300b lovers

This is probably the best, most informative thread on Audiogon. Just insane amounts of obscure technical information and experience being shared. 

Not that I really understand it. 

Looking forward to the PAF opportunity to hear what this all sounds like!

I am super excited that I will be able to be at PAF on Jun-23 to hear this amp and meet Don, Lynn, and others.

Frankly I had pretty much written off Agon due to the continuous sniping and politicalization (either way) of all topics UNTIL this thread!  Best thread and info I have seen in the last couple years.  Thank you to all whom have contributed.

@donsachs 

I didn't tell him to start a thread and certainly had no idea it would go on this long!

Understood.  If a thread topic generates quality insight, information and perspectives from genuinely knowledgeable and earnest contributors then this is what happens. People can discern quality discourse from intentional agitating trolling.

Charles

As I said, there are subtle complexities in what appears to be a really simple circuit when you first look at the schematic.  I am sure the symmetric Reichert sounds great, but I am sure it cannot touch the current build due to all the improvements we have made in the past year.   It has been quite a fascinating project for me because I thoroughly understand all the vintage circuits having rebuilt more vintage tube gear than I can count, and then building my own with improvements to the power supply and signal path, but basically the same approach as all the classic tube gear.  This approach is completely different and I have learned a lot in the past year.  The result is an amplifier, and a preamp that don't sound like anything I have ever heard before.  Well, really, they sound less like anything at all and more like music.  @whitestix has the first pair of mono 300b amps that I let leak out into the world and he started this thread.   We have never met, but talk on the phone now and then and he has had a version of pretty much everything I have ever built over the years.  So I thought it fitting that he should hear these.  I didn't tell him to start a thread and certainly had no idea it would go on this long!

By the way, my own little hypothesis about output impedance of SET amps varying over the signal cycle could be completely wrong. Transformers do store energy, which is why the primary easily swings well above B+ voltage. So the transformer itself might keep things constant over the signal swing. Not 100% sure about this.

However ... sharp Class AB cutoff transitions can generate ringing in the output transformer, which is why setting bias in a PP pentode amplifier can be fairly critical. Maybe the real reason partial triode, or so-called Ultralinear, was developed so the AB transitions were better behaved.

But the same applies to transistor amps, as well, setting bias can be temperamental, with the added entertainment that incorrect bias can destroy the output stage (transistors have a positive temperature coefficient and can "run away", carrying more and more current until the bond wires melt and the whole thing shorts out). Fortunately, vacuum tubes are designed to operate at high temperatures and don’t have temperature coefficients like transistors.

(Transistor parameters vary with voltage, current, and temperature, which is why a lot of local degeneration and current sources are used to stabilize the circuit.)

What’s fun about knowing Don and Thom is they have different ideas that bounce off each other. Way back when, Thom had some pretty gruesome-sounding amplifiers, and in my usual blunt and ham-fisted way, I said as much. I may be the son of a diplomat, but I’m not always diplomatic when it comes to audio.

Hey Thom, try this, and showed him the original Reichert SET schematic. You can skip the DC heating, because the one I reviewed for PF magazine had an AC balance pot and it sounded fine. You can improve it further by using TWO separate B+ supplies, one for the input+driver and the second for the output tubes. And use damper diodes instead of the usual 5AR4 or 5U4. And if you really want to get fancy, use LC coupling instead of RC coupling for driver/output interface. You just need a big audio-grade choke (not power-supply drek) and a decent coupling capacitor. Plus, you’ve got two good Magnequest single-ended output transformers laying around, why not put them to work?

Thom built each amp on a wood plank ... literal breadboards ... and they wiped out the overpriced audiophile amplifiers (no, I’m not telling which one). Thom sold the audiophile amps and never looked back, improving the prototypes over the years, until they became the NiWatt amplifiers they are now, many many refinements later. I’d give hints every now and then, sometimes in a heavy-handed way, but every improvement took it further, and 90% to 95% of the improvements came from Thom, who has a very sharp ear. A win-win, from my perspective.

And it’s a lot of fun working with Don, who has the nerve to restore Citation amplifiers, which are notorious in the industry as one of the most complex and eccentric designs of the Golden Age. Stu Hegeman was seriously out there, compared to everyone else. I would feel faint just looking at the underside of those things, never mind trying to correlate the schematic with the maze of point-to-point wiring used back then. It would be like troubleshooting a 1963 RCA color TV with 28 tubes and weird, complex setup procedures. Complete respect on my part.

I’m frankly amazed Don took on my amps. They are NOT for beginners, with demanding layout requirements, and no relationship to Golden Age amps at all. There are no points of familiarity except to seriously obscure amps of the 1930’s. One of the dirty secrets of the high-end biz is that most medium to high-power PP amps are warmed-over Golden Age designs, with a few extra regulators or cathode followers thrown in here and there. Full respect to Don for even looking at the schematics. Back when I started this in the late Nineties, I had people laugh in my face when I told them what I was doing. The big thing then was single-ended EVERYTHING, and as a speaker designer, that just felt wrong.

For one thing, tubes (and transistors) work by varying their resistance. That’s all they do, there’s no little elves inside that give them special properties. With a SE amp, that varying resistance (powered by B+) drives an output transformer, which faithfully reflects it down to the speaker. The transformer is fully passive and has no special properties; it multiplies current and divides voltage, about 28:1 or so. (The current/voltage multiplication is the same as the turns ratio, but the impedance ratio is the SQUARE of the turns ratio.)

I don’t feel comfortable about the speaker being driven by a varying impedance ... that means the damping factor is merely an average value over time; at any one instant, it can be anywhere. If I want a steady, constant impedance, what can I do?

A unique property of Class A push-pull is nearly exact symmetry between the two tubes. When the resistance of one tube goes up, the other goes down, with a precision of a percent or so. It’s a exact see-saw action, and it is unique to Class A push-pull triode. The curved grid lines you see in tube manuals (for SE circuits) nearly exactly cancel out, leaving parallel lines.

As far as the speaker is concerned, it is being driven by a low-value resistor, not a wildly varying source impedance, thanks to the precise complementary action of the two tubes. The biggest mismatch you are likely to see is 5% or so.

Is this true of Class AB? No. When one tube cuts off, which is typically around 2 to 5 watts, we’re back in the single-ended situation again. A Class AB amplifier has three regions of operation: upper tube ON, both tubes at once, and lower tube ON. Only in the small central region is there true Class A operation. If you are not careful, the sharp cutoff associated with Class AB transitions can even generate ringing in the output transformer.

It gets worse with pentodes. The grid lines of pentodes have higher-order curvature, so the complementary action does not fully cancel, so the summed grid lines are wavy and not straight. This was understood when pentodes replaced triodes in the late Thirties and Forties, but feedback was always used to straighten out the mess. But it was controversial at the time, with triode fans holding on to their beloved 45’s and 2A3’s (300B’s were not for sale to the public, and were never used in consumer electronics).

So there is only one class of circuit that has a constant, unvarying output impedance: Class A push-pull triode. All the rest (tube or transistor) require feedback to synthesize that impedance. In fact, of the famous Western Electric 300B theater amplifiers revered by Japanese collectors, only the single-ended model (the 91A) uses feedback. The two push-pull 300B models do NOT use feedback; Western Electric used a very unusual circuit called the Harmonic Balancer, which had been completely forgotten by the Fifties, and in following years. It wasn’t re-discovered until John Atwood and I referenced it in Vacuum Tube Valley magazine in the late Nineties, sixty years later.

This is why I never joined the SET bandwagon, but wasn’t interested in Golden Age pentode amplifiers, either. (Yes, you in back, I see you raising your hand. The paired 6V6 beam tetrodes in the driver stage of our amplifier are triode-connected, which gives a very close approximation to a 45 triode. Plus, the 6V6 pair remain in Class A push-pull under all conditions, including heavy clipping. The interstage transformer re-balances and sums the error terms before the drive signal reaches the 300B grids.)

Don mentioned the subtler aspects of the Karna Mk II’s, or Statement 300B amplifiers. This is just one of them.

The most recent "coincidence" occurred last Friday (literally within hours of each other, according to Lynn), where we made parallel revisions to our 300B cathode's circuit.

... Thom @ Galibier

Long live the 300b! 😊

Charles

Weirdly enough, both Thom Mackris and Don Sachs came up with the 6V6 driver within hours of each other. Thom was hacking around with his SET 300B amp and swapped in the 6V6 driver and was jumping up and down about it, and Don calls the same day. OK guys, you win, tell me how it’s going.

Thom plays electric guitar and has built his own guitar amp, as well as his 300B SET for his hifi system, so he’s really into tone. The 6V6 is famous throughout the guitar community for its tone. And Don’s a big fan of the 6V6 too. I’d been casting around for a replacement for the unobtainium 45 (in matched pairs no less), and both my friends say the same thing on pretty much the same day. And yes, Don and Thom chat on the phone on a regular basis. I think they have an ESP thing going.

My NiWatts started off as 6SN7 --> 6SN7 --> 300B.  You could think of the Reichert as being the launch point, although through the years, the only remaining similarity between the two designs is two tubes (the 6SN7 input and the 300B).

I was pondering switching to a 45 for a driver for quite some time, and thought, why not start with a 6V6?  I have to say that I can't find a compelling reason to change.

I let this amp sit in a 90% state of completion for several years.  I had too many irons in the fire. 

Anyhow, Lynn's conversations with Don motivated me to pop the hood and finish the job.  The spooky thing about this process is how many things the two of us discovered in parallel, and even more spooky - within days of each other.

I could swear that an audio muse was injecting thoughts into both of our heads.

The fact that we settled on so many similar design concepts (while the two designs are so radically different) still has me shaking my head.

The most recent "coincidence" occurred last Friday (literally within hours of each other, according to Lynn), where we made parallel revisions to our 300B cathode's circuit.

... Thom @ Galibier

I prefer preamps with output transformers too. Luckily, there are decent line output transformers around that are not too expensive, certainly cheaper than exotic caps. But sadly preamps with output transformers are not often offered by preamp manufacturers.

Yes I am well aware of the debate and tradeoffs in interstage coupling in amps, but I am talking about the output transformer in a preamp here. It has numerous advantages over cap coupled output provided the transformer quality is superb and the turns ratio and design is well mated to the circuit.

@donsachs 

 So if done right, there are lots of advantages to a transformer coupled preamp.  I can say that I don't miss coupling caps at all, note even the really good ones I was using previously.  So again, for me it is about seeing what was possible and then refining it.

This has been a topic of long standing discourse. Premium grade coupling capacitor versus high quality interstage transformer.

Charles

I will also add that for the preamp, I built one with off the shelf Lundahl transformers and then again, I got Dave Geren at Cinemag to wind me the best small output transformer he could after I showed him the circuit.   Better.......   Very transparent and no coupling caps anywhere!  But plenty of current drive to make cable choices far less important, which is one of the features of the Raven circuit.  Plus the transformer coupling allows for XLR or RCA output and it will happily drive headphones.  Will easily drive even a 10K amp.  So if done right, there are lots of advantages to a transformer coupled preamp.  I can say that I don't miss coupling caps at all, note even the really good ones I was using previously.  So again, for me it is about seeing what was possible and then refining it.

As I am retiring from constant production of tube gear I wanted to see what was possible.  How good could it sound?  Lynn got me to look at the symmertic Reichert circuit and it looked interesting and off I went.  Then he showed me the Raven preamp circuit, which like that amp is deceptively simple at first glance, but there is a subtle complexity to both.  The Raven took some doing to get it to take XLR inputs as well as RCA and to do so with the remote contolled Khozmo attenuator that I favor in my previous preamp.  There were things to figure out.  We did the same dance with power supplies, building it several ways and then decided to combine both the modern regulated approach with the VR tubes.  The combination is quite astounding.  So all of this is about having fun and seeing what is possible.  I could build a DHT preamp with the 26 tube, but there are few to buy.  I kind of wanted to see how good a 6SN7 can sound, and the answer is exquisitely good if done right..... without all the hum potential and microphonics, and cost of the DHT apapproach.  So it is about balancing things, using tubes that are readily available and seeing what is possible.  Now someone else can build them in commercial quantity!

I usually hang out at DIYaudio, and am the originator of the notorious "Beyond the Ariel" thread in the "Multi-way" speaker forum. Gary Dahl, my neighbor when I lived in Silverdale, Washington State, finally retired the thread when he built his own speaker system. What Thom Mackris and I are doing is a variant of Gary Dahl’s system. Most of the back-and-forth on the "multi-way" DIY forum is discussion about speakers, not amps. (You have to log on to see the graphs and illustrations.)

Brief description: 2-way high efficiency (97 dB/meter) large-format system, with Altec/GPA 416 Alnico-magnet 15" woofer, Athos Audio Yuichi A290 wood horn, 1.4" large-format compression driver, and a 4.2 cubic foot closed-box low-diffraction cabinet. Crossover will be 2nd-order at 700~800 Hz, with level-setting L-pad or tapped autoformer. The speaker is in same studio-monitor format as the 1938 Lansing Iconic, 1966 Altec Valencia, or 1978 Altec Model 19. Thom and I are having the bass module cabinets built here in Denver, and we have most of the other parts on hand.

In 22 years of membership here, reading this thread has provided more knowledge base and common sense, respect and integrity also come to mind, keep on track😉

This degree of insight is so interesting and appreciated. I do recall the introduction of the Galibier 300b SET a few years back. Three highly regarded audio veterans sharing thoughts, ideas and concepts. This is precisely why I believe that there will always be a place for high quality well implemented tube audio components. If done right, sublime sound is the successful result.

Charles 

Weirdly enough, both Thom Mackris and Don Sachs came up with the 6V6 driver within hours of each other. Thom was hacking around with his SET 300B amp and swapped in the 6V6 driver and was jumping up and down about it, and Don calls the same day. OK guys, you win, tell me how it’s going.

Thom plays electric guitar and has built his own guitar amp, as well as his 300B SET for his hifi system, so he’s really into tone. The 6V6 is famous throughout the guitar community for its tone. And Don’s a big fan of the 6V6 too. I’d been casting around for a replacement for the unobtainium 45 (in matched pairs no less), and both my friends say the same thing on pretty much the same day. And yes, Don and Thom chat on the phone on a regular basis. I think they have an ESP thing going.

Actually Lynn said if I built the Symmetric Reichert it would be the best amp I had ever heard.  I like a challenge.   Then I decided if I was going to build it we would figure out the best way.  So we ditched the driver section and came up with a much better one, and I prefer power supplies done my way, to the max so we worked on that approach too.  A year later here we are.....

Last but not certainly not least, my collaboration Don Sachs, which began with phone conversations and many, many emails after I bought Don’s preamp. Naturally, my preamp had to be different, with a balance control ... the dual-mono Khozmo volume control. One thing led to another, with Don curious about both the published Karna amp and the hard-to-find Symmetric Reichert, which some folks in London had built. The project evolved from there, gaining in refinement each step of the way.

Don was also curious about the Raven, so we discussed how it could be improved and brought up to date. To my surprise, he went ahead and built it, and we agreed on a cascaded power supply ... Don's favorite followed by my favorite, a VR-tube shunt regulator for each channel.

I am extremely pleased the Amity, Raven, and Karna designs are having their public debut. The only people who have heard them before were very adventurous DIY builders, but now, thanks to Don and the team at Spatial Audio, we are moving steadily towards production. Don has taken the amplifiers a long way on his own initiative, and it’s been a lot of fun having him as a collaborator.

I look forward to seeing you all at the show, and even more collaborations and products in the future.

People ask me about a phono preamp. Well, no. That’s an entirely different set of skills.

Speakers? My neighbor here in Colorado, Thom Mackris of Galibier Designs (turntables), and myself are completing a high-efficiency large-format 2-way loudspeaker loosely based on the Altec Valencia and Model 19. Will it be manufactured by anyone? I have no idea.

I have a friend in the UK who has built an SQ quadraphonic decoder based on my Shadow Vector patent (1975), except realized in software and with 8 spectral bands operating in parallel. It would be a real blast from the past to hear that again ... I lost track of the handbuilt Audionics prototype in 1976.

I occasionally fantasize about a quadraphonic system using Shadow Vector, a quadraphonic Raven, and four Karna triode amplifiers powering four of my speakers. That would be fun, with sound probably much like Todd-AO 70mm movie theaters from the early Sixties.

It’s been a fun journey. A summer job at NASA in 1969, during the Apollo 11 mission, running 16mm sound projectors, the Altec sound system, and working in the darkroom. Inventing Shadow Vector, which got me the job at Audionics. Going to the UK and meeting the BBC quadraphonic team and Laurie Fincham at KEF. Building the Shadow Vector prototype and designing several speakers at Audionics. Working at Tektronix as a tech writer in the Spectrum Analyzer division, and meeting Rich Cabot, who would go on to found Audio Precision. Landing on my feet after the 1988 mass layoffs and becoming a tech writer and editor for several magazines, designing the Ariel speaker, Amity amplifier, Raven preamp, and Karna amplifier. Moving to Colorado after thirty years in the Northwest, and meeting Thom Mackris, who is my neighbor only a couple miles away.

@pindac  

Hi, the amps are not even released to the public yet.   Perhaps when they are in full commercial production someone can take one to a local audio group.  I agree that is a great way to let more people here them.  One step at a time....

cheers,

Don

I have to say this thread has more information, insight, and history about amp topology and component interaction than anything I've ever read.  What a wealth of information!  Thanks @lynn_olson and ​​​​@donsachs for a stunning primer in current amp design, and tube and tranny interaction.  Normally, I would say I'd love to be at the show (I'd still love to be just to shoot the breeze with Lynn, Don, and the guys from Spatial) but I'm on the other coast.  Thankfully I can get a big helping of what's being discussed by just going to my music room and having a listen to my DS 300b stereo edition along with the DS2 preamp, Lampizator Golden Gate 2 DAC, and Musica Pristina streamer (built by another great designer).  The music is like nothing I've ever heard and I've had a LOT of equipment in front of me over the years.  Having said this, I've already made arrangements to have a pair of these new monos, and the new pre in my system as soon as they become available.  (Yeah...I know I need a CAT scan or something, but at least I've come to accept the fact I have this incurable disease to keep advancing my musical experience).  I know these amps are going to draw a ton of interest and subsequent orders, so I figured I'd get in tbe queue now.  Happy listening to all.

When individuals that have a keen interest in Audio and are time served, make a discovery that is able thoroughly impress, there should be little doubt as to the devices capabilities when properly partnered with supporting ancillaries.

Even though these Amp's are yet to receive a name, I know they will capture many when encountering them for a Demo', and a fair amount of talk should follow on. I know this experience personally through using my system for Public Attended events where demo's of my system and multiple other owned systems have been exhibited for demo' purposes.    

I wish the Amp' was available for one of my Local HiFi Groups future get togethers.

I'm sure all the owned and respected Sources and the selection of Quad ESL Speakers will be partnered to it for numerous hours of indulgencing in the delights it can conjure. 

I look forward to hearing this amp at PAF. I'm currently running 300B PSET Monoblocks (transformer coupled) and love the way they sound. 

Wish  the amp was going to be at T.H.E. Show in SoCal as i would love to hear it.

@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.

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. 

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 👍

@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....

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.

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 

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.

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

..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.....

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.

FULL MUSIC makes decent 45 and 50 tubes.

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:)

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.

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.

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 

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

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.

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.             

@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
 

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.

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.

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?

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.

@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.

Enjoy !    That sounds like a great amp.