300b lovers

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. 

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.

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.

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.