300b lovers

I have been an owner of Don Sachs gear since he began, and he modified all my HK Citation gear before he came out with his own creations.  I bought a Willsenton 300b integrated amp and was smitten with the sound of it, inexpensive as it is.  Don told me that he was designing a 300b amp with the legendary Lynn Olson and lo and behold, I got one of his early pair of pre-production mono-blocks recently, driving Spatial Audio M5 Triode Masters.  

Now with a week on the amp, I am eager to say that these 300b amps are simply sensational, creating a sound that brings the musicians right into my listening room with a palpable presence.  They create the most open vidid presentation to the music -- they are neither warm nor cool, just uncannily true to the source of the music.  They replace his excellent Kootai KT88 which I was dubious about being bettered by anything, but these amps are just outstanding.  Don is nearing production of a successor to his highly regard DS2 preamp, which also will have a  unique circuitry to mate with his 300b monos via XLR connections.  Don explained the sonic benefits of this design and it went over my head, but clearly these designs are well though out.. my ears confirm it. 

I have been an audiophile for nearly 50 years having had a boatload of electronics during that time, but I personally have never heard such a realistic presentation to my music as I am hearing with these 300b monos in my system.  300b tubes lend themselves to realistic music reproduction as my Willsenton 300b integrated amps informed me, but Don's 300b amps are in a entirely different realm.  Of course, 300b amps favor efficient speakers so carefully component matching is paramount.

Don is working out a business arrangement to have his electronics built by an American audio firm so they will soon be more widely available to the public.  Don will be attending the Seattle Audio Show in June in the Spatial Audio room where the speakers will be driven by his 300b monos and his preamp, with digital conversion with the outstanding Lampizator Pacific tube DAC.  I will be there to hear what I expect to be an outstanding sonic presentation.  

To allay any questions about the cost of Don's 300b mono, I do not have an answer. 





The Mark I Karna's thrived on a matched quad of Emission Labs 320B-XLS, but that was also a serious investment. The European super tubes are something else


I had early production Elrog 300b tubes that sounded wonderful but had reliability issues. Fortunately that has all been sorted out with the current production Elrogs under the ownership of Thomas Mayer.

I have used the EML XLS 300b tubes over a 9-10 year period and they are utterly rugged reliable workhorse tubes that sound absolutely splendid. Expensive but worth every dime.


Considering that each Blackbird B+ regulator can crank out 500 mA peaks, and 400 mA steady-state, there’s plenty of current the super tubes can use. The only thing limiting the Blackbird is peak emission from the 300B filaments ... nothing else.

400 mA might not sound like much until you realize it’s at 480 volts, and the output transformer multiplies the current 28.7 times (on the 8-ohm tap).

(The 4-ohm tap has 40.2 times current multiplication, for those with current-hungry electrostatic speakers.)


400 mA might not sound like much until you realize it’s at 480 volts, and the output transformer multiplies the current

To give context to these stated  parameters is the Blackbird easy, average or hard on the 300b tube? How is tube longevity affected by the circuit’s operational points?


The operating points are conventional, well within standard 300B specs, with the plate at about 75% of max rating (which typically gives maximum life). But if you really want to slam the amplifier, well, you can. There’s nothing stopping you. The amplifier gradually turns into a limiter, with the output current saturating on peaks. If you treat it like a guitar amp, running into heavy distortion for hours at a time, the output tubes will wear out more quickly (the same as a guitar amp). Momentary overload, as in music playback, is benign.

If peak power, and particularly, peak current, is important to you, the enhanced-rating European tubes are the best choice. Instead of 40-watt plates of the classic 300B, they have 65-watt plates, and peak current emission is 50% higher, or even more. Although they are related to 300B’s and bias the same, they are in the KT120 class, not EL34 class. Unlike the 845, which relies on voltage to deliver more power, the super tubes provide more current thanks to higher emission, and plates are more generously sized, keeping them cooler.

(User note: Not recommended or guaranteed for guitar amp use. 300B’s are physically fragile and can be damaged by high vibration, excess heat, or operation with a tilted chassis. If you own an electric guitar, get the right amp for the job.)

Unlike the 845, which relies on voltage to deliver more power, the super tubes provide more current thanks to higher emission, and plates are more

Thank you Lynn for your reply.

The EML tubes are a bit larger and heavier compared to my other 300b tubes. The glass envelope is thicker as well. It does possess a heavy duty aura and appearance.


What separates the Euro super tubes is massively higher electron emission. For example, what separates the EML 320B XLS from the EML 300B XLS is 50% greater heater current, with correspondingly higher emission. But even the EML 300B XLS, although consuming the same heater current as a standard 300B, has significantly higher emission.

Same story for the thoriated-tungsten Elrog tubes. Thoriated-tungsten is used in high-power transmitter tubes, not "radio tubes" with the typical coated filaments. The peak emission characteristic will sound more like the Eimac giant transmitter tubes than a 2A3 or 300B.

What’s fun about these is they are plug and circuit compatible with classic 300B’s, but they are not replicas. Not at all. They are modern high-power designs, optimized for both linearity and peak emission. And they sound like it ... powerful and blazing fast.

But ... the amp circuit has to match the peak power and speed, or you never hear what they can really do. They sound like just another 300B with generic RC-coupled drivers, for example, and the potential is mostly wasted. Give them a driver that is a powerful Class A amplifier that is transformer coupled, and whoa, stand back. It’s not a meek little flea-power amp any more; put on Mahler or Mastodon and frighten the neighbors.

One unique feature with transformer coupling is 97~98% of the plate power is presented to the following grid. The plate power doesn’t disappear into a resistor, choke load, or MOSFET transistor current source, or jump through a plastic-film dielectric in a capacitor. It’s right there at the grid, with only tiny losses in the transformer.

This is audible as vivid and tactile tonality, speed, and power. Basically, less electronic cruft in the sound, which is what you’d expect. Don and Whitestix will attest to that.

The regulated filament supplies for each 300b can deliver 3 amps, and the tubes draw 1.2 amps, so they are coasting along, like the B+ supplies.   As Lynn described, the amp is lightning fast.   I would love to audition the Elrog 300b in it.  I have to admit the Linlai replica WE300b is just superb though.  I cannot imagine it needing more slam, but my speakers are 97 dB and an easy load, so the amp is just cruising along and is really not stressed.   Whitestix told me he tried a pair of Dynaudio C1 on his shoebox earlier edition amps and they drove them with no trouble at all in his smaller room.  That is the third or fourth time someone has put a more difficult speaker on the mere 27 watt amps and they had no trouble.  I don't suggest you power some horribly inefficient speaker with these amps, but don't let the 27 watt rating fool you.  Any rational speaker load is fine.

I suspect any electrostat is fine. The weird capacitive/inductive load will not bother a zero-feedback amplifier.

What will not work is something like Wilson Audio or B&W speakers, with low efficiency, a band split into three or more drivers, complex crossovers, and big woofer arrays. Or MBL. They really do need 200 to 500 watts with a high damping factor (lots of feedback). Transistor amps, in other words.

Hi @lynn_olson ,

According to your comparison between RC coupling and IT coupling, IT coupling between input and driver stage should increase slew rate (dynamics, trancients, speed) similar as it does between driver and output stage. Isn't it?
On the other hand, the input stage doesn't have a big voltage swing, and as result of it a linearity issue. It shouldn't drive the driver stage in class A2. 6sn7 with IT has less wide bandwidth compared to 6v6, 6f6, 45 that can be used in driver stage (in my amplifier 6sn7 18Hz-35KHz vs 6f6 6Hz-95KHz -3db ).
Is It wars to try interstage transformer after the 6sn7 input stage in my 300B SET?

I certainly confirmed that the 300b monos will drive the Dyn's to the full measure of their sound in my 13'X26'X9' room.  Lynn points out the need for speakers of a certain design, as he listed examples off, to have lots of power (current) to those speakers with amps which have a high damping factor.   My main speakers are Cube Audio Jazzon single driver speakers which by design, require an amp with a very low damping factor, such as most tube amps I can imagine and ss amps like the Enleum 23R and most of the Pass Labs First Watt amps.  

I am very happy that these 300b monos drive both my Cube speakers as well as the Dynaudio C-1's so I conclude that these monos are much more compatible with a larger variety than probably typical ~8wpc 300b amps would drive. 

I started this thread with reference to my Willsenton r300 300b tube amp with 8 wpc.  I swapped that into the rack to try to drive the Dyn's and it ran out of clean power pretty darn quickly, while the 300b monos cruised along easily and cleanly at very high SPLs.   It is a very unfair comparison, but here it is... a 300b tube amp that will drive a lot of speakers that many others likely would not.  This fact dramatically broadens the selection of speakers one could pair with these fantastic amps because having a tube amp that only pairs with high efficiency speakers is awfully limiting as one's choice of speakers today might not be the same speakers you have down the road.  

No, the comparison isn't really fair.  The little Chinese amp is a nice value for the money, but it is a single ended design made of, shall we say, "cost effective" parts.  It probably makes 7 or 8 watts per channel.   The mono 300b amps are built with cost no object parts and custom wound transformers, with state of the art power supplies, and they really are about 27 watts/ch.   They cost multiple times what the Willsenton does.   The Willsenton has a chance on efficient speakers.  It doesn't have a prayer on a speaker presenting an inefficient and difficult load.

Reflecting on a recent phone conversation with Don (he’s in BC Canada and I’m in Colorado, a bit north of Denver), I suggested that SET amps are kind of like a paint-box, and a much more fun way of tuning a system than messing with cables. If you are DIY’ing, there are many ways of changing the tone color ... which coupling caps, what kind of passive power supply, which rectifiers ... the options are endless. And a lot of fun if the amp is on a breadboard and you can solder in new parts in a few minutes.

The characteristic SET sound works in your favor, giving a lot of leeway with parts selection. And the amp is fundamentally simple and easy to understand, a godsend when you are tuning with many variables. Like I said, a paintbox. When you learn painting, you learn color harmony and the art of mixing. Nothing teaches faster what XYZ cap sounds like than heating up the soldering iron and swapping parts.

A balanced amp is a harsher taskmaster. Yes, more transparent, potentially by 20 to 30 dB, but not nearly as forgiving. Colorations can sound pretty ugly if the wrong part is in the wrong place. And there is no feedback to tidy up the mess. Maybe more like working with an airbrush, or transparent watercolors, instead of pigments. There’s still balancing to be done, but the high level of transparency, and lack of feedback, exposes everything. I found this out the hard way with the original Amity amplifier back in the Nineties.

And the colorations from different part selections are not the same as SET. This makes sense when you reflect on it ... the balanced circuit is cancelling most, but not all, colorations, and the residue left over can be unwelcome and surprising. The SET experience can be a very rough guide telling you which parts sound really awful, but it will not tell you which sound the best.

This mirrors working with speakers. As transparency goes up, tolerance for coloration goes down. In the absolute sense, this is wonderful, because now you’re really hearing the music. In a way, I’m not surprised the simplest topology won ... less to go wrong, and with the most efficient plate-to-grid coupling.

I have explored all sorts of variants of this basic circuit for about two years now, and have learned what I like and understood why.   This is a deceptively simple circuit, except that there is subtle complexity.   Often, we get caught up in the conventional way of doing things, and then spend all our time refining them to try and hide all of the problems and colorations.   Sometimes we need to think outside the box.   I left traditional power supply design for tube amps probably 5 or 6 years ago.  There are huge threads on the sonics of various capacitors in speaker crossovers and amplifiers.  Trust me, I have heard many of the top caps.  The best cap is no cap.  It took time to arrive at that notion.  Time that was wasted auditioning top capacitors instead of thinking outside that box.  As we discussed earlier in the thread, your alternatives are to either directly couple tubes, or use transformer coupling.  Both have advantages in different sorts of circuits.  In this circuit, transformer coupling is best because of the balance at all stages, which is the key to this amp.  Think of it as constantly canceling distortion and balancing itself.   But you cannot just buy off the shelf transformers from company L or H or E or whomever.  For this circuit you need interstage transformers that are optimized for the loads they see and can give flat response from below 20 Hz to above 20 KHz with little or no phase shift.  That is no easy task, and you have to work with a very experienced winder and it takes some prototyping and testing.  Well, over a year of prototyping and testing.   It is not an accident that the best of the vintage tube amps had killer transformers.  So, as Lynn elegantly stated above, this circuit is totally revealing and even though you have relatively few parts in the signal path, they have to be really good parts, and some of them have to be custom designed to get the best performance.  The audio circuit looks trivial, except that it isn't trivial to get that hand full of parts to work really well together.

Two years ago I would never have built a cathode biased amp, and I resisted that notion at first.  Then I thought outside that box and understood what this circuit was doing and why the cathode biasing was best.  I had the notion that all cathode biased amps sound slow and syrupy and deliver far less power than a fixed bias alternative.  Well, in a conventional circuit that is correct, but not in this one.  

The power supply is also highly optimized and we use some tricks to further isolate it from the audio circuit, and ways to isolate the input tube from the driver circuit power supply.  Of course the 300b supply is separate from the driver and input supply.   The filament supplies also are regulated and isolated from each other.   So there is a fairly complex, yet very conservatively rated and reliable power supply that drives this subtly complex, but fairly simple circuit.   If you change one thing you instantly hear it.  So, in this amp there was quite a bit of tuning, again, to remove coloration.  The result is a transparency I have not heard in any other amp to date, plus the ability to drive quite an array of speakers.  Again, don't let the 27 watts fool you.

Is this a perfect amplifier?  Of course not.  There is no perfect amplifier.  Again, if I were to magically create a straight wire with gain and play it for 100 folks, half of them would love it, and half would probably hate it.  This amp's hallmark is utter transparency and tonal correctness.  The piano is in the room with you and you can easily tell a Steinway from a Bosendorfer.  I realize that most everything I have heard to date is quite colored, or if fairly neutral, lacks the resolution of this circuit built this way with these parts.  This circuit is uncolored, transparent, and highly resolving, and has a boatload of driving ability.  It packs serious punch.  Unlike a single ended amp, where the idea is to tune the coloration inherent in the design to suit your taste, this project was about removing the coloration so the circuit could really shine.  Trust me, it doesn't sound cold and clinical.  It sounds like music with all over the overtones in the instruments, the inflections of the voices, etc...  It is not sterile sounding at all, but rather it invites you into the music.  

To each their own, but this amp is wonderful to my ear and ready for production and this thread has been an insight into our design choices, and the journey.  Others make other choices and that is as it should be.  I have no desire to build a 200+ watt amp to drive a very difficult speaker, but I do want to drive most speakers in most rooms.  

And the colorations from different part selections are not the same as SET. This makes sense when you reflect on it ... the balanced circuit is cancelling most, but not all, colorations, and the residue left over can be unwelcome and surprising.

SETs produce a quadratic non-linearity, which in turn makes for a fairly prodigious 2nd harmonic. If set up properly (if you see what I did there) the succeeding harmonics will fall off on an exponential curve.

A fully balanced amp will have even ordered harmonic cancellation, so the resulting non-linearity is cubic in nature. So the 3rd harmonic will be dominant, but at an amplitude slightly less than the 3rd is when seen in an SET. Succeeding harmonics should also fall off on an exponential curve, but it will be one with a different exponent- the harmonics will decrease in amplitude faster as the order of the harmonic is increased. The reason for this is distortion is compounded less from stage to stage throughout the circuit. Since distortion obscures detail (in addition to altering the tone colors of instruments) this makes for a more detailed presentation, with less harshness and brightness than an SET can manage, which is saying something. In either case of SET or fully differential, the lower ordered harmonics will mask the higher orders.

The ear treats the 2nd and 3rd in much the same way- in that it finds them innocuous. FWIW, a properly functioning tape recorder will produce a 3rd harmonic as its primary distortion component also, so we have a pretty good indication on that alone that the 3rd isn’t a problem.

Its also worth mentioning that a fully balanced circuit, running zero feedback, will produce a greater percentage of usable power- close to or exceeding 95% of full power, while an SET is doing well to make 25% usable power.

Of course setting the correct operating point is critical in either circuit. In a balanced differential circuit, the best operating bias point will usually be just above the maximum gain that the differential circuit can do, with symmetrical clipping. If this rule of thumb is followed, there will be no unwelcome ’left over residue’. To achieve this, a proper Constant Current Source circuit should be used- a simple resistor to B- is inadequate owing to the rather low mu that most tubes have. It will be found that the current sensing resistor that is tied to B- is quite critical. I usually set it slightly high to allow for variations in the tubes themselves.

A good quality CCS cannot be made using a single tube or transistor- you’ll need at least two devices. Semiconductor CCS circuits can work exceptionally well and offer the benefit of no likelihood of tube damage as the tube warms up due to cathode filament arc-over concerns. If you work with differential circuits you find out quickly how important the CCS really is. In most solid state amps I’ve seen the CCS leaves performance on the table. If it is not optimized, the differential circuit will not achieve its best gain, distortion or Common Mode Rejection. So its important to get this bit right.

Hi @donsachs ,

I have two questions about your new amplifier:

1. When you decided to go to an interstage transformer after the input stage, why did you stay with 6sn7 and didn't move to something like 5687? With 5687 you can get even wider bandwidth especially on bass. What is the advantage of 6sn7 over 5687?

2. Did you try direct coupling between input and driver tubes?

With 5687 you can get even wider bandwidth especially on bass.

@alexberger All tubes have response to DC. If you hear a difference with the ability to play bass, its not the tube type that's causing it. The reason we like to use 6SN7s is there are several that are new manufacture and most of the NOS types are great so there's a lot out there to support the design. 5687s OTOH aren't being made AFAIK. I like the plate curves of the 6SN7 better too- a bit more linear which is helpful in a zero feedback design.

Here is an example of direct coupling:


Is 6f6 grid load easy enough for SRPP?

ECC82 can be changed for 6sn7.

What do you think about this schematic?

Hi @atmasphere ,

I meant that tube with less internal impedance with a transformer load with the same inductance has wider bandwidth in bass. I agree unfortunately 5687s OTOH. 

Here is an example of direct coupling:


Is 6f6 grid load easy enough for SRPP?

ECC82 can be changed for 6sn7.

@alexberger It needs grid stop resistors. Its usually a good practice to bypass the output of a regulator like the LM317 with some kind of capacitance to improve transient response; 1uf is a good typical value. You might want a bit more capacitance after the 180 Ohm resistor since any voltage variation where the regulator meets the driver transformer can cause intermodulations. The 6F6 grid is no problem for the SRPP and you should be able to sub a 6SN7 since their characteristics are so similar.

Here's something to think about: many tubes perform better when a cathode bypass capacitor is employed. Yet there isn't one in the output stage. To install one you would need two parts, one for each side of the filament and its 30Ohm resistor (plus pot). It would not have to be a high voltage part but it would need to be a fairly high capacitive value- perhaps about 2,200uf on each side. 10Volt parts should work nicely since the voltage across the resistors and pot might only be 2 Volts.

Hi, Alex!

The original Karna, designed by me, and built in four-chassis format by Gary Pimm (of Portland, Oregon) in 2003, used a 5687 class of input tube. (The 6900, 7044, and 7119 all have the same pinout and similar operating points. The current production JJ ECC99 is similar but has a different pinout.)

I selected that tube for the Amity, back in 1997, and also for the Karna because it had a low plate impedance ... around 2K ... and pretty decent linearity, much better than a 12AU7, which is quite poor and not really suited to driver duty. But I was never entirely happy with the 5687 or the other similar types. I tried just about all of them ... I have quite a stash of 5687, 6900, 7044, and 7119 tubes ... but there was always a bit of glassy, hard quality, nowhere as bad as a 6DJ8, but still there.

There’s nothing wrong with them, again, far better than any 6DJ8, but these are commercial tubes never intended for audio use, and never used in any Golden Age amplifiers, tuners, or TV sets. They were designed for analog computers, commercial radio relay use, and aerospace ... high-end commercial and military applications, at high prices, and not sold in consumer retail channels.

These days they come from military surplus stocks, and only produced in consumer format by JJ as the ECC99. So supplies are getting a little dodgy, twenty years on. Not really suitable for consumer use unless you already have a substantial stash of them, in the hundreds, and all tested and matched, of course.

The 6SN7, and its single-triode predecessors, like the 6J5, 6C5, etc. etc. are famous for their linearity, and they were designed for radio applications in the audio sections of the receiver and power amplifier. Millions were made, in varying quality, but all of them were more linear than the 12AU7 successor, or the quite different 6DJ8 (which was an RF tube never intended for audio). So there’s nothing rare or exotic about the 6SN7, unlike the 5687 family.

I mention "designed for audio" as if it is something special. Well no, not really. But if a tube was originally designed as an RF amplifier, it would never be checked in production for linearity, since RF circuits don’t care about linearity. Nowadays, of course, 6DJ8’s are never used for RF circuits, and only for audio, mostly high-end audio, not guitar amps.

This has the practical effect that vintage (NOS) stocks of authentic 6DJ8’s can be all over the place for in terms of linearity, since that’s not a controlled manufacturing parameter and would have no effect on its performance plugged in to a 1965 RCA color TV set or FM tuner, the task for which it was designed.

In practice, using Gary Pimm’s custom-designed spectrum analyzer with 140 dB resolution, we found that upper-harmonic (5th on up) spectral shapes mostly reflected a given manufacturer, and was surprisingly consistent from year to year. Gary Pimm and I have both worked in manufacturing for big and small companies, and we surmised that consistency reflected the special jigs that aligned the grids, and different manufacturers used slightly different techniques to align the inner structure.

Although tube models are intellectually useful in a design phase, they model ideal tubes that are only available as beautiful Platonic Ideals in a store somewhere in Heaven. Sadly, we humans on Earth have no access to that store. No Platonic Ideals for us.

The tubes we can actually buy were, and are, hand-made by skilled human beings, not robots. The grid pitch is not perfectly uniform, the grids are all tilted just a little bit, electrons escape out each end of the structure, the list of imperfections (and departures from ideal models) goes on and on. These tiny imperfections result in high-order harmonics that can be seen in a high-resolution spectrum analyzer, and heard in a good audio system.

Surprisingly, these departures from perfection are consistent with the manufacturer. That’s why Gary and I surmised it came down to small variations in assembly technique, or even the individual assembler. Again, tubes were never assembled by robots, and still aren’t today. The assembly was, and is, semi-automatic at best.

Frame-grid tubes, like the 6DJ8 or more exotic WE417A, are even more difficult to make consistently, and it doesn’t matter in a high-gain RF circuit anyway. Using them in an audio circuit is a roll of the dice, especially if there is no feedback to tidy up the mess. Harmless in a preamp at millivolt levels, not so good in a power amp.

For all these reasons, Don and I decided to move away from the 5687 family. (Neither Don nor I are fans of miniature 9-pin tubes anyway.) True, the 5687 family greatly simplifies the interstage transformer design, since the plate impedance is about three times lower than 6SN7, but that low plate impedance is the result of high transconductance and more difficult assembly procedures. Part of the reason that direct-heated triodes have a much cleaner spectra is they are big and easy to assemble ... as dumb as that. We’re talking late Twenties to late Thirties technology here ... precision assembly was very difficult back then, especially on a production basis.

Effectively, Don and I took the ultra precision out of the tube and put it into the transformer designer and assembler. That’s where the 21st Century tech comes in. These transformers could not have built in 1939, when the 6SN7 first came on the market (replacing single triodes). The 5687 family dates from the mid-Fifties, with transformer design still in the build-and-try phase, like the loudspeakers of the day. Computer modeling was still decades in the future.

To sum up, we have a circuit with big, simple tubes designed no later than 1939, combined with 21st-Century transformers and power supplies. In that sense, it is a hybrid amplifier, spanning 84 years of time and technology.

(If you want the Blackbird to fly even higher, look to the Emission Labs 320B-XLS or the ELROG 300B with thoriated-tungsten filaments. Those are 21st-Century 300B’s.)

Alex, the Shishido 811 circuit is basically uncopyable, since it relies on DC flowing through the secondary of the custom interstage transformer that goes into the 811 grid. Unlike nearly all other audio circuits, this circuit operates the 811 power tube ONLY in the positive grid region ... from zero volts to a substantial positive voltage.

When I met Shishido at the CES back in the Nineties, as technical editor of Glass Audio, I pressed him on this point. In Shishido’s "Inverted Interstage Transformer" designs, the grid voltage swings from zero volts to a higher voltage. It never passes through the zero-bias region (according to Shishido).

This requires DC current to steadily flow into the grid, while the grid is an extremely nonlinear load for the driver stage. There’s only two ways to pull this off: a powerful MOSFET driver with a paralleled current source (MOSFET likewise), or a very special interstage transformer that can tolerate a lot of DC going through the secondary, while current goes through in the opposite direction in the primary. If you did it with MOSFETs the chances of a spectacular explosion would be pretty good. You don’t mess around with transmitting tubes.

Brilliant but the weirdest thing I’ve ever seen. A (very) custom interstage with bidirectional DC current flow. Zany doesn’t begin to describe it. My worry would be matching the current flows to the exact values. Tubes love to drift ... they are not well suited to DC circuits. Tektronix scope designers went to insane lengths to DC-stabilize their vacuum tube scopes, and this amplifier would also require a complex DC-stabilized supply.

How did it sound? I preferred its big brother, the monster 833 amplifier, which was the top-of-the-line Wavac IIT amplifier. That used a hand-selected vintage KT88 from WAVAC private stock as the driver. When you bought the WAVAC 833, they set aside several vintage KT88’s (real British Genelex) just for replacement purposes. Shishido told me that, and I believed him.

I also loved the stunning solid aluminum NC-milled chassis and custom safety glass enclosure for the insanely hot (and very dangerous) 833 transmitting tube with the top cap at many kilovolts. That probably doubled the price, but man, it looked really cool and high-tech.

Transmitting tubes are in the "look but don’t touch" category. In real transmitters, they are behind thick safety glass, with interlocked steel cabinet doors. If they blow up, it’s no joke. The steel doors and safety glass are there for a reason.


Thanks for the exceptionally educational and interesting posts. ! Very much appreciated. Your summation of the simplicity of the DHT tubes really explains their longevity, continued desirability and enduring superb sound quality. Talk about withstanding the test of time.


Hi @lynn_olson ,

Thank you for the detailed lecture about tubes!

Here is Shishido version with 300B. No DC on secondary of IT. 300B is in a regular cathode bias.


What do you think about SRPP is this schematics?

Investigated in some depth in this 1997 Glass Audio article by Matt Kamna (designer of the Whammerdyne 2A3 amplifier) and myself:

Hidden Harmonics

We found that transformer coupling had the most favorable distribution of harmonics ... by that, the smoothest and fastest drop off. Other forms had more harmonics, with more uneven distribution. Test conditions: 6SN7, single section, 50V rms out, with several different circuits, with and without cathode bypass capacitors. Noise floor with this setup was -118 dB, and harmonics out to the 11th were investigated.

To my knowledge, this was the most thorough examination of vacuum tube harmonic generation at the time, using direct measurement instead of reliance on tube models. Standard assumptions about local feedback from cathode degeneration, and SRPP distortion cancellation, were proven wrong. RC coupling, in particular, was shown to have quite high distortion, while transformer coupling was the lowest.

Nice article! Too bad direct coupling wasn't tried. That would have been fun to see.

Direct coupling would have no effect on tube loading, which is responsible for the spectra shown. If we were to re-do the article, we’d try a MOSFET cascode current source load, as well as transformer coupling, SRPP, and RC coupling.

We were surprised that cathode degeneration doesn’t work, and creates some nasty high-order terms instead. Separating the data into even and odd-order terms was essential to unscrambling the chaotic results of the spreadsheet ... a legitimate way of looking at the data, since the underlying transfer curves of odd-order (S-shaped) and even-order (C-shaped) distortion are fundamentally different.

Nowadays, we have the computer power to discover the actual shape of the input/output transfer curve, and exaggerate it enough to be visible. The regrettable drawback of FFT spectral information is that phase is usually discarded, so the underlying transfer shape cannot be found (although it can be inferred).

(What I mean by this is the phase of the distortion harmonics is important. For example, a square wave and a triangle wave look exactly the same on an FFT spectral display; the only difference is the phase of the harmonics. The magnitudes are the same. In real circuits, square waves and triangle waves are created by entirely different mechanisms, so this is important data.)

Which is why, on my highly resolving system, I could clearly hear the differences between CCS loaded RC coupled, choke loaded RC coupled, and interstage coupled versions of essentially the exact same amplifier.  IT coupling won hands down in all areas and is the way these amps are built.  The preamp too.  No RC coupling anywhere in the signal path....  Of course you need really good transformers.....which has taken a year.

Yup. Wires, transformers, and 1930's vintage tubes. That's the entire signal path, from preamp input to speakers.

Typo above, choke loaded is clearly not RC coupled, but rather LC coupled, sorry.  At any rate, full interstage transformer coupling was easily the best sounding.

Which returned the Blackbird to the original Karna topology, with far superior power supplies, and the luxury of interstage transformers specifically designed for the Blackbird amplifier.

I give Don full credit for doggedly trying every possible form of coupling, optimizing each circuit with the most favorable operating point, and giving it a serious, I’d even say exhaustive, evaluation. While I sat back with original 20-year-old Karna circuit and criticized from afar. I’m sure I annoyed the hell out of Don more than once.

The selection of an IT for the output section is obvious. The driver, which has to swing a lot of volts at very low distortion, gets to transfer all of its power to the DHT grids. If the DHT grid swings into Class A2 and starts drawing current, no big deal. The power is there, and there are no caps to charge or discharge. Recovery time is instantaneous, unlike RC or LC coupling, and there no risk of DC-coupled failure propagating from driver to output, as there is in solid-state equipment. It really is ideal.

The input tube was another question. In principle, at the lower working voltages, there shouldn’t be much difference between any of the methods, with RC coupling as the obvious and cheapest method. Unfortunately, that’s exactly what it sounds like.

The more serious auditioning over the last year was between current-source + cap coupling, inductor loading + cap coupling (LC), and straight transformer coupling, with no coupling caps or grid resistors involved. And that sounded the best.

Also the simplest. Six parts ... two custom inductors, two good-sized and quite expensive caps, and two grid resistors ... are replaced by one reasonably compact, purpose-designed transformer. The folks who own the "shoebox" format amps, as demonstrated at the show, can be upgraded to the new circuit, which actually opens up space under the chassis. All new amps will have the new circuit, of course.

Charles1dad, thanks for the compliment ... much appreciated. Don and I put a lot of work into these seemingly simple amplifiers.

DHT’s had a rather short reign in audio (much longer in transmitting tubes). It was only from the early Twenties ... the dawn of radio ... to the late Thirties. Once the 6L6 and 6V6 came out (they were designed by the same team), that wiped out the 45, 50, 2A3, 300B, 211, and 845. Even Western Electric abandoned the 300B by 1940 when they designed their new generation of amplifiers around push-pull 6L6’s (WE350). Since the 300B first came out in 1935, it wasn’t in favor all that long.

300B’s have now been in production longer than they were in the Thirties and Forties, rather odd when you think about it. It was the vogue in Japan, Europe, and finally the USA in the Nineties that created the continuing demand for the type and DHT’s in general. It’s been thirty years now, so I think it’s safe to say they are here to stay, along with their pentode cousins.

Class D GaN amplifiers will continue to erode Class AB transistor amplifiers, but I think vacuum-tube amps have an enduring appeal that continues to grow. They now dominate high-end audio, which was not true thirty years ago. I remember going to some CES shows with hardly any tube amps at all, never mind DHT’s, and now they are everywhere.

Now you see quality record players, and tube amps, in movies as a marker of good taste. The movie viewer gets a little buzz when the tonearm descends into the groove, making that distinctive vinyl "click" sound, then you see a tube amp quietly glowing in the background, and wonderful music comes out. The camera pulls back, and you see the protagonist, looking contemplative, and out-of-focus city lights in the background. That alone sets a mood.

I’m really pleased about this. In an era of superb all-digital, all-solid-state 4K HDR video, tube amps continue to make new friends because they sound so good, on all types of music.


I’m really pleased about this. In an era of superb all-digital, all-solid-state 4K HDR video, tube amps continue to make new friends because they sound so good, on all types of music.


I believe that class D GaN amplifiers will definitely continue to grow market share and popularity. This seems undeniable. Yet I also believe that there will remain a certain number of music listeners who find that nothing satisfies their musical/listening desires as well as high quality tube products. The two will peacefully coexist.

Both can be excellent while simultaneously being distinctly different. I do not share the gloomy “demise of tubes “ forecast . Time will certainly tell.


First, loving the conversations here - delving into the correlated technical stuff. Everyone should have a Tektronix RTA for this hobby, lol. But the last time I used one they were over $50k. That's more than a good pair of interconnects🙄.

It's also intriguing reading that these amps will run Monolith output iron. Kind of curious which series they are based on, but maybe that's asking too much.


Over the years I've used quite a few different tubes from NOS to current production - Shuguang, Psvane, JJ, Sophia, Linlai, KR (Kron), EML, Takatsuki, WE. Some brands aren't worth mentioning. In the past I preferred KR in amplifiers as they are wideband, dynamic, and have solid bass. But depending on the system, can come across as a little unrefined/aggressive. I liked them in what I had. (PX4, PX25, 242, and 5u4g)

But with 300b in what I've owned:

Art Audio - They make heat and conduct electricity. About the nicest thing I can say about them

Shuguang 98b - Meh. Pretty darn average across the board. Puts me to sleep.

Shuguang BT - Pretty good in a VAC Renaissance P/P amp. But other places are bright, lacking midbass weight, and little bass

Linlai 300b E - A good step up, decent bloom and detail, a little light in the midbass and down low

EML 300b XLS - Pretty good. Not as nuanced as the best. Depth and width OK, but not the best. Good low end drive. Much better than everything listed above.

Takatsuki - Excellent tube. Best bass drive. Good Detail. Depth and inner resolution good, but not the best.

WE 300b new production (2022) - Excellent detail, inner resolution, nuance. Holographic. Quiet. Appropriate bass weight and definition. Not as punchy as the Tak or the EML. That will depend on your system. 5 year warranty and projected long life. Love this tube in my SET amp. Well worth the money.

So I find a huge performance delta in the list. Spending the money will be rewarded, probably more in SET than P/P, but that's speculative.


Oh, regarding comments about DAC's made earlier, I concur with Don who has a Lampi Pacific 2 with DHT output, my Lampi Horizon dac is the most significant piece in my system. But there are no unimportant pieces.


Very curious to hear user comments once the production Blackbird and Raven hit the wild.


EML 300b XLS - Pretty good. Not as nuanced as the best. Depth and width OK, but not the best. Good low end drive. Much better than everything listed above.

Takatsuki - Excellent tube. Best bass drive. Good Detail. Depth and inner resolution good, but not the best

In my experience I’ve found that the preference for a particular upper echelon 300b is very dependent upon the specific amplifier in use. Some of these top quality tubes are simply better fits in certain amplifier/systems than our others. It’s very difficult to draw an all encompassing generalization.



They now dominate high-end audio, which was not true thirty years ago. I remember going to some CES shows with hardly any tube amps at all, never mind DHT’s, and now they are everywhere.

While it is correct that there are more tube amplifier producers (many of whom show at audio shows) than there was in 1958, its incorrect to say that tubes dominate high end audio (most of these producers are very low volume). The larger players who produce solid state products are still the bigger sellers.

I should have been more clear when I mentioned direct coupling as a method. Specifically, I meant using a cathode follower which is direct coupled to the power tube so that the bias of the power tube is obtained from the driver tube. For this one would need a B- supply, but if the best reproduction is a goal then that cost isn’t important. IMO using a driver tube in this manner does not result in higher ordered harmonic generation, at least insofar as our OTLs seem to measure out. In them the higher orders fall off at a faster rate than any SET I’ve seen. They sound smoother too. One thing we sorted out from doing this over the years is that the plate needs to be heavily bypassed to prevent IMD and harmonic generation. In our amps we found we had to use bypass capacity values an order of magnitude higher than one would typically expect; after trying lesser amounts we found the seemingly excess amount made a difference to the distortion. IMO this aspect of cathode followers is poorly understood.

@tuckia08 Thanks for your comments.   I have heard a number of 300b now, but not the new WE 300b and I have not heard the European expensive tubes.  When I tried the Linlai WE300b copy I stopped looking.  I can get them for about $750 a quad from my supplier and so far the 4 or 5 quads have all been perfect with no issues.  As for sound.. well they stomped all over all the $400-500 per quad tubes from China, and the Gold Lion reissue, which is a nice tube, very well made, but just cannot match the clarity of the Linlai WE300b copy in this push pull Blackbird.

The Monolith OPT is based on their standard summit series core.  I don't want nano or amorphous.  The amps have incredible micro detail and the tonality and timbre of instruments and vocals are spot on to my ear.  They are rich and full sounding.  I am happy with the summit cores.  I was playing records last night because I am burning in a phono stage for a customer and shipping it today.  I had not used phono with this final version of the amps and I have to say that I heard subtle details in the familiar records that I had not heard before, even with earlier prototypes of these amps.  The summit series transformers do it for me:)

I too will be interested in user comments.  We hope to be making them in November!

Yes, the Lampi Pacific is outstanding.  Mine is modded a bit and I am considering sending it in for their upgrade.  They will put the Horizon digital engine in there for about $4K, and do a few other things.  Spendy.  Once the 300b project is off the ground maybe I will gamble the $4K:)   But the Pacific is just stunning as it is.  I run a quad of NOS 46 tubes with adapters, so they are essentially a 45.  The clarity is astounding.  I wish we had this quad at the Pacific Audio Fest in that Pacific that Fred, the NA distributor for Lampi loaned us.  Anyway, enjoy your Horizon.  An end game source no doubt!  Once you get a DAC that good... you cannot go back.



Back when I was playing around with 300B’s, I found that each brand had its "sweet spot". The authentic 300B’s, and the exact reproductions, seemed happiest between 65 and 75 mA. The European super tubes, between 72 and 85 mA. The monsters with giant 65-watt plates are probably happy between 80 and 95 mA. Power supplies need to deliver at least twice the quiescent current, preferably more.

If the current is a little low, they are rich-sounding, but also murky and dull, and if too high, super detailed but also wiry and hard-sounding. Very much like setting VTA in a phono cartridge ... there is a correct setting, and you know it when you hear it.

There is also a tiny range of allowable filament voltage, from 4.85 to 5.05 volts. This can change the entire character of the tube, from very dull but rich sounding to wiry and hard. For longest life, it should be exactly 5 volts, and leave the subjective tuning to setting the quiescent current.

The B+ voltage only had a minor effect compared to the other two parameters ... well, three, counting tube swapping. The other indirect-heated tubes aren’t as temperamental ... put them in the usual range, and they sound fine.

Don thanks for sharing what you are hearing with the OPT and tubes. I’ll have to hear these somehow someway. 

I share my tube experiences because I know how hard it is to get all of these expensive things in one session so they can be A/B/C/D compared. Some I owned, some were loaned. Goes without saying that everyone will find their own preference. But meaningful differences exist. I roll till I find the right ones then stop. Some never do.

I was mistaken. Thought you did the PAC 2 upgrade already. From what I hear (from Fred) it is money well spent. If this was an option when I had my Pacific, I would have gone to 2 and stopped. End game. The PAC is DHT, the Horizon is not. Some consider the 46 as the best option. I never had any to try. I can say the EML 45 globe mesh are pretty special in a Thomas Mayer amp and the Pacific. But rare unobtainium now. But my everyday Pacific tubes were KR RK PX25. I ran these in my amps too, which is kind of funny.

The Blackbird amps are running the 300b tubes at about 400V B+ (plate - cathode voltage), and just a wee bit under 80 mA. About 31.5 watts at idle. They sound wonderful and should last a very long time. The tubes each have their own independent regulated filament supply and are right at 5 volts. I have measured a lot of them and it is always 4.99 - 5.01 volts. All the supplies are regulated so the tubes stay at that operating point. It seems to be a sweet spot for the 300b tubes, although I suspect that dialing the operating point up a bit for the Euro super tubes would be an option. The supplies are run very conservatively and could easily handle the extra current.

One of the maddening things about the original Karna amplifier was sonic variability. Some days, it would be a glimpse of Heaven, and other days, nothing special. It was always extremely transparent ... that’s the nature of the circuit ... but the tuning came and went. Most of the time it was quite good, but every now and then, it was extraordinary.

By contrast, SET amps are usually consistent, any time of day, due to massive 2nd harmonic masking all the high-order harmonics. Tuning a SET is straightforward ... which parts complement the dominant 2nd harmonic most gracefully.

One of my experiments was bringing out the filament circuits on a separate power socket. I bought the Audio-GD AC regenerator, and powered the filaments with that. Sure enough, the variability went away, and I discovered quite small variations in filament power had a big effect on the sound, overwhelming any other tuning decision, including tube swaps. But the Audio-GD liked to run its fans, despite the easy load.

So I was open to Don’s approach of regulating everything, using the proprietary regulators he’d been using so successfully on his 6L6 and KT88 amps. Sure, why not? I’d already been splitting the high voltage B+ supplies between input+driver and output section (to prevent crossmodulation between sections), and using high-quality regulators for a balanced drive for the filaments and heaters made a lot of sense.

(I should mention regulating filamentary tubes is not trivial, and the usual 3-pin low-voltage regulators introduce unacceptable colorations. Filament and cathode nodes are extremely sensitive to coloration, and balanced discrete circuits are required.)

Sure enough, variability gone, vanished, like the morning mist. No change in sonics depending on the time of day. No annoying and objectionable regulator coloration, which is the bane of high-end audio ... that obnoxious grainy transistor sound, coming out of an otherwise good tube amplifier. Not a trace of that, thankfully. Reliable, too, which goes with good regulator design.

This alone justified re-naming the amplifier. I suggested Blackbird (because Red-Winged Blackbirds are a common sight in Colorado), and to my surprise, the name appeared in the Pacific Audio Festival show guide. So Blackbird it is.

Yes, these amps sound very good after 10-15 minutes and are very consistent.  They sound the same every day.  I have the preamp and amps plugged into a Puritan supply, which does clean up grunge in the AC line.   Basically, you turn them on and they sound quite good as soon as they can play music in 30 secs or so, but in 15 minutes or so the tubes are all warm and then they are pretty much on song until you turn them off.  They may sound a wee bit better after 30+ minutes, but really 15 gets you almost all of it.  Plus they run quite cool.  You can leave your hand on the transformers or anywhere on the top panel even after the amps have been on for an hour.  No, it is not a class D amp that barely warms, but they run quite cool for a class A tube amp.


This has been an interesting thread.  I didn't start it, nor did I expect it to have this long a life.  It is very clear that there are many ways to build amps and preamps.  I just want to restate my goal for this project.  I am semi-retired.  I wanted to build the best sounding preamp and amp combination that I could.  A setup that I can happily listen to the rest of my days and that will drive a reasonable subset of the speakers out there.   Within reason, I didn't care at all what it cost.  Once done it will be produced by a partner and priced according to what it actually costs to build plus enough to cover their labor and margin to stay in business.  It will be sold directly to customers though with no distributor markup.  But it costs what it costs to build because the main goal of the project was to produce the best sounding preamp and amp that was possible without have some four chassis amp or gold plated case and $50,000 starting point.  The thread has shown why we made the design choices we did and where it led us based on a combined (gasp) probably 70 years of experience with amplifier design and builds between Lynn and I.  He is the historian as well, and I have rebuilt literally hundreds upon hundreds of pieces of classic tube gear, so I guess that makes me sort of a hands on historian.  

At any rate, I thank everyone for their civility and free discussion of audio ideas that has made the thread enjoyable.  I hope to meet a few more of you next year at the Pacific Audio Fest when I will be there with actually production versions of these for you to hear.  Trust me, they are FAR better than the prototypes shown this past June.


Spatial Audio Lab will be making the amps and preamp.  All parts are ordered and I hope to be going to Salt Lake City in early November to train them on the builds:


If you have interest, then contact them.


I’ve been following … no … have been massively influenced by Lynn’s Karna amplifier and the reasons behind the topology. My first effort is on Lynn’s site as the PP813 Grand Aurora so I’ve been at this for about 20 years.

It has now morphed into all DHT with finals of GM70 so not part of the discussion in a 300B thread.

These types of amplifier are not easy to get right. Even though the signal path is ridiculously simple, it is still challenging and the supporting power supplies are anything but easy. Finally after all this time, I’ve succeeded in getting to the sound I was after.

Probably as I’m not in the position to commission a super IT and maybe as my current one is not so super, I’ve ended up DC coupling the 1st to the 2nd stage in a similar way to Kevin Carter’s method; choke loaded 1st stage.

Don mentions in the thread DC coupling was not an option for a commercial product, but I’m left wondering, was this method evaluated in the many permutations of coupling configurations for the front end?

I’ve also got a line stage, which again, is heavily influenced by Lynn’s designs; the Raven but using 12B4; initially used as a headphone amplifier. The system is better for it being in the chain than out despite having no gain.

Many thanks to Lynn for providing oodles of thought-provoking posts over the years to fuel the ideas leading to the success I’ve had.


The problem of DC coupling vacuum tubes in a balanced circuit is maintaining DC balance ... during warm-up, in steady-state operation over hours, and as the pair age over the life of the amplifier. A small DC imbalance error in the first stage becomes very large in the second stage, resulting in a massive current imbalance in the second stage.

This can be servoed out by a housekeeping circuit, using a bit of analog logic, but if that ten-cent opamp fails, it takes out the entire amplifier. I have seen that happen while I was sitting in the listening room of the editor of the magazine I write for, Positive Feedback. A cheapo servo circuit in the preamp took out the entire power amplifier and the bass driver. $50,000 worth of damage in a few seconds. I don’t care how it sounds, that’s just bad design.

DC coupling without a servo basically doesn’t work. Small drifts become big ones over time, and the circuit will have to be manually re-balanced by the user whenever tubes are replaced, which will happen many times over the life of the amplifier.

I think the horror of transformers has been taken much too far. There’s a reason they have been used so widely in studios for the last eighty years. They are problem solvers. Output transformers take the pint-size currents of output tubes and multiply them 28 times, or more. Input transformers reject common-mode noise and RFI, presenting a clean, quiet signal to the input grids. Interstage transformers sends the power of both driver plates, summed together, to whichever grid needs it the most (grids take turns going into Class A2).

Looking at the driver section, I don’t see the appeal of a cathode follower drive circuit. Adding an additional stage is not exactly direct coupling, and it requires another regulated power supply with oddball voltages for both plus and minus. It’s not a simplification, it’s considerable added complexity, and for what gain? There’s no improvement in slew rate, which is controlled by the current available to drive the Miller capacitance of the power tube grid. The Blackbird has 32 mA of current from each side of the driver, far more than the usual 8 mA of many other amplifiers. The driver can even enter Class AB for a half-second or so, so 32 mA is not the upper limit for grid drive.

The least necessary transformer is between the input tube and the driver tube. The driver grid is relatively easy to drive, and no clipping is seen in that part of the amplifier. The biggest annoyance is the slowly drifting DC imbalance of the input tubes.

In an AC coupled circuit, it doesn’t matter ... it’s only a few volts out of 150 or more. In a DC circuit, though, it controls the bias of the driver tubes, which is a big deal. You really don’t want one tube at be at 50% power while the other is at 90% power, and you don’t to burden the user with meter and knob twiddling on a regular basis. Most of all, you never want to give the user the power to destroy their own amplifier with a thoughtless knob twist.

The 6SN7 DC balance will drift ... not by much, but by a few volts. I do not want it controlling the 6V6 bias point, and most of all, I do not want a solid-state servo circuit to control the 6V6 bias point. That circuit will fail sooner or later.

One option that was considered was a center-tapped inductor for the 6SN7 plates, with direct coupling to the 6V6 grids. But the performance of the inductor, against expectation, was actually worse than the dedicated transformer, and the transformer completely eliminates DC imbalance at the 6V6 grids. Like all transformers, DC is not getting through.

Don and I tried all the more complex options that would give supposedly better operation. They were worse. Removing current-source coloration is non-trivial and difficult in a very transparent amplifier. Bipolar transistors and MOSFETs are audible, even as plate loads.

All of the coupling caps were colored, some much worse than others, but they were all colored sounding. (Once you hear capacitor coloration, you cannot unhear it. Just ask Don.) That was a source of great disappointment. The high-value load inductors had their own set of issues, mostly excess stray capacitance that could not be removed.

I did not expect the transformer to win, honestly. Don and I tried everything else, and the more complex options were always a step downward. In a zero-feedback circuit, you hear every single part. It’s a dumb truism in audio, but simpler usually does sound better. Not that I’m a fan of 2-stage amplifiers or full-range drivers ... there’s such a thing as too simple. Every designer has to find the balance point between simplicity and complexity.


It’s a dumb truism in audio, but simpler usually does sound better. Not that I’m a fan of 2-stage amplifiers or full-range drivers ... there’s such a thing as too simple. Every designer has to find the balance point between simplicity and complexity.

No doubt a painstaking balancing act to conquer.


The thing about the transformer coupling is that once done correctly, it just works.  It will not fail.  It protects against a tube that may develop a grid short.  Rare in smaller signal tubes like the 6SN7, but I have seen it.  So for a commercial amp I just want something that works and the owner never has to worry about.  If he or she gets a hum or odd noise from the system, a quick tube change will most likely solve the issue.  Direct coupling has the chance for a tube failure to propagate to the next stage.  Very rare, and if it was just my own amp that I could repair, then I probably wouldn't worry about it.  But if you put 100 amps out into the world....sooner or later a rogue tube will appear.  Lynn's point about servo circuits is also valid.  It adds complexity and again, a possible failure point.  If my own amp in my living room, then fine.  I can fix anything.   But if I put 100 amps into the world I don't want to see one fail.  My business philosophy would be to immediately take care of the problem for the customer, but I would rather just avoid the issue entirely.

Every component and coupling method has a sound.  I found the well designed transformer to have less of a sound than any other method, plus it is totally reliable.  I find it more transparent than any other method as well, with subtle detail more audible.  It has less coloration than anything else.  I think transformers get a bad name because there are a lot a mediocre ones out in the world and that is what most people have heard.  If you go all in on good ones they are quite spectacular.   So that is the way we went.

As to Lynn's comments about capacitors, yes, I have heard a very large subset of the best caps available, bypassed and unbypassed, etc...  You can happily live with many of the best ones, but the transformer coupling kills them all to my ear in this circuit.  Once you hear the really good IT you realize that none of the caps can produce the tonality of the transformer.  The instruments all sound just a little wrong with the caps.  The transformer does the "piano is in the room" thing a LOT better in this circuit built this way.  You may have a different experience in a different circuit built with a different power supply.  RC coupling is certainly easy.  LC coupling a little more difficult because you need a good anode choke and you have to physically find room for one.  Direct coupling has advantages, but again, there are failure modes and colorations. The hallmark of this circuit is the absolute transparency and I just found the transformer sounded best, once a really good one was wound, and it will be trouble-free.