Don has improved upon with his preamps. I can't speak to the gain question your raised, sorry, but Don's DS2 mates perfectly with his new 300b amps. His integrated amp will be commercial available in the near future, but none will be in the offing in kit form to my knowledge.
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Oh my, thanks for chiming in, Mr. Olson. Don has regaled me with the story of your joint development of the 300b monos and my ears at the present moment, listening to your creation, leaves me just delighted with the sound of my system. It is just simply stunning, driving my Spatial Audio speakers. I am going to attend the Seattle show largely to meet both Don and you in person. I am sure that your room with the newest Spatial Audio speakers will be spectacular! Thanks for participating in this thread.
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@lynn_olson
It’s a fully balanced circuit, with 6SN7 input, 6V6 driver, and 300B output. It’s a high current, high speed design.
Thanks for taking the time to post on this thread. I have appreciated various articles written by you over the years. Quite some very insightful participation presented here on this topic. This amplifier from you and Don is genuinely exciting and intriguing. I wish you both a terrific audio show presentation.
Charles
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Being a continued user and lover of Don Sachs's preamp... where & with whom do I sign up to get one of these amps?
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For now, Don tells me he is focusing on the Raven preamp and Statement power amp. We expect to transfer production to Spatial Audio, with product names and prices to be determined. The circuits, parts selection, and overall construction technique are pretty much done, so what you hear in your home (the first pre-production run) and at the Seattle show, will represent what's coming.
Will Don continue to make his current preamp and KT88 power amp? That's his decision, not mine. My focus right now is the Seattle show and completing the large-format high-efficiency 2-way speaker Thom Mackris (Galibier Designs) and I have been working on for the last couple of years. (This design is completely independent of Spatial Audio, and an outgrowth of the "Beyond the Ariel" thread elsewhere.)
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As Lynn was saying above, I have spent the last year working on this 300b amp project and the matching "Raven" balanced preamp. Spatial Audio will decide on final naming and pricing and I will help them get started on production. The cosmetics will probably change a bit, but the circuits are stable and that is what will be shown in Seattle. This thread has mentioned many fine amps and as I said, there are many paths to audio nirvana. Lynn and I have had fun with this project and we want to see it out in the world. It is cost no object, sort of within reason. The gear isn't going to cost $50,000, but it also isn't going to cost what the current Kootenay amp and preamp cost either. These are much more expensive to build and more involved. If you can hear them in Seattle please drop by. Honestly, I would love to hear people's opinion!
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Let’s discuss driver tubes. Back in the Nineties, my Tektronix friend Gary Pimm built a distortion analyzer that could measure each distortion harmonic, out to the 10th harmonic, down to an astounding -120 dB. Maybe even -140 dB. It was way, way down there.
So naturally Gary and I went and measured a bunch of tubes, and ran them at realistic drive levels (50 volts RMS or so). I knew from previous experience that audibility of 2nd harmonic is quite low ... I can barely hear 1% second harmonic, and audibility of 3rd harmonic is around 0.25%, if it’s mixed with somewhat more 2nd harmonic. (3rd by itself is harsh, but sounds pretty nice mixed 1:4 with 2nd harmonic, giving a richer, more fun sound.)
But the high-order harmonics are not nice sounding. Crowhurst suggested back in the Fifties weighting audibility of the order by the square of the order, or even by the cube of the order. High-order harmonics are what give electronics their nasty "electronic" coloration, and contribute very strongly with a forest of IM distortion clutter on the noise floor.
Unfortunately, unless the design is very poor, the relatively innocuous 2nd and 3rd order harmonics will dominate the THD measurements, to the point where THD is nearly useless for assessing sonics. When 2% THD is just barely audible, no, that is not a useful measurement. By that standard, all amplifiers are perfect.
But a spectral analysis is a lot more interesting, providing you pretty much discard the 2nd harmonic, and take a good, hard look at the rest, particularly the 4th on up. In a good amplifying device, you want 4th through 10th as low as possible.
We measured 300B’s from several vendors and vintage 45’s. Of all the tubes, these were closest to perfect. High-order harmonics were nearly absent, and of all the 45’s, none were visible at all. For 300B’s, they were way down, -60 dB or less, depending on the vendor.
That was one surprise. Most tubes, with a few exceptions, had comparable THD and 2nd harmonic, but the high-order spectra would vary within a given tube type by 20 dB, a huge difference. Twiddling the operating current made about 2-3 dB change ... not much ... but changing the brand would show much larger changes, 10 to 15 dB. We were plainly seeing differences in manufacturing technique ... uneven grid structure, tilted grids, etc.
The DHT’s were the clear winners in all categories, but the vintage 2A3’s were 10 to 20 dB worse than the 300B’s and 45’s. And the indirect-heated tubes were another 10 dB worse, or comparable to the 2A3’s. The worst tubes of all were the 9-pin miniatures, with the 12AU7 and 6DJ8 at the bottom of the heap. The 12AU7 had the decency to have lots of 2nd harmonic, which masked the clutter higher up, but the 6DJ8 had less 2nd harmonic and a harmonic spectra more like a transistor, with abundant high-order terms.
The 6DJ8 was a frame-grid tube designed for RF front ends in TVs and FM tuners, where linearity is of no concern at all, so you can’t really hold that against it. And the 12AU7 was designed as a compact replacement for the older 6SN7, operating at the same bias point and voltage. But ... high-order distortion is 10 to 20 dB worse than any 6SN7, even the worst. But you can’t hold that against it; 20 dB of feedback was universal when the 12AU7 was introduced. Loop feedback will reduce distortion in all tubes in the circuit, so it hardly matters.
The 6SN7 family is the tube of choice if you care about high-order distortion, and a DHT alternative is out of the question due to cost, size, practicality, microphonics, and severe filtering requirements for the filament supply. Of the 6SN7 family, the antique single triodes were the best ... 6J5, 6C5, etc. but there certainly were good 6SN7’s, both new and vintage. And differences were consistent within brands, reflecting manufacturing technique, and not always visible from the outside. Were the fancy MILSPEC ones better? Not really. More consistent, which is what MILSPEC is really about.
Don is correct that octals are (nearly always) better, unless you need a mike or phono preamp tube. That’s the 6DJ8 (or WE 417A) niche, where signal levels are very low, and noise and low microphonics are the primary concern.
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I am another of the lucky guys to get one of Don's 300b stereo amp. After many years of transferring components in and out of my system, this amp simply puts everything into place.
And, of course, being matched up with the DS preamp doesn't hurt either. Stunning sound. Everything in its place. Great soundstage and holographic to boot. The only other amps I may consider in the future are the new monos when they become available.
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Enjoy ! That sounds like a great amp.
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@tinear123 The pricing and availability isn't set yet. Most likely Cloud Sessions of Spatial Audio Labs will keep a wait list and they will have to decide on whether there is a deposit required, final pricing, etc... Once we have it all ironed out there will be an announcement on the Spatial Audio Labs site. First the official unveiling of the prototypes at the Seattle show on June 23.
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Hi @lynn_olson ,
Thank you for very interesting information.
Do you have any data high order distortion in 6F6 and 6V6 in a thiode mode? A friend of mine use 6BX7 double triode for driving. Is there any information about 6BX7?
I heard 2 SET amplifiers built by one person using James Audion iron. Both have 6sn7 input. One had a 6sn7 driver and a kt88 in triode mode output. The second amp used 6F6 in triode mode driver and 300B output. Both amplifiers used RC coupling between input, driver and outputs. The first amplifier sounded too slow. The second one sounded more fast and dynamic. It lead me to the conclusion that 6sn7 is not strong enough to drive even kt88! And to drive 300b even harder. I don't talk about the cathode follower connection of 6sn7; it is a totally different output impedance of the stage plus direct coupling.
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I meant to post this question the other day, but it apparently disappeared into the ether -
Who, on this thread, plans to attend the Pacific Audio Fest this year?
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RC coupling, although fast and compatible with overall loop feedback (highly desirable in pentode amplifiers), throws away about 1/3 of the potential swing (through voltage divider action) and also creates a steeper load-line, which increases distortion by 2 to 3 times (the load-line swings down into the low-current region).
But Job One in any feedback amplifier is bandwidth, otherwise the thing goes unstable. Can’t have that. So the rule of thumb is no more than one coupling cap in the entire forward path, plus the output transformer needs at least 60~80 kHz of bandwidth. The Williamson, which dominated from 1948 to 1956 in this country, was marginally stable unless you used the specified Partridge transformer, which had an astounding bandwidth extending to 120 kHz, and down to 4 Hz. So your options are limited if you want to wrap feedback around the amp ... only RC coupling, and only once per side, not twice, like the Williamson.
If there is no overall loop feedback, you can have any kooky topology you like, since phase margin no longer matters. But ... triode-connected pentodes have somewhat higher output impedances than DHTs (2 k instead of 800 ohms), and aren’t as linear. But this depends on the pentode. Don and I selected the triode-connected 6V6 because 1) 45 tubes are near-unobtainium these days 2) 6V6’s are reliable, been around forever, much loved by guitarists for their famous tone, and oh yes, sound good too. Last but not least, the 6V6 was purposely designed as the replacement for the 45 in radio use, so operating points are very similar.
Part of amp design is deciding what "tone" you want. Because it’s going to have a sound, no matter what technology is there ... Class D, MOSFETs, bipolar transistors, pentode, triode, or DHT. No matter what you design, it will have a sound, no matter what you do, or how clever you are. The perfect component does not exist. The designer needs to steer that little touch of coloration in a good direction. Fortunately, Don and I are on the same wavelength ... I’m looking forward to meeting him at the Seattle show, in person. Should be a lot of fun.
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As they say, if you literally built a straight wire with gain half the people wouldn't like it:) Everything has a sound. I will say that these amps have far less of a sound than anything I have ever heard... or not heard... or whatever... They just seem to remove themselves and just leave this cloud of music in your room. Pinpoint imaging, but it is like omnimax for the ears. That is my take. I will be interested to hear what other people think in Seattle.
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@lynn_olson
Part of amp design is deciding what "tone" you want. Because it’s going to have a sound, no matter what technology is there ... Class D, MOSFETs, bipolar transistors, pentode, triode, or DHT. No matter what you design, it will have a sound, no matter what you do, or how clever you are. The perfect component does not exist. It is the responsibility of the designer to steer that small residue of coloration in a desirable direction.
Yep!!!
Tubes and transistors both present some degree of colouration but with different signatures.
Charles
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Like @whitestix I am quite comfortable with my very limited understanding of 'Math' for Electronics and 'Hands On' practical exercises with Electronics, even though the latter is easier to comprehend.
The above has never been a limitation or a constraint for me in relation to having Devices produced for Audio that are Bespoke and unique, as is the Design of Amp's under discussion in this thread. I have created relationships with EE minded individuals that are very adept, and one relationship/friendship extends back 30 years and I still contact the EE a few times a year and send a Bottle of Drink for them in the New Year.
The Math is one thing, how to utilise the Math and adopt it to be used for the Practical Exercises being undertaken, 'will always be a better undertaking', when a time served experienced individual, who really has a understanding of how to interface their design hooks are doing the Math and Circuit Design.
I have 6SN7's actually 'Half 6SN7's' in a Driver Stage on 845 Mono's. I have been fortunate to have had a variety of these Tubes from modern to vintage tested for being matched pairs. These have then been used for Tube Rolling and as the subjective evaluation, I have become very attached to the Vintage KR VT 231 Black Glass for the SQ when the perception of a lean Bass and projected Mid and High is the desired presentation. If a Richer Presentation is desired, the Vintage Sylvanias do this, to various degrees, even though some will swear the details.
As a subjective evaluation I have found that the modern Tubes EH and Genalex have proved good for a period of time, a distortion can be detected to develop and even though not too much of a detractor, it is enough for me to want to end the use of the Tubes when it has been detected.
I have been extremely happy with what I have learned about the use of a 6SN7, I have also found the Tubes I am Wed to.
Additionally, due to the rarity and costs associated with vintage tubes and then the concerns for a accurate matching, I have looked at other modern alternatives and there is a Linlai Thread on this forum that presents a very positive assessment for this Brands Tube.
As a Tube Roller and attendee of other Tube Owners rolling experiences, I am very aware a magic can occur as a result of a change of a tube. When Don's 300b design is up and running, I am sure there will be a time when an owner will become curious and the stock 6SN7's are to be swapped, there is nothing to suggest differently, that a Valve might be discovered that really 'hits one out of the ball park' for the perception of Amp's performance.
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Pindac brings up a good point. Designing a good amplifier is a lot of work, and there’s just as much work getting a product market-ready. The input and output tubes were obvious choices, based on subjective sonics, low distortion, and continuing availability. The challenge was the driver.
Low-powered DHTs have a very small niche market, so have extremely high prices, and it is an open question if they will still be available five or ten years from now. That was a question about the 300B back in the Nineties when the Triode Revival began, but it has been answered ... the 300B has created its own market, and is manufactured in several countries. It has joined the 6V6, 6L6, EL34, EL84, KT66, KT88, and 6550.
The requirement for this amplifier was a wideband driver producing about 2 (or more) watts of very linear Class A power. The first Karna amps used a quartet of hand-matched vintage 45’s. If they were scarce in 2005 when Gary Pimm built the original Mark I Karna’s, hoo boy, they are insanely rare now. Or built only in Germany at frightening prices. Yes, there several Euro-made low-powered DHT’s. All different from each other, with different bias requirements, and some with different pinouts. All very niche products.
The Chinese 2A3’s are interesting. They have single plates, unlike the twin plates of the RCA original, and look a lot like 300B’s with 2.5 volt filaments and somewhat de-rated emission. I’m not quite sure where they fit in the DHT world, but people seem to like them, and demand appears steady.
And then there are the real odd ducks, the Chinese 101D’s. Wow. These are really out there, a tube that hasn’t been in production for ninety years, and extremely scarce in the collectors market, far more so than 45’s. Will they still be produced five or ten years from now? Who knows?
Don and I played it safe. There are zillions of old-stock 6V6’s out there, and a large continuing market for them in guitar amps, much larger than the hifi market. Whichever tubes we selected for this amplifier, and the new preamp, we wanted replacements to be available well into the future, and entertainment value in pawing through old stocks, if the owner wants to do that. All we ask is the pairs be well-matched, both for sonics and optimal performance.
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It’s my understanding that KR audio is now manufacturing a 101D tube in the Czech Republic. But yes, in the big picture a relatively rare tube. I use the Psvane W.E.Replica 101D in my preamplifier. It’s very good sounding and fortunately has proven to be very reliable.
Charles
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Hello, all.
A big THANK YOU to Don and Lynn for their collaboration in developing these new 300B amps. I'll be at PAF from Thursday through Sunday, and will set aside plenty of listening time in the Spatial Audio/Don Sachs 'experience' room. I must admit to being giddy with excitement in anticipation of how these amps sound with Clayton's speakers.
I'm looking forward to putting faces to names at Pacific Audio Fest.
Michael
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As Lynn mentioned above, we chose the 6SN7 and the 6V6 because both are in current production and there are very good modern versions of each being built. Obviously there are lots of 300b choices as well. I favour the Linlai E-6SN7 beause it sounds wonderful and is reasonably priced. The JJ 6V6 sounds quite good. I love the old stock Russian black bottle 6V6 in here and you can still get them for reasonable prices out of Ukraine, Romania, and other places without sending money to Russian vendors at this time. You do need pairs that are within maybe 2 mV of each other for full bass performance in the amp. Matched pairs of 300b for the same reason. They are all readily available, and the amp runs tubes at sane operating points so they will last a long time. We didn't want to produce an amp that required really expensive tubes that could be unobatinium in a few years.
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It has been well debated 20 years ago during golden years of direct heated power triodes.
Based on tests on ALTEC/JBL horn systems:
300B is middle of the road of all those small/medium signal direct heated tubes.
It has good/mellow middles, lose/weak lows and not very sweet highs. 300B has longest life among those DH tube.
PX25 has best low end quality among all < 8W tubes.
VT25A/10 has best high end, I built and love the sweet sound. However, the tube is very fragile, did not last.
if you want to best balanced sound, push/pull 45/50 can give you what you need.
For small signal (pre-amp) tubes, 26 sounds the best, and same time is hardest to build the amp due to sensitive to hum and very low heater voltage.
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I run 46 tubes in my Lampi Pacific DAC and they are wonderful. You can still find some. The 45 is now really scarce unless you want to spend a fortune for new production. I have some 26 I collected for a potential preamp project. So many tubes, so little time:)
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FULL MUSIC makes decent 45 and 50 tubes.
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As mentioned above, the Mark I Karna used a quartet of 45 tubes, but new production is limited at this time, with future availability an open question. Five years from now? Maybe, maybe not. Ten years? Maybe, maybe not. I can only go by popularity in the market and how many different vendors, in different parts of the world, are currently making them. Not just one factory in one country.
For a DIY hobby amp, availability of tubes not a real concern. But if people are buying a commercial product and expect service (warrantees are a legal requirement in the USA and Canada), availability of new-production matched pairs is a major concern. Don and I looked at the market and we both decided, nope, not yet, still a pretty specialized niche product, and NOS and old-stock supplies have all disappeared into the collector market. When the Mark I Karna was built, they were scarce, but could still be found. Now they are very scarce indeed.
As for sonics, for anyone that’s designed an amplifier from scratch, as I have, the sonics of the 300B are largely determined by the linearity and current delivery of the driver stage. Most of the commercial 300B amps I see at hifi shows have driver stages that are under-designed by a factor of 2 or 3, which results in not enough current to drive the Miller capacitance of the 300B grid. They aren’t as hard to drive as an 845, which is nearly impossible, but they are only second to an 845. A good driver needs high current, high linearity, high speed, and enough headroom to drive the grid 20 volts positive and then recover in microseconds.
Most drivers can’t do that, so the driver and output both clip at the same time, and then both have different recovery times, which can last an appreciable fraction of a second for RC-coupled amps. A typical SET will have RC isolation for the B+ supply between stages, and that RC filter has a characteristic recovery time once a stage saturates. So four things have to recover after saturation: the RC coupling between stages, the RC isolation B+ filter between stages, and cathode bypass caps for each of the cathodes. Typically, they all have different recovery times. Then we get into the subtler issues of capacitor coloration, which can overshadow tube coloration if the caps are not well-chosen.
Most of the "sound" of a conventional SET amp is simply a driver that has run out of linearity. The same is true of preamps, by the way. They run out of linear current to drive the interconnect cable capacitance and then the power supply folds down as it is saturated.
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..and what Lynn is not saying is that these amps address these problems. Without going into proprietary detail, I will say that there are two separate power supplies in each amp and that every tube plate in the amp has a healthy inductance between it and the power supply that feeds it. These things add up:) The matching preamp also is transformer coupled and has a ridiculously overbuilt power supply.
The 300b is incredibly extended at both frequency extremes when you can drive the heck out of it....... It is not slow and syrupy at all, but rather it has lighting transient response and stunning bass. If driven correctly.....
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Great discussion. My current amp has 3 heater voltages so I can use 300B, 2A3, or 45s. I've tried all 3, settled on some 2.5V 300b's because I found good tubes in stock.
This amp is a 6SN7 driven amp so now I wonder is the 45s or even the 2A3s are easier to drive.
I'll put my 45s back in for a while (kenrads) and re-evaluate them.
Jerry
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One of the really fun things about designing an amplifier from scratch (not from 1950’s schematics) is you can hear for yourself when you make a change. Reviewers can only treat these things as "black boxes", and customers can only work at the margins with cable swaps. Don and I can change topologies, power supplies, tubes, basically everything.
Every change is audible, but some are far more important than others. Driver design is right at the top of the list. That’s what makes or breaks an amplifier. I wish more designers understood this.
I shouid add there is no 1950’s or 1960’s tech in the power amp or preamp. All of the tubes were designed in the 1930’s, and the circuit is a combination of 1930’s and 21st-century tech. No relation to Golden Age tech, in other words.
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Lynn,
I have a burning question that you, Don, or Ralph might be able to answer. So you start with a new design with caps and resistors and power supplies, etc. and then you put it on the bench to test it out and of course give it a good listen. Conventional wisdom is that caps etc take a while to run in so do you have to run in the whole design for a a while to get the true sense of it and then tinker with the component parts thereafter.... and then wait for them to run in again before making an evaluation? I would think this would be an even more difficult process with speaker drivers... which clearly need to loosen up. My Spatial Triode Masters sounded awful for the first 50-100 hours but gradually came to life. At the risk of teeing up a disputatious topic, I often wonder if it is our ears that break in or the audio components. Breaking in speaker wire just seems silly to me, something I have never honestly experienced. Any insights would be welcome on this tangential issue. which would give us a glimpse into the design process
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Fortunately, Don and I know where the "bad actors" are, and it’s no secret. Caps. Specifically, film caps. They’re the parts that need the 10 to 50 hour run-in. I do not like this ... it suggests the plastic film is undergoing a very slow chemical change while it is being charged, and a further change when the charge is modulated by an audio signal.
But direct-coupling is a whole another can of worms. The worst is that failures in one part of the amplifier cascade through the amplifier, and can even destroy loudspeakers. This is the typical failure mode of solid-state amplifiers ... a DC servo circuit, or voltage regulator, fails, and then destroys everything that is DC-connected to it. Cap-coupling or transformer coupling limits the failure to one part of the amplifier, and limits the scope of the failure to no more than a few parts.
Also, Don and I have prior experience and expectation to guide us when we try XYZ change. Is it audible at all? Is it better or worse, or just different? I don’t know about Don, but I can hear changes in ten seconds or less, and have a better, same, or worse reaction, or sort of a mixed feeling.
The mixed feeling is a danger sign and indicates that there’s something I don’t like but can’t describe or pin down. That’s pretty common at the serious design level, by the way. You often hear or feel things that just make you uneasy, which is a sign to turn it off immediately and take a good, hard look at the circuit and see what you missed. Maybe a wiring error. Look again. Stop, think, reflect, do something else. It happens to us all.
Yes, fresh film caps often sound dull, flat, and dynamically compressed. There are often subtler colorations I describe as "glossy" or softened, a kind of over-processed Photoshop impression. You hear it a few times, and you know it immediately.
But compared to an outright circuit error, that’s nothing. Those sound much worse, grainy, shrill, shouty, almost always something very very wrong, and almost always in the high frequencies. Does it sound "electronic"? Yeah, that’s bad. Find out what’s wrong. Look hard enough and you’ll find it.
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@whitestix
I make a change and live with it for at least a few days or a week before deciding. Yes, the film caps have to run in. With this amp project I built a stereo version first because I had a few of the Kootenay amp cherry cases laying about. So I made it all fit. There are different topologies to try. The first was full "silicon assist" where the driver tubes each have a CCS on the plates. I took this version to Spatial Audio a year ago and we all agreed it was the best amp any of us had heard. But then I tried LC coupling and it was better to my ear. So then I had Dave Geren of Cinemag design me a custom interstage transformer for just this circuit. As Lynn said above, the 300b is a hard tube to drive. Dave likes a challenge, and his interstage transformer sounds better than either previous version. The IT version is more "musical" and "relaxed" sounding, yet has an insane level of detail. Then we decided to go to mono blocks because we could improve the power supply considerably with more space to play with. So my point is these things evolve over a year or more and I listen to each variation for quite a long time. The mono 300b amps going to Seattle for the show are the result of a year of listening to different ways to build the circuit. You have a pair of them and they are pretty special to my ear. It took a lot of experimentation, guided by theory to get to this point. That and patience of course....
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Guys,
Just love reading about different type topologies that went into this 300b design and it’s quite amazing the time, effort and dedication it took to come into fruition...
I know this is going to be a special 300b as I am a current owner of Don’s Kootenay 120 ...😁
Wig 👍
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Gents,
This was an excellent glimpse in to the design process, voicing the sound of the electronics, that has always mystified me. Since getting Don's first preamp many years ago, he did improve upon the sound of the preamp with a few capacitor upgrades... not rushing right to the very pricey Dueland caps, but sleuthing out caps with similar SQ characteristic at affordable prices to the benefit of his customers' pocketbooks. V-caps in the DS2 are very nice improvement. As Lynn mentioned and I am sure Don agrees, there are go-to components for the gear. I know from visiting with Don how much of a revelation it was to get the Cinemax transformers which is proof positive of his relentless search for excellence.
I always think that Don's prime aim was designing his gear for his own listening pleasure, and after 15 years of knowing him, I trust in his ears and know that others enjoy the fruits of his labors. That you two collaborated on these 300b amps indicates to me that he is willing to stretch out with his designs, involving Lynn and working with Cinemax to get the properly designed components. Often one doesn't see the whole picture and two heads are often better than one.
I have had your mono's running since dawn and I am inclined to wonder if my Spatial Audio Triode Masters have somehow been swapped for the excellent MBL speakers that I hear at every audio show... providing a holographic sound image, with amazing inner details and significant heft in the LF... and of course the HF is ethereal. I suppose I exaggerate to my peril, but the presentation of the music is just 3-D remarkable. The sound is neither warm nor cool tonally, it is just true to the source.
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@whitestix
When I "voice" amps and preamps I am always trying to make them sound like they are not there. I have a Lampizator Pacific DAC so the front end is killer. I am always trying to have the amps vanish. That is the goal. They should just sound like music and not be warm or cool or anything. Lynn's circuit is amazing. If you put a really good power supply in front of it and really good output transformers behind it you get the vanishing act......
I "voice" things mostly with recordings of acoustic instruments and vocals of course. Does the piano or mandolin sound real? So I have my favorite 100 test tracks. Of course I enjoy Pink Floyd or a live Dead show, but you don't really know what the electric guitar is supposed to sound like. That sort of music will tell you if your amp rocks, but not if instruments sound convincing. I know what a mandolin or stand up bass sounds like. I love David Grisman's acoustic disc label. I love the last recording of the Goldberg variations that Glenn Gould made. If I can easily follow the left and right hands with no strain, then the amps are good. I really like Myriam Alter's compositions. She plays on some of them, but they are always interesting music with hints of Klezmer and all sorts of other influences. They are usually 3 to 5 instruments, like piano, clarinet, bandoneon, and interesting percussion. If you can easily follow an individual instrument or switch to listening to the whole, then the amp is good. I find this circuit to be FAR better than any amp I have ever heard. The reason is that it avoids many sorts of distortion that plague other circuits. You don't really know you are listening to all of that until it is gone, or at least very minimized. So what you hear is really the lack of all the other things you usually hear in other amps. That allows only the music to remain, floating in the air in your room. I find it quite astonishing after all the years of restoring classic tube amps, and then building my own, to hear what this approach brings. Very enlightening.
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Wish the amp was going to be at T.H.E. Show in SoCal as i would love to hear it.
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