300b lovers

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

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 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.

@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 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.

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.

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.....

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.

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 

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😉

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.

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 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.

@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

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.

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.

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

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

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.

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.)

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!

@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

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.

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!

 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?

@whitestix Usually a thing like a coupling cap will reveal its character fairly quickly. We've been doing this since the 1970s and in that time have yet to see a coupling cap change so dramatically during break-in that it exceeds the character of another, better sounding cap with the same time on it. So you can audition them easily right out of the box. So far the Teflon dielectrics have proven themselves over and over again. A regard paper and oil as very nice sounding parts as well, but they can develop a voltage drop across them which can throw off operating points in the design; IMO not worth the return shipping and frustration!

@lynn_olson I've been harping about the audibility of the higher orders for a very long time. Nice to see some agreement in this regard.

@donsachs @lynn_olson @thom_at_galibier_design 

I confirmed a theory I had about OPTs over the weekend. You might give this one a try; its inexpensive. Replace the bolts that hold the OPT together. Typically these are made of steel and are insulated from the transformer by fiber or plastic shoulder washers.

You can get non-magnetic stainless bolts to replace the steel bolts. Its not a big change, but in the case of SETs or lower powered tube amps in general, every drop counts. I measured about a 7% increase in amplifier power. I suspect this will vary depending on the transformer design as well as the specific alloy of stainless bolt used.

The shoulder washers are supposed to take care of the problem of a magnetic short of course and for the most part they do. But they don't do the job perfectly and I suspect that since this technique is 70-80 years old, tradition has set in and caused no-one to look into it further.

I found out decades ago that the mounting bolt in a toroid transformer, commonly made of common steel, would heat up more than the transformer because the actual toroidal mag field was sloppier than theory. So the bolt was a short to the field. By replacing it with non-magnetic stainless the transformer ran at a lower temperature.

I've been working on a low power PP tube amp recently so decided to give this a try.

I just scanned through this thread and while some is over my head, it is extremely informative. I love reading the journey behind the development of a new product, especially when the main driver is to create the best sound instead of the most profit. I am only sad because I was planning on going to PAF this year since I missed the first one, but I now have a work event that will not permit me to attend. If I were to attend I think I may be signing up to order these amps/preamp. Hoping and expecting those who attend will post their reactions here, thanks.

Fascinating read. I'm definitely stopping by the Spatial room at PAF

You want a technical presentation, here’s my talk at the 2004 European Triode Festival:

2004 ETF Presentation

As for the lengthy previous post, I am not sure about the impact of dynamic variations in output impedance. One way to measure the impact would be to measure distortion harmonics at a constant voltage output, compare 4 and 16 ohm loads, then repeat again into a reactive loads.

The key point stands: Push-pull Class A triode has the most linear interface to loudspeakers. Nearly all loudspeakers generate significant back-EMFs, which are resonant in character, that reflect back into the output devices. Other topologies have nonlinearities or discontinuities that interact with these resonances, which exaggerates speaker coloration.

There are planar-type speakers that present essentially resistive loads, but the reason for this flat impedance curve is very low magnetic coupling between the diaphragm and the magnetic system. As the magnetic coupling (BL product) becomes more intense and efficiency increases, resonances appear in the impedance curve, as well as smaller narrowband divots that reflect stored energy. Regrettably, speakers are inherently resonant, particularly as efficiency goes up. It has to be kept in mind that most speakers are stupendously inefficient, ranging between 0.3% and 1.0%.

@lynn_olson 

FYI,  the guy who published DH tube comparison more than 20 years ago uses the same driver and same speakers as well the same signal source during his blind listening tests.

I can confirm his report that PX25's bottom is much solid and deeper than 300B. 300Bs could not even match 7A1's deepness.  The common sense is 300B could not be driven into A2 region w/o distortion, so I consider 300B is a good candidate for pre-amp/headphone amp which always worked at A1 region.   Also, the measurements of output impedance from 300Bs is way higher than PX25s.  FYI, my 6336 SE amp beats my 300BXL (beefed up 300B) amp hands down at bottom spectrum.

You are right about driving stage quality is the bottleneck of MOST commercial SE DH amps.  but the reason is not what you stated.  The real secret of making best SE amplifier is to SHUNT REGULATE POWER SUPPLY for the driving stage. Push Pull amps are less critical of regulated power supply for its driving stage.

Many roads lead to Rome...

As to DHT shootout, using the same driver may suit certain output DHT tubes better than the other ones. Different DHTs also need different output transformers, different B+ voltages and filament supplies, etc. So AB test is not that easy as more than one elements have to be changed for each DHT output tube to have a chance to perform at its best potential. That said, there is no bad DHT in my personal view - assuming adequate power output. So, many good choices are around!

Don,

Oh my, I was not aware that I had the good fortune to be the first in line for the mono's, but I do recall that I was a very early owner of your first preamp as well your DS2 preamp, both remarkable preamps.  I am gratified to see revealed on this thread the thinking that went into the creation of these stellar monos and the collaboration that brought it about.  The participation of Ralph and other knowledgeable audiophiles, along with Lynn for sure, has given us all an appreciation for the bounds of your joint inquisitiveness that has wrought such a stellar component.  Obviously, it like nothing I have ever heard before, just as you said it would.  The Spatial Triode Masters (which you encouraged me to buy nearly 6 years ago before you yourself had your own pair which was a stellar recommendation as all owners of them know) sound amazing with your amp and my soon-to-be-delivered Cube Audio Jazzon speakers clearly will love to be driven by your monos so expect a report back on that soon.  

Finally, as others have said, this has been among the most informative and civil threads I have ever participated in on the forum and I am among many who are excited to hear your room in Seattle next month.  

This place needs more info like this, I almost resigned sludging through some pretty pathetic posts that all went south and should have been erased.

The 300B ... all of them ... quite happily accept at least 20 volts of positive grid drive. This is not secondhand info gleaned off the Internet, I’ve seen it for myself on a Tektronix scope screen back in the Nineties. I was frankly surprised, because there wasn’t even a trace of a kink or a glitch as it went from negative to positive grid drive. I was expected more drama from the Big Bad Positive Grid Drive, but nothing, no drama, and no signs of grid or plate overheating, either. The 300B is a pretty tough tube, even the bargain-basement Chinese tubes of the day, back in the Nineties. The only reason I stopped at 20 volts is I lost my nerve at that point, and rolled back the gain. I actually have no idea how much power can be pushed into the 300B grid.

That’s when I realized why this amp, or rather the Amity precursor, sounded like a 60-watt tube amp, or a 150-watt transistor amp (I compared it to a Crown Macro Reference and it played just as loud). It just acts like a compressor when things get hot and heavy, and the separate B+ supply for the driver sails right through the heaviest output stage overload. Best of all, the interstage transformer recovers instantly from overload, nothing like RC coupling which requires the cap to re-charge once grid current flows.

An interstage transformer is certainly a benefit for a single-ended amp, thanks to the efficient power transfer and smooth entry into the A2 region, but the benefit is much greater for Class A push-pull. You see, in "normal" or Golden Age amplifiers, only one driver plate is available when the power tube grid goes into A2 and current starts to flow, or if heavy current is needed to overcome Miller capacitance.

The power tube grids never demand A2 at the same time; they take turns. This is important when a push-pull driver, coupled to a balanced interstage, needs to deliver A2 current into a 300B grid. Thanks to summing in the interstage, both sides of the driver circuit are available to push current at every moment, and not only that, because it is a symmetric circuit that always remains in Class A, it is far more linear than a single-ended driver. It really is a small, very linear power amplifier in its own right, so it shrugs off the demands of the DHT grid.

The key principle in a non-feedback amplifier is the lowest possible inherent distortion, coupled with immediate recovery from overload. A balanced, low-distortion driver that uses a well-balanced interstage transformer offers the greatest voltage swing, greatest linearity, and greatest immunity to a reactive load. And the requirements aren’t trivial: one grid needs to swing up 100 volts while the other grid swings down 100 volts, and this needs to happen at 30 kHz with no slewing, transformer saturation, or power supply sag.

This is far beyond the drive requirements of any pentode (35 volts typical), and double anything seen in a SE amp, unless it uses an 845 transmitter tube. (If you’re wondering why so many 300B amps sound mushy and dull, there’s your answer. It isn’t the 300B. It’s the driver falling short.)

That’s also why I discarded the SE driver -> PP output topology. I built an amplifier that used this approach, but the demands on the interstage transformer were too severe, and it never had the clarity or the sense of unlimited headroom of the balanced driver approach. That’s also when I switched from 5687/7044/7119 driver tubes to push-pull 45’s ... actual power tubes with 25~32 mA of current going through each one. Since the 6V6 was specifically designed to replace the 45 (they bias up pretty much the same) in an era when feedback was not universal, the performance of the 6V6 had to be good enough to replace the 45 in hundreds of thousands of radios in the mid-Thirties. The guitarists adopted it and the rest is history.

Which I guess leaves the question why does the input stage have the topology it does. The quick-and-easy approach would be a Mullard-style long-tailed pair or differential stage, or maybe borrow from Williamson or Dynaco and direct couple a half-6SN7 input tube to a "concertina" or split-load inverter. It would certainly be cheaper, and is the approach of just about every Golden Age amplifier.

This is the intuition part. There is something wrong with the sound of Golden Age amplifiers ... hard to describe, and it’s not there in SE amps. Something to do with diminished low-level detail, subtly flattened tonality, and a lack of air and "shimmer". The folks at Sound Practices were confident that this "PP" coloration was inherent in push-pull itself, and that’s where I parted company with the common wisdom.

I became convinced the problem was the phase-splitter tube. For one thing, the three approaches to vacuum tube phase splitting (split-load inverter, long-tail pair, floating paraphase) sound quite different, and they all have varying levels of that "push-pull sound". So why not take a passive approach? Studio transformers have been around a long time, and if they are good enough, retain phase integrity through 20 kHz. Then the rest of the amplifier can simply be fully balanced, with none of the circuitry devoted to phase splitting, just amplification. Do one thing, and do it well.

Sure enough, even in the first version of the Amity in 1996, the coloration was gone. It didn’t sound push-pull, and it didn’t sound SE, either. It sounded like itself, and not like anything else. The rest of triode community went their own way, off in SE-land, and I did a lot of historical research for Glass Audio and Vacuum Tube Valley, while thinking of the next steps beyond the 2-stage Amity amplifier.

I’m sure this will sound wonderful - but I can’t stop and think about why not simply run 845 SE, what are the sonic factors driving the adoption and development of 300b push-pull?

I do understand the desire to avoid working with high voltages however.

The 845 is not happy at 500 volts. It is a (low-power) transmitter tube, and is designed to work from 800 volts (minimum) to well over 1000 volts.

Once you go over 500 volts, construction, and the parts required, are a whole different world. It requires ham-radio transmitter technique. Parts are air-spaced, wires DO NOT lay on each other, circuit boards are out of the question, and special-order high-voltage power and output transformers are required. Electrolytic caps have a hard upper limit of 550 volts, and 1 kV film caps are industrial parts, not audiophile specials. In short everything is different. Consult a 1950’s American Radio Relay League (ARRL) handbook to see what safe construction technique looks like. It is nothing like audiophile practice.

Sure, the builder can ignore safe construction technique and build it the regular way, but that’s a very serious safety and fire hazard. You do NOT want an amplifier exploding and then catching on fire. Transmitter technique takes us out of consumer electronics and into the realm of professional high-voltage equipment ... interlocked chassis doors, special start-up techniques, status lights, etc.

Yes, I see audio equipment at shows with hard-core Eimac transmitter tubes that light up the room. I would never allow anything like that in my house, unless it was in an outbuilding. The companies that build these high-voltage amplifiers have no track record of building ham or pro radio gear ... they’re just winging it, despite the curved glasswork and the pretty CNC chassis.

By contrast, the 300B lives in KT88 territory, with similar voltages and operating currents. Standard hifi building technique, but still not a plaything. The voltages in the B+ caps are quite lethal, so no poking fingers where they don’t belong.

Best thread on Audiogon in a long, long, long time.  Great information, and has me wishing I could attend the Seattle show.

@lynn_olson

The usage of 845,211,805,GM 70 and other high voltage transmission tubes have been manufactured and sold by very reputable and established brands for quite awhile now. They’ve been enjoyed by owners for years without reliability or hazard issues. So it appears they’ve addressed the real world problems you logically cautioned about.

I will say however that this new approach/design of utilizing 300b tubes in push-pull is highly interesting and frankly exciting. Certainly in terms of acquiring higher tube power (Thus expanding speaker choice flexibility). I’m really looking forward to reading listener feedback from those attending the Pacific Audio show.

Charles

The output transformer for 845 SET should be at least 10K Ohm. And it can't be as wide a bandwidth as a 3K output transformer for 300B. Another issue is that driving 845 tubes is much more difficult.

A well designed 845 SET will always be inferior to a well designed 300B SET in sound quality.  

@alexberger

A well designed 845 SET will always be inferior to a well designed 300B SET in sound quality

I am a thoroughly happy long term 300b SET owner. However, I do know that there are excellent sounding high voltage transmission tube SET amplifiers. Do you believe that the output transformers in these amplifiers is the overwhelming factor that informs your opinion?

Charles

To distill much of the above.  What struck me (well, Clayton and folks at Spatial Audio as well) with the first build of the circuit in a stereo box with a CCS on the plate of each driver tube was the purity of the sound.  It didn't have a sound per se. The amp just got out of the way.  The first prototype of the Spatial X4 fully passive speaker with the new crossover was there.  It was about 87 dB and 4 ohms.  The amp drove it with ease.  One of their techs, who is a lot younger than I, liked it LOUD.  I went out to grab lunch and when I came back, Ryan had it cranked so loud I could not be in the room.  No strain, no clipping, no nothing.  It just got louder.  He said it played as loud as his Schiit mono block SS amps, and sounded FAR better.   It indeed sounds like a 60-100 watt tube amp for drive, but it sounds far better than any other amp I have heard.  Of course we have spent a year improving that prototype considerably.  There are a number of reasons and they are all covered above.  Circuit, power supply topology, coupling methodology, inductance of one form or another between every tube plate and their power supply.   Real world modern production tubes, with many NOS versions available.   It all sums up.  What you hear is what you don't hear... various sorts of subtle distortion from other topologies that give amps a certain grayness.  Trust me, I have heard a LOT of other amps.  You don't even realize that it is there until it is gone.  That is what these amps sound like.  Or don't sound like....  Been a fun journey with Lynn, working out real world builds of the amp and preamp circuits.  If you can come to Seattle, please hear them and give your opinion!

Hi @charles1dad ,

I don't have practical experience. But theoretically 300B is winn-winn vs 845.

1. Output transformer has wider bandwidth.

2. Easy to drive. Driver tube and driver transformer.

3. Easy to build a high quality PS for 400v vs 1000v.

The only one but significant advantage of 845 is more power. But it looks like a parallel 300B SET or 300B Push-pull can be a better solution if you need more power.

The 300B ... all of them ... quite happily accept at least 20 volts of positive grid drive. This is not secondhand info gleaned off the Internet, I’ve seen it for myself on a Tektronix scope screen back in the Nineties. I was frankly surprised, because there wasn’t even a trace of a kink or a glitch as it went from negative to positive grid drive. I was expected more drama from the Big Bad Positive Grid Drive, but nothing, no drama, and no signs of grid or plate overheating, either.

@lynn_olson Back in the 1990s we built an experimental OTL that used four 6300bs (a graphite plate variant of the 300b) per channel. The plate voltage was only 150V since higher than that is impractical in an OTL. To get the tubes to conduct properly we biased the tubes at +15V as their operating point. We played that amp at CES that year. The only reason we didn't produce that amp was it was impractical- that's a lot of money to spend on power tubes for a 15 Watt amplifier! We could get slightly less than double the power using four 6AS7Gs which could be had for less than the cost of one of those 6300bs.

Do you believe that the output transformers in these amplifiers is the overwhelming factor that informs your opinion?

@charles1dad I've said it many times in the past. The greatest limitation SETs have is getting bandwidth as the design is scaled for more power. The OPT is the defining issue.

@atmasphere   So many paths to audio nirvana.  So many tubes, so little time:)

@atmasphere

I’ve said it many times in the past. The greatest limitation SETs have is getting bandwidth as the design is scaled for more power. The OPT is the defining issue.

Yes you have and with clear explanation. I was specifically asking @alexberger because he was quite emphatic. A thread with this caliber of participants stimulates further inquiries from posters. Informed commentary is valued. 😀

Charles

@donsachs 

So many paths to audio nirvana.  So many tubes, so little time:)

+1

Charles

A thread with this caliber of participants stimulates further inquiries from posters. Informed commentary is valued. 😀

@charles1dad Such is the nature of the internet I suppose, where fact and opinion freely co-mingle. The physical nature of SET transformers are governed by physical law FWIW and that law isn't interested or caring about opinion. It simply is.

Since the 300B fits into the KT88 ecosystem, the same choices for power and output transformer apply, except you’re not messing with ultralinear connections, and some thought (well, a lot of thought) needs to be applied to the filament circuit, which is a very critical node sonically.

I agree about the output transformer. That’s the make-or-break part. Fortunately, we have many good PP output transformers, going back to the Partridge in 1948. Lots of good ones today, too, as long as they are PP and you specify the allowable offset current. The SE world has not quite as many choices, but there are still lots of vendors making good parts these days. Many more than the Nineties.

The interstage is tougher. There are fewer choices, and it is a more difficult design assignment, since impedances are higher than the output transformer. Don’s industry connections came in very handy here, so we had a custom unit designed for us, with outstanding performance.

The original Karna had two interstages, which was frankly over-the-top. Requirements for the first one bordered on impossible, since impedance from the first tube was much higher than the driver. When I switched over to the 6SN7, I decided enough was enough, and went with simpler inductor loading instead.

@lynn_olson 

The original Karna had twointerstages, which was frankly over-the-top. Requirements for the first one bordered on impossible, since impedance from the first tube was much higher than the driver. When I switched over to the 6SN7, I decided enough was enough, and went with simpler inductor loading instead.

I love the candor.

Charles