300b lovers

Hi @atmasphere , 

McIntosh MC30 has the similar cathode follower driver 12ax7 to drive 6L6 directly. The output tube works in fixed bias in this schematics. Isn't it? Can you explain how the output tube bias is self adjusted?  

Don is being modest. The last year, going through the present, really made Don pursue every obscure byway of amp design, building and listening as he went, every step of the way.

Don started with an obscure version that I called the Symmetric Reichert, which was literally a Reichert 300B done twice, with a phase-splitter transformer at the input. All RC-coupled. He built that and called me out of the blue, about a year ago.

Don then tried separate B+ supplies for the input+driver and output section, and an interstage transformer between the driver and 300B’s. A few months later, Don used a triode-connected 6V6 instead of the hard-to-find 45 driver. Thom Mackris and Don independently tried this at just about the same time, pretty much on the same day. Don (but not Thom) then used active current-source loads for the input 6SN7, instead of resistor loads. That was the Stereo version Don built and shared with the Spatial team and the first customers.

Next, replacing the active current source loads with custom Cinemag inductors designed for the purpose, and using the shoebox-format monoblocks that became the show amps. My Colorado neighbor, Thom Mackris of Galibier Design, has been following along in a parallel project, with a SET architecture, but with passive CLC B+ supplies and damper diodes for rectification.

That’s where all of us were a month ago ... Don Sacks, the team at Spatial, and Thom Mackris. The latest from Don is an IT between the 6SN7 and the 6V6, replacing six other parts with a much simpler approach ... provided the IT was up to the task, which it is. The IT has turned out to be superbly designed, exceeding expectation, and also making our lives simpler. Don and I have gone full circle, and re-invented the Karna (after trying every alternative), with far more advanced power supplies that were not available in 2003.

Don really has tried every topology, one after another, and carefully measured and auditioned each one. RC coupling, active loads, LC coupling, and now, IT coupling. By lucky coincidence, Thom has been walking a parallel path with his SE topology. All four groups ... Don, Thom, Spatial, and myself, have been exploring this zero-feedback approach for several years now.

If other folks want to build transistor Class A, Class AB, or Class D, more power to them. Those designs have an entirely different set of challenges that have nothing to do with triode amplifiers. In triode amplifiers, the devices themselves are exceptionally linear, and the appropriate circuits take advantage of that.

Lastly, I will say that internet forums are both a blessing and a curse.  There are people who tout one amp topology over another, or whether they prefer SS or tubes, or interstage transformer coupling over LC or RC, etc...   When you read these things consider the source.  There are lots of people who have strong opinions about what sort of coupling to use between tube amp stages, or even whether they should be direct coupled.   We all have our biases.  You need to figure out whether that poster has actually tried custom transformers for example or whether the opinion is based on some off the shelf midrange transformer.  Or the person who states that coupling cap quality doesn't matter much, or they love this or that cap.  The question is what have they heard before, what circuit is it going in, etc.... I have had several cherished notions overturned by this project, simply because I experimented thoroughly with different approaches and let my ear by the final arbiter.  I started out with the silicon assisted tube amp approach and ended up with all IT coupling, like the original Karna amp.  So my point is that you have to keep an open mind.   I am definitely a tube guy and solid state makes me unhappy when I listen for long periods of time, but that said, if a pair of Ralph's latest and greatest Class D amps appeared in my living room I would give them a serious audition and chance to shine.  I hope to hear them at a show some day.   As I said way up above many pages ago, there are many paths to audio nirvana, and we all may have our own.

Again, basically what Lynn said.   At each stage of the project whether in the preamp or amps I discussed the exact parameters and even shared the relevant part of the circuit with Dave Geren at Cinemag, who is a very experienced master winder of small transformers.  He models everything and makes a prototype and then I build with it and listen. Then I give my impression and measurements of the frequency response to him and we do round two where he tweaks the design and that is that.  Dave is also a maestro of core materials and interleaving.  The result is that all coupling caps are now gone from the input to the preamp to the output of the amplifiers.  Certainly you can accomplish this with direct coupling in places.  Every approach has strengths and potential pitfalls.  One advantage of transformer coupling is the complete isolation of every stage and the banishment of hum, and also far greater immunity to ambient RF.   Also, if a tube goes south, or a customer does something odd like put the wrong tube in the wrong place (yes I have repaired things where people did this), or pulls a hot running tube, any potential damage is limited to a very small portion of the amp.  Direct coupling can lead to a daisy chain of failures, cascading through your amp.   These are considerations for commercial gear, where numerous units will go to many environments over which you have no control.  I can build whatever I want for my living room because I know how it works, and I can fix it.  But if a commercial product I want reliability.  Transformers are very reliable.

I will say that I have had the luxury of semi-retirement to spend a over a year with these preamp and amp circuits.  That has allowed me to try pretty much every permutation and combination of power supply and coupling topologies.  We have finally settled on everything and the final tweaking is about done.  When you are trying to make a living selling gear you generally get something that works and sounds quite good, and is reliable and then you make them.  When you are semi-retired you can go down every rabbit hole until you find the exact sound you seek.  Good enough isn't good enough.....

So my comments above are based on this approach.  I cannot tell you whether you will like all transformer coupling vs. RC or LC using your favourite capacitors.  I much prefer IT coupling, but I had everything custom wound by a very experienced winder, and we did a prototype and final version in each case.  I can do this because I have time and am willing to devote some money to the project.  For your one off amp you are forced to buy off the shelf transformers, but there are some very good winders out there and if you communicate what Lynn discussed with them, you may well get a very good solution.  Just don't expect to pick a transformer off some web site and have it work perfectly.  You will have to communicate with the winder.  I wish you all success.  If you persist, then you can probably get a great transformer for your circuit.

 I will also say that if you heard the gear at the Pacific Audio Fest in June, you heard prototypes and the set in my living room is considerably better, both preamp and amps.  Now they are about done.  The cases will be redesigned by the folks at Spatial Audio and the amps are getting physically larger so we can fit one more mod under the hood, which I expect to bring the sound up yet another notch.   Then they will be commercially available, probably late Q4 or early Q1.  There are already 4 or 5 folks on the wait list at Spatial and I would expect them to have gear very late this year or early next.  So if you are on that list, your patience will be rewarded with gear that is considerably better than what was shown in Seattle.

I apologize for making this sound confusing, but I wanted to give the readers of this forum a taste of what Don has been through. There are many possible ways of getting a zero-feedback amplifier wrong, and from the standpoint of mainstream audio engineering, all zero-feedback amps are wrong ... not just in practice, but in principle.

An all-transformer coupled amplifier is especially wrong. The accepted path is DC coupling throughout, using transistors, with lots of excess gain for plenty of feedback. Modern feedforward techniques (Bruno Putzey, THX, et al) can get distortion into the parts-per-million range, so why look elsewhere?

Unless your goals are subjective, and you have a weird hypothesis about linearizing each stage, as much as possible, without using feedback. That’s why Don took a gamble on the Karna topology, a circuit out of the late Twenties and mid-Thirties.

This is entirely up to the transformer designer. They need to know the Zout, or Rp, of the tube driving the primary, and the load on the secondary, which will either be a pure capacitance in the 60~80 pF range, or paralleled with a load resistor, typically 100K or so.

The method of extending HF bandwidth is interleaved windings, and this falls into the realm of modern computer modeling. Back in the old days, this was cut-and-try, now, it can be modeled. Interleaved windings extend bandwidth, but the interleave pattern has to be carefully chosen so there is no HF ringing into the intended circuit. (This is why they need to know the Zout of the preceding stage and the load of the following stage.)

They will want to know your expected bandwidth, power handling within that bandwidth (particularly below 40 Hz), and how much square wave overshoot you will accept. And the DC parameters ... if SE, how much quiescent current does the tube run at, if the circuit is balanced, how much DC imbalance do you expect from the pair of tubes. This affects core gapping, which in turn dictates core size and transformer size. A small air gap linearizes the transformer, but can also double the required core size, which in turn affects HF bandwidth. The DC parameters are critical for the entire transformer design.

As you can see, this isn’t a matter of selecting an off-the-shelf part, but consulting with the transformer designer and telling them what you need (and what they can do). They *might* have an off-the-shelf part, or they might not. If not, what is the minimum order, and how long will that take?

I haven’t mentioned sonics yet. Aside from meeting minimum technical specs (which you and the transformer designer both agree on), there’s the matter of subjective sonics, and how it fits with the sound you are aiming for. This might sound trivial, but if the amplifier designer has no subjective sonic goal, you will not get there. "Perfection" is not a goal, it’s a marketing term, like "Perfect Sound Forever" for CD’s back in the Eighties.

Are you familiar with the subjective difference between RC coupling, LC coupling, active current-source loads with capacitor coupling, and interstage transformer coupling? (With this amplifier design, Don built and auditioned each one.) This is very useful to know as the amplifier is tuned subjectively.

Similarly, the driver stage design has a major effect on amplifier sonics, aside from inter-stage coupling. The driver section affects slew rate, HF distortion, and subjective colorations in the mid and upper frequency range. It’s useful to know the sound of a 6DJ8, 12AU7, 6SN7, and a power-tube (45, 6V6, KT88) driver ... they sound quite different from each other, and can dominate the sound of the entire amplifier.

What is the minimal inductance is acceptable for 6sn7 interstage transformer ? 80H, 70H, 60H?

The problem is transformers with high inductance have narrow high frequency bandwidth. But if interstage transformer doesn't have high enough inductance with 6sn7 the low frequency will be cut off. 

See page 21 for a construction article on a push-pull, zero-feedback 300A/300B amplifier. This is about the same time as Columbia introduced the LP microgroove record, so it’s very early days for high fidelity. What surprises me about this article is that 300A’s and 300B’s were even for sale to the public; they must have been pulling them out of prewar theater equipment

At the time of printing, the only records you could actually buy were 78’s, and FM radio was very new. Note the primitive state of tonearms and phono cartridges ... also, 78’s had no standard equalization, so preamps had to cover several curves, including "acoustical" for pre-electronic records.

The reign of the monophonic LP record was surprisingly short; ten years later, almost to the month, stereo LP’s were announced by all the major labels, and stereo cartridges and stereo preamps were also on the market.

Audio magazine, July 1948

Audio magazine, August 1958, the Stereo Issue

Audio magazine, November 1982, first CD player

Agree 100%. Don is describing what I hear.

The metaphysics are optional, but they have a good toolkit for describing perception.

Basically what Lynn just said.  Once you hear it you cannot go back.  Even the best caps cannot touch it.  Now, that said, you will not hear this if the rest of your system is not up to the task.  You put a $500 DAC in front and a mediocre preamp that uses small signal tubes, and a mediocre pair of speakers behind and you will mask all of this.  You are not going to get this with cheapo ITs either.   The Lundahls I played around with last year sounded good, but rang at 15KHz.  That doesn't mean Lundahls are bad, just that they were not designed specifically for this circuit.  So if you get really good quality ITs that are designed for your circuit, then you will hear what Lynn described.  These replaced VCap ODAM, and I also tried top shelf Miflex and Duelund JDM copper foils.  All are very good caps.  None has the realism of the custom wound Cinemag transformer.  I have no doubt the Monolith iron is superb and I am about to try their OPTs whenever they arrive.   But you need iron of this quality.  You wouldn't judge all coupling caps based on a cheapo Solen, nor would you judge all ITs based on something like a Hammond or Edcor.

There are levels and to my ear in our circuit (important caveats), the top quality IT is better than the top quality coupling cap used in the LC with a very good anode choke.   My 2 cents and feel free to disagree, but it is what I hear.

What you will hear in your circuit .... I don't know, and it doesn't make me right and you wrong or vice versa.

These are the kind of conversations I used to have with Harvey Rosenberg, who definitely "got it". Unless you knew him, you didn’t realize his clowning around, and occasional jaunts into speculative metaphysics, was an act designed to chase away mainstream audiophiles (which it did). Of course, I can speculate about metaphysics for hours on end, as folks who met me at the PAF discovered. But the clown act was pure Harvey ... I’m too sobersided to joke around like that. But we did discuss the subtler aspects of musical perception, and how they interacted with culture and worldview.

I am not sure where capacitor coloration comes from, or what’s causing the hours-long "burn-in" process. It can’t be measured, and the plausible mechanisms are pure guesswork. But it’s there. In a positive sense, direct-heated triodes, in the right circuit, have very special qualities.

Not everything is revealed by the spectrum analyzer, and the usual audiophile lingo falls short of describing what triodes really sound like. Sanskrit or Japanese might be better, but I don’t speak either, although I am conversant with a few basic Zen, Taoist, or Vedanta concepts. English is not particularly good at discussing subtle aspects of perception and consciousness.

Now that Don has built a version with zero coupling caps, he’s having the same experience I had when I first heard the Amity in 1996. The Karna was a development of that, and the Blackbird is a development of the Karna.

Umm, hard to describe. More "there" there. More sense of a physical presence, and a better feel of the performer’s musical intentions. More hall sound. Most of all, a feeling of the performer being in the room instead of somewhere "out there".

Not the usual audiophile verbiage, but it’s immediately evident when you hear it.

By now, the Blackbird is very close to the original Karna, but with greatly improved power supplies and a much more practical monoblock construction, which also improves performance. Full credit to Don for pulling off what I thought was impossible.

As you approach the highest levels of audio, the sound kind of takes a right-angle turn from the usual path of improvement. Instead of just getting more and more clear and vivid, suddenly, there’s a physical sense of musicians being right there in the room, instead of sounding like a very good recording. The instruments have physical size and feel like you can reach out and touch them.

A piano sounds like it’s five feet away, and it sounds BIG. Even "bad" recordings sound like this. Is something being added? No, I don’t think so. Instead, a mechanical quality is no longer there.

This phenomenon is well known amongst triode practitioners. It’s not a secret. Unfortunately, it is almost never heard at hifi shows, so don’t expect it there. I’ve heard it several times at the advanced DIY level. Never heard before from any mainstream or transistor system, which is why I started working with direct-heated triodes in the early Nineties, after hearing and reviewing the Audio Note Ongaku and the Herb Reichert Silver 300B.

Hi Lynn and Don,

It is very interesting to hear more details about IT versus LC coupling between 6sn7 and 6v6. What is a difference in sound?

I have to agree with Eddie about the ST-style 45. Aside from driver use, the charm of the 45 is that it is really easy to make a superb, low-power amplifier with it. It has the cleanest distortion spectrum of any tube I’ve measured (closely followed by the 300B), but unlike the 300B or 2A3, it is super easy to drive. This makes two-stage amps simple ... in fact, I’ve yet to hear a bad 45 amp, while I’ve heard plenty of not-great 2A3 and 300B amps. Good 300B amps in particular are quite difficult.

45 amps also sound louder than you’d expect ... they easily keep up with 2A3 amps, despite what the numbers say. If 45’s were more abundant and at different price points, they would quickly find a market.

I have a dark suspicion that modern 2A3’s are simply below-spec 300B’s, and not related to original 2A3’s at all. Of course, that’s not really true, since 300B’s have 5V filaments while 2A3’s have 2.5V filaments. Modern 2A3’s are quite different than the bi-plate RCA originals ... I’m not sure where they fit, actually.

Returning to the 45 triode, I really think people would be surprised just how good 45’s sound, and how crisp and vivid they are. They are very different sonically than 2A3’s or 300B’s. They actually sound more like 845’s than anything else. Very clean and fast, no murk at all, and nothing like any pentode (no hash or grain). If you can get your hands on a good pair (or quartet) of 45’s, they are quite impressive and worth exploring,

The Raven is something else, quite unexpected, really. Don did a superb job on it, no question. The Khozmo volume control, with a L/R balance control thrown in, is the extra deluxe touch.

@donsachs Yes, new tube types are a daunting investment from the manufacturer’s perspective. My top wish list, without a doubt, would include an ST style 45 (which is the driver of choice in my personal PP 300B build). Followed by a round plate 6SN7, and then maybe an 801A (have not tried Mayers versions yet, but the originals are spectacular).

Regarding iron, I saw Monolith Magnetics mentioned here. I have had great success with everything I’ve tried from them (OT’s, interstages, and plate chokes). I can’t recommend them enough. Their IT02 in particular is stunning, and defies what I thought was actually possible with magnetic coupling.

Getting back on topic, I’m elated to see a fresh PP 300B amp hitting the market. The PP Triode topology has been a personal favorite of mine for many years, and this was most certainly inspired by Lynn’s work/publication of the original Karna, well over 20 years ago now!

I have to say I’m really looking forward to hearing the production Raven :)

@eddie138 Thanks so much for chiming in.  Really interesting post and the economics are of course as you stated.   It is not like getting a custom capacitor made where you only have to have 100 of them for an order.   I guess that explains why manufactures stick with the known tube types where there will be a market.   Now if someone would start cranking out moderately priced 45 tubes......:)  But you would have to sell a lot of them to make it worthwhile.

I am impressed you took the project that far, but the minimum requirement for 10,000 units on an unknown tube is a steep hill to climb. Even at OEM prices, that’s a half-million dollars on a gamble. The "X" tube would have to be very very good, and very very popular, for that gamble to pay off.

In other news, the all-IT, no coupling-cap version of the Blackbird is the best version yet. The LC coupling on the 6SN7 is going away and getting replaced with a custom IT with 18 Hz to 35~40 kHz bandwidth. This gets rid of six parts - a pair of 100 Hy inductors, a pair of copper-foil coupling caps, and two 220K grid resistors. No RC coupling, no LC coupling, and no current sources, either as plate loads or in the cathode circuit. The signal path is copper wire, high-nickel magnetic cores, and vacuum tubes.

As Don mentioned on a recent phone call, there is no direct electrical coupling between any of the stages, which filters off any RFI incursion before it gets amplified. Homes are much noisier in the RF spectrum than they used to be, with Bluetooth and WiFi everywhere.

Tube lineup remains 6SN7, matched pair of 6V6, and matched pair of 300B. Tube rolling is welcome so long as pairs are matched.

One of the prototype triodes described above. Note the thin internal shape of the box-plate which is similar in dimension to the 300B pic in the datasheet. Also note the added material folded over on the plate clam shells for heat sinking, 40W in that diameter/size bottle is asking a lot:

Hi Lynn and Don (and the OP!), it’s been great fun watching this thread. I heard from several PAF attendees that the Spatial room was a highlight of the show. 

A friend and Audiogon member directed me to this thread (Hi Mark!). In reference to Lynn's suggestion regarding a cathodic triode, several years ago I worked directly with JJ Electronic on an octal based cathodic style "300B". This was originally a joint project with Matt Kamna, who also desired an easily implementable cathodic 300B derivative. We tried modifying all the usual suspects (KT88 included) but the best candidate was their EL509S. With that tube we could simply remove G2 and the beam forming plate, tweak the G1 pitch, and you get a viable power triode, with a hell of a cathode to work with...

Unfortunately, without further tweaking the electrode spacing (which requires new dies for a new mica design), and gently massaging the plate shape/design (also requires a big investment in tooling), the best we could get is as follows: H=6.3@2A, 300Vp, -56Vg1, 60mA-Ip, 2.6mA/V-Tc, 1.5K-Rp. Which makes it a bit of a pig and does not provide much linear swing to work with at reasonable B+ potentials. I will try to add photos of the curves to illustrate this.

Grid Curve 300Vp fixed:

Plate Curve -56Vg1 fixed:

On the Biz side of things, to develop a new tube type you're looking at a minimum of 10,000 production units to make any headway on the investment. As the investing company, it’s also helpful to see existing applications. In this case the only way that happens is if the tube is a viable plug-and-play substitute in existing amplifiers. With neither of these requirements met the project was shelved. Which is not to say it couldn't be done.

Cheers,
Eddie Pletka
Eurotubes

P.S. Thank you and kudos to all the builders that have contributed to this thread. To see everyone communicating on a forum in such a civil and constructive manner is virtually unheard of. I never post on forums. But seeing the discussion here was motivating :) I hope I’m not derailing it!

Funny you talk about filament sag.  I used to restore tons of vintage tube amps when I collaborated with Jim McShane.  So I worked on many amps that used the EL84 type tubes.   There is a Russian variant that is quite a good sounding tube and you could get them for peanuts.  The only thing was that for a series of years they made that tube, they suffered from filament sag.  They were tough as nails and worked for years, but it you tipped an amp on its side to work on something while it was running.... bbzzzzt.  Fried output tube and usually the cathode resistor as well.  You could run any other EL84 type in any direction, but not those Russian ones of that decade or so!  You could run them upside down without an issue, but not on their sides.

The sonics would be interesting. The target device would of course be the 300B, but with an octal socket with a 6.3V indirect heater, and KT88 biasing.

The 300B is more physically fragile than a KT88 because of filament sag, which can happen if the amp is tipped on its side while the filament is hot. If it sags enough, the filament will touch the grid, and ZAP! the tube turns into a full-power diode, which destroys it and the cathode circuit.

Indirect heated tubes have the heater coiled up inside the cathode box, or cylinder, so it won't go anywhere if the amplifier is tipped on its side. This is why guitar amps are so rugged ... it takes enormous abuse to damage a KT88.

The customer base for a TR88 would be much larger than a 300B. If the sonics and harmonic structure were like a DHT (not a KT88), a lot of people would be interested, assuming the price would be in the KT88 range. If it had a 300B price, that would greatly diminish the market, since it would be far outside the KT88 class.

nice wish list....   unfortunately, the power race is on to build even more powerful KT88 to get KT120 and now KT150.   They generate more watts if the amp can be biased to run them, but they don't sound as good as a good KT88.  I think it is the watts race that is driving development though.  What Lynn suggests would be really interesting and I would certainly try working with such a tube.

@lynn_olson : that's such a good idea! I'd buy a set for my pair of  Dynaco Mk3's! An octal-base power triode would sell well!

People have gotten the weird idea that somehow the filament of the 45, the 2A3, the 300B, and the 845 are responsible for the ultra-low distortion, and the super-vivid tone color, of the DHT family. Wrong. It isn’t.

It’s the grid. DHT’s have a physically large grid, well spaced away from the whirling cloud of electrons called a "space charge". Surprisingly, electrons are not directly emitted, pass through the grid, then strike the plate. Instead, they whirl around in the space charge, find a passage through the venetian-bland repelling field of the grid, and are accelerated to the plate.

It’s the grid geometry that sets not only the DC characteristics of the tube, but also its linearity (especially high-order terms). This is the most critical part of the entire circuit. If you need pentode or beam tetrode characteristics, fine, you’ll have very low Miller capacitance, very high output impedance, and easy drive characteristics. This makes an excellent RF modulator, where distortion doesn’t matter.

But if low distortion comes first, and you’re not asking for 20 to 50 dB of feedback to linearize the whole amplifier, a true purpose-made triode should have the lowest distortion. Since there are already lots of octal sockets in PP power amps, why not make a special triode tube just for them? There is some design work to optimize the grid structure so DC biasing is the same as a triode-connected pentode, but the absence of all those other grid wires should help. If the design is good enough, it could rival the 300B without the hassle of direct heating and the complex filament circuit.

@lynn_olson - that’s a cool idea Lynn. There are certainly plenty of audiophiles out there with KT88 amps that are running them in triode mode that could benefit from these, not to mention folks with KT88 amps that don’t currently have a way to run triode connected. 

i don’t currently have a KT88 amp, but would probably build one if such a tube were available. 

Since nobody is reading this thread right now, I’m going to throw out my wish list, my note in a bottle, to the wilds of the Internet:

* I’d like to see LinLai or JJ or any of the other tube vendors, try something a little out of the ordinary. A true triode, using an octal KT88 socket, that biases up exactly like a triode-connected KT88. An indirectly heated triode, in a KT88 package, with only three elements ... cathode, control grid, and plate. With no screen or suppressor grid, and the control grid correctly spaced so the whole tube mimics a triode-connected KT88, so it can plugged directly into a KT88 socket in an existing amp and work right away.

What is the benefit over a standard KT88? Well, with no useless screen or suppressor grid, the one remaining grid can be optimized for lowest distortion ... in particular, the lowest proportion of high-order harmonics, like a direct-heated triode.

It’s not the direct heating of the M-shaped filament that’s responsible for the very low distortion of DHT’s (compared to triode-connected pentodes and beam tetrodes). It’s the clean, uniform grid structure, and the carefully chosen spacing from the cathode (or filament). So there’s no reason a purpose-designed true triode can’t be designed to fit a standard KT88 (or EL34) socket that has the same DC bias characteristics as it’s more complex brother, but also much lower distortion.

Literally, a simple plug-in improvement for all the hundreds of thousands of conventional PP-pentode amps out there. No change in bias, no change in cathode circuit, no change in fixed-bias operating point, just lower distortion, ideally approaching DHT performance if the grid is correctly designed.

You could call the new tube a TR88 to distinguish it from a KT88, while signifying it is plug-in compatible (thanks to the same DC biasing). Or TR34 if it replaces an EL34.

Just a quick update that Don and I are continuing to refine the Blackbird, with a bit of Raven and Karna Mark I thrown in. The chassis will be 18" wide to give a more spacious layout, a simplified build procedure, and visually match the Raven.

The circuit continues to be balanced throughout, with a tube lineup of a 6SN7, a pair of matched and balanced triode-connected 6V6, and a pair of matched and balanced 300B’s. The power supplies (both of them) have a slow-start circuit that protects against hot-start transients if the AC power flickers off for a second or two, as well as controlling tube warm-up.

Sonically, Don and I are prioritizing depth and realism of tone color, like 300B SET amplifiers, combined with a clarity and directness usually associated with high-performance Class A and Class D transistor amplifiers.

How would you compare the sonics of your 300B amp to the the Class D amp you make now? You’re the creator of both, so you’re in the best position to evaluate and compare. I only spent a half-hour of casual listening to the Purifi at the show (and Audio Group of Denmark), so I’m hardly an expert on the subject.

Its been a long time so my comparisons have a set of our M-60 amplifiers in between if that makes sense. The M-60s were overall less colored by distortion with wider bandwidth and a greater sense of palpability. Peter Moncreif had us do a direct comparison at the show.

So the M-60 compares to the class d in that they have a similar tonal balance- with no grain or edginess in the mids and highs. The big tell that we hear and that customers report is that the class D is more focused in that its easier to hear what’s going on in the rear of the sound stage, pick out details and that sort of thing.

And if you really want to get hardcore, make sure all the cathode circuitry, of each section, comes down to a single star ground on the main ground bus-bar.

If I can add to this, make sure that the grid circuit and cathode circuit of each tube employ a single wire that goes to ground for both of those circuits. If you are using terminal strips, to do this the grid resistor and cathode resistor would tie to the same point and then a single wire to ground is used.

Tubes amplify differentially, which is to say the grid and cathode are out of phase with each other. So if a single wire is used for ground and noise is injected into that wire, it will be rejected by the tube. If you use separate wires the tube will be more noise susceptible.

Yes, you do star grounding, and you always do "sub" stars for each stage.  At least that is how I do it.....

Yes, plenty good enough for the job. Don’s comments above are right on the mark.

What you want is isolation. Stage-to-stage, and isolation of the critical filament supply.

And if you really want to get hardcore, make sure all the cathode circuitry, of each section, comes down to a single star ground on the main ground bus-bar. So, star ground for 6SN7 cathode components, a few inches away, the star ground for all 6F6/6V6 cathode components, and another few inches away on the main bus-bar, all the cathode components for the 300B. The idea is keep all audio currents local to that stage, and to that stage only, and only have DC return currents on the main ground bus-bar. You would be surprised how few high-end components do this.

The presentation below is for advanced students. You know who you are:

European Triode Festival Presentation

ETF Part Two

if you can fit a toroid in there, then look at the antek site.  They have small filament transformers and may have what you want.  AN0205 is 25VA with dual 5V windings so will give you 2.5 A x 2.   They are fine for what you want.  they also sell transformer covers.  If you want to regulate the filament supply for each 300b then you can get a 2 x 7V.  

Hi Lynn,

I found online only one filament transformer for 5V - Hammond 546-166MS. This transformer has only one 5v 3A tap. So I need 2 such transformers. I use Hammond in my DIY phono stage. But IMHO Hammond transformers are built cheap compared to Lundahl, Hashimoto, AN or even James Audio. 
Is these Hammond good enough for this task?

You can do a lot of sleuthing just by listening to the spectra of noise. Magnetic induction is going to be pure 50/60 Hz and fairly hard to hear. Capacitive coupling is high frequency only, and will sound like buzz, usually harmonics of 100/120 Hz switch noise from the rectifiers and transformer secondaries.

Ground loop noise can be isolated by shorting the input plugs of your preamp or power amp. If the input is shorted, and the noise persists, it is inside the component itself, and is usually a design or layout error.

If the noise is the result of two components connected together, that is a ground loop. This can be confirmed by disconnecting the interconnects between them, turning both on (with volume down), and using a DVM to measure the AC voltage potential between the two chassis. Scrape through the paint or anodize if you need to, then measure.

The AC potential between the two should be less than 1 or 2 volts. If it is more, then you have a ground loop. This is caused by capacitive leakage from the power transformer to the chassis. It can cured by reversing the AC polarity going into the power transformer on ONE of the components, but this is not a DIY job.

What causes this is that consumer AC power is not balanced; instead, there is neutral, which is only 1 or 2 volts away from safety ground, and hot, which is 120 volts in North America and 220 to 240 volts elsewhere. Power transformers are not symmetrically wound; one side has lower capacitance to ground than the other, but unfortunately, the leads are not marked, so they can randomly assembled in production. Ideally, the low-capacitance side of the primary should go to HOT, and the high-capacitance side of the primary to NEUTRAL.

If all your components were assembled this way, you would never have ground loops. Unfortunately, the phasing of the power transformers is random. The capacitive leakage from primary to transformer case will let the chassis float to a high value relative to safety ground, which is the true ground. The only real solution are medical-grade power transformers, which have extremely small leakage to chassis.

Short of that, you can hire a skilled technician to wire all of the power transformers in your system for minimum AC HOT to chassis leakage ... which is a good idea from a safety perspective anyway. No more little shocks when you touch a component (which should never happen in equipment built to code). Safety code requires that the fuse, then the power switch (in that order), always be on the HOT side of the line.

The 300B filament circuit is a very delicate circuit node. The high voltage windings of the B+ transformers see switching pulses of hundreds of volts with very steep rise times. It only takes a few pF of winding-to-winding capacitance to transfer that 120 Hz switching noise straight into that expensive 300B. Unless you know that the low-voltage winding is electrostatically screened (with copper foil), don’t do it. Use a separate transformer just for the 300B alone.

You wonder where low-level buzz comes from? Winding to winding stray capacitance. Whenever you hear buzz instead of low-frequency hum, that’s a capacitive coupling, not magnetic. The spectrum gives it away.

I would NEVER use vintage caps in a power supply. Never never never. Use modern parts. They’re not that expensive, and used in air conditioners all over the world. Vintage is OK in a crossover, where failure is no big deal. In an amp, just say no.

Not sure I see the merit of mixing films and electrolytics. If you have the space, use the industrial parts, and bypass caps to personal taste. In terms of location, the cap bank can be several inches away from the tube socket, but the little caps (0.1 uF or less) need to be close by, an inch or less.

Hi Lynn and Don,
What do you think? Is it a good idea to use driver-input power transformer taps to feed 300B filament? This power transformer should be less pushed then the 300B power transformer.
I don't use only electrolytics capacitors for B+. My DIY friend taught me to use a mix of electrolytes, polypropylene and vintage industrial oil capacitors (Siemens MKV, Tesla, KBG-MN and new Obbligato). All this capacitor bank is bypassed by 0.1-0.22uF PIO and 0.001-0.01 uf Soviet silver mica. This bunch of capacitors doesn't smare sound. The only drawback I can hear is the relatively long warm up time (at least 2-3 hours). Probably a 200+uF set of good quality vintage industrial oil capacitors will sound better. But it will be more expensive.

Since this is (mostly) a 300B thread, I’m still curious about the general circuit of Ralph’s 300B amp. He’s been doing this a long time, so his design choices are of considerable interest.

I have my own way of doing things, and that was strongly influenced by my research when I was writing for Glass Audio and Vacuum Tube Valley. John Atwood, in particular, showed me the Bell Labs archives and other primary sources. Charlie Kittleson, the magazine’s founder, had a treasure trove of working 1930’s electronics, which sounded very different than anything I’d heard before ... not like the Fifties sound at all. They clearly had different priorities back then.

John Atwood and I have a lively interest in early technology, particularly early monochrome and color TV in the USA, the UK, France, Germany, and Russia. Developments in color TV filter technology went on to influence Neville Theile in Australia, modern crossover design (Laurie Fincham in the UK), and the time-switching technology used in the GE/Zenith FM Stereo multiplex system.

Much appreciated.  I will investigate further.  Vinnie is a well respected designer of note.

cheers,

don

I tried to paste my screen shot showing the 300 B line stage in the chassis. Hybrid so not a purist design like yours. Being a biased owner  with other Amplifier ownership over some years it is a worthwhile audition if convenient.

Best Regards 

@bcurtis1 I looked that piece up.  It has absolutely no tubes of any sort in it at all.  Can you enlighten me?  Did I miss something?

Yes, it is quite surprising how sensitive these two circuits are to parts choices.  Much more so than any I have ever worked with.  I am used to coupling cap differences, and I have my favourite types, but there are several film caps in here that make as much or more difference.  Also, I have been through several versions of anode loading, and it is clear in this circuit that a custom designed interstage transformer walked all over the other choices in the amps.  We developed a new twist on the regulated supplies for the preamp, and now we will extend that idea to the power amp and then we are done.  As Lynn said, I think we are reaching the limits of what the circuits can do.  Also, I am trying the excellent Monolith Magnetics transformers as soon as they arrive.  I will see if this pushes the amps over the top!  As I believe I said earlier in the thread, this is pretty much a cost no object project to see what is possible with these circuits.   They sound unlike anything I have ever heard or worked with.   The better you make them, the less they sound like anything at all, which is about the highest compliment I can give an amp or preamp.

Vinnie Rossi Brama ✅

Oh, that wasn’t technical enough for you? Well, chew on this, the way I got my first job at Audionics, by inventing this little gizmo:

Shadow Vector Quadraphonic Decoder

Sadly, only one prototype was ever built, but it did get demonstrated at EMI and the BBC in 1975. After that, loudspeakers, Tektronix, and various magazines.

Believe it or not, there’s a programmer in the UK who actually built this in software a couple of years ago. Now, that’s impressive. The 1975 hardware prototype took nine circuit boards plugged into a backplane ... the UK programming genius took it to the next level, and made it into an eight-band decoder, all working in parallel, thanks to the wonder of modern DSP.

These pictures give me a bit of a chill. It isn’t a Raven, not quite, but it’s pretty dog-gone close. Both date from the Twenties, and they come out of Bell Labs. In the first, note the archaic nomenclature for the direct-heated tubes and the weird little capacitor ... and what the heck is it doing, dragging the lower grid off-center?

Western Electric 7A Repeater Amplifier

This might look familiar - the WE 42A amplifier

Build either with modern parts, and it sounds like it’s from outer space. It’s like discovering a working spaceship under a long-forgotten tomb.

On a more technical level, here’s the relevant discussion, a deep dive into audio archeology:

Western Electric, the Rosetta Stone of Audio

OK, I didn’t know how much Don was keeping under wraps, so I was a little vague about our continued progress.

The new full-size chassis of the Blackbird (compared to the models at the show) gives us the freedom to "open up" the Blackbird ... ultra performance caps in critical locations, a bit of Raven tech in the front end, and more rigorous isolation between the high-voltage power supply and the audio circuitry. All of these need more room, which is why the production chassis will be 18" wide. Sonically ... well, I haven’t heard it yet, but they’re all good things that move in the same direction as the past year of collaboration.

One the things about the Raven/Karna/Blackbird that is frustrating, but also very gratifying, is the circuit is extremely transparent and revealing. The frustrating part is that parts quality is revealed in a relentless glare, at least in the critical nodes of the circuit. This is the downside of any zero-feedback design; there is no clean-up crew of servo feedback to tidy up afterward. You hear things as they are.

But the transparency is also a gift, because "minor" substitutions are immediately apparent in the first minute of listening. I think Don, Cloud, and the rest of the Spatial team will agree on that point. I feel we are fairly close to the upper bound of what the circuit can do, but I keep being surprised.

The Raven, which I had never heard before, took me aback ... that was not what I was expecting. It is super fast and resolved, with sounds flying out of dead-black space. You can practically see the shapes of the notes as they fly by. No exaggeration, no tipped-up HiFi sound, no artificial edge sharpening, but boy, it’s all there. If it was on the recording, you will hear it.

I find it kind of shocking that a late-Twenties Bell Labs/Western Electric telephone repeater, built with modern ultra-wideband parts and quiet MOSFET cascode power supplies, sounds like that. There ain’t nuthin’ retro about the sound at all.

Actually, I have already improved the preamp considerably with a few subtle changes, and the power amps will get improvements to the power supply with the larger case, as well as higher quality and physically larger film caps in what we now know are 3 really important places.  I could not fit some things in the previous case that Lynn and I agree should be added.  I would expect the whole system to sound noticeably better than what was at the show.  Same tonality, but a bit faster, with even better imaging and separation of elements in the sound stage, and tighter and deeper bass.  Experiments are already showing these improvements.  Production by the end of the year is the goal.  When we have production models there will be photos posted on the Spatial Audio Lab site.   I really appreciated meeting people at the show and getting their opinions!

Alex, I wish you every success with your revised 300B amplifier. An isolated power supply for the input+driver, and replacing all the electrolytics with arrays of 440VAC (630VDC) industrial motor-run caps, will make a difference that will astonish you. Not joking here. It will definitely take it into the top class of SETs.

Be prepared for a pretty large and heavy chassis for an 8-watt amplifier. 18" x 18" and 50 lbs or more would not be out of line.

As for cap-value tuning, set the RC (or LC) frequencies between 3 and 4 Hz. The beat of most music is between 1.3 and 2 Hz, and you do NOT want any of the RC networks interfering with that. Anything slower than the beat of the music will give a kind of seasick, unsteady feeling, so don’t go there.

Leave the banks of electrolytics to the DC-coupled transistor guys. A lot of their tuning (and amplifier sonics) comes down to the brand of the electrolytic. (Oops, did I let the cat out of the bag? Sorry, guys.)

Don accomplished a miracle of miniaturization for the show, but we’re dialing it back a bit for the production models (to simplify assembly). We’re expecting 18" wide, the exact same width as the preamp, and maybe 16" to 18" deep. Weight ... yeah, maybe 50 lbs or so. Depends on what the transformers weigh. Specs and overall performance will be the same as the show models, so if you like what you heard, that’s what you’ll be getting.

This most genial, thoughtful, educational, respectful and generous thread of all time deserves to be a "sticky". Not only is it full of wonderful history, a person can learn a lot from it.

A little ways up @donsachs used the expression "nutshell" in reference to one of Lynn’s posts. Hah! I think it is more like an advanced panel discussion between Don, Ralph and Lynn and some of our other technical gray-beards. Or maybe it can be likened to a graduate seminar in esoteric Amp design.

Wonderful. My nomination for thread of the year, 2023.