300b lovers

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

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

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

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

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

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

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

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

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

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

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

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

Hi @atmasphere , 

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