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. 




Let’s talk a bit about distortion, particularly in a DHT driver. It defeats the purpose of using a very low distortion 45, 2A3, 300B, or 845 if the driver has more distortion than the DHT itself. You might as well use a much cheaper power pentode or beam tetrode and save yourself a lot of trouble. Not only that, but pentode and ultralinear-connected output sections requires feedback across the whole amplifier, which cleans up the mess from the input and driver sections. This is why the driver section is an afterthought in Golden Age PP pentode amplifiers. In the Dynaco circuit, there isn’t even a driver ... the power tubes are driven from the split-load inverter, or "concertina" stage.

But ... if the goal is the most linear possible amplifying stage, with zero feedback, a completely different approach is required. True, push-pull substantially reduces distortion, but in reality it only reduces even-order distortion ... 2nd harmonic, 4th harmonic, 6th harmonic. etc. It has no effect on odd-order distortion ... 3rd harmonic, 5th harmonic, 7th harmonic, etc. Most tubes have dominant 2nd harmonic distortion. But not all. The 6DJ8/6922, an RF tube, has dominant 3rd harmonic. Tubes designed for video amps can also have dominant 3rd harmonics, since distortion in a video tube has almost no effect on the picture (even 5% distortion would be barely visible).

Aside from the subjective sonics of dominant 3rd harmonic, this has consequences for a PP amplifier. The PP circuit will have no effect on 3rd harmonic; it will not reduce it even 1 dB. So tubes with dominant 3rd harmonic should be avoided.

What about more "normal" tubes with dominant 2nd harmonic? Well, one of the requirements for PP cancellation is the magnitudes and phase of the cancelled harmonics line up with each other. The magnitudes of the (even) harmonics from each tube should be within 1 dB of each other, and the phase of the (even) harmonics should be within 45 degrees of each other. Normally this would be of no concern with reasonably matched tube sections, but it does matter if the capacitive loads are not the same. The odd harmonics, of course, do not cancel.

The notion of harmonics having phase might seem a little odd, but keep in mind that square waves and triangle waves have identical magnitudes of harmonics; the only difference between the two signals is the phase of the harmonics. So the phase of the harmonics is not insignificant, and actually reflects a different transfer curve. In graphical terms, you want complementary transfer curves; if kinks appear in the curve of one tube but not the other, that kink will not cancel.

The full scope of the driver requirements for DHTs is probably now coming into view.

* Three times as much swing is required, compared to pentode or beam tetrode.

* Linearity should be better than the DHT itself.

* The load is mostly capacitive, consisting of the 60 to 80 pF Miller capacitance of the DHT grid. Reactive loads increase distortion at high frequencies, where it is most audible, and also reflects the power back to the plates of the driver tube.

* If the driver is PP, 2nd harmonic should be at least 10 to 20 dB greater than 3rd harmonic, and the two loads should be symmetric to keep the phase of the driver harmonics the same.

* There should be at least 3 dB of driver headroom so the amplifier doesn’t all clip at once. More headroom is desirable.

* Momentary sags, or program-correlated noise, in the power tube B+ supply should not interact with the driver tube B+ supply.

Since rolling out of bed in the morning, your amps have been filling my room with amazing sounds, and I appreciate greatly the design thought that went into them, the technical derivations of which is way over my head.

The amps, now with the excellent new Linlai WE300b tubes also run in, create a sheer wall of sound, effortless in its presentation, heft in the LF, delicate highs, and a mid-range to that sounds exactly right.   Even compared to Don's excellent Kootenai KT88, these monos are in a whole different realm.  The sound with my Spatial Audio speakers just hangs effortlessly the room.  

This thread, with both the participation both you and Don, has been most informative, and I am an early beneficiary of your design which as been to elevate my listening enjoyment to degree to a level that I never anticipated possible.  With your monos, there is no amp at all... just joyous music, heard in a way I never thought possible with no sonic signature at all... just pure clean vivid sounds.  Perhaps that ought be the design goal for all... create front end gear that sounds like nothing at all...  You gents have succeeded in that respect. 


I am very happy for you. What you wrote expresses genuine joy and satisfaction listening to music. Isn’t this the ultimate objective? You are fortunate to have received this early opportunity. I know you are very grateful.👍

It’s been so much fun and interesting reading about the development of this new 300b push-pull amplifier and what it took to get here.



How about using SE as opposed to PP in the first stage and a SE to PP interstage transformer between the first and second stage? 

Well, I tried that with an amp I once called the Aurora. SE input, and if memory serves, the conversion to PP prior to the driver, which was PP. The problem is the input tube has a high output impedance, which enormously complicates the transformer operation.

You see, the input transformer of the new amplifier, as well as the Mark I Karna’s, is driven with a low source impedance ... the preamp. There are a handful of ancient preamps with a Zout of 8k or so, but they are hard to use because a Zout that high makes them very susceptible to rolloff from cable capacitance. Most tube preamps have cathode follower outputs in the 400 ohm range, and if feedback is used, quite a bit lower. Transistor preamps can be as low as a few ohms.

Transformers like to see a low impedance on either the primary (input) or secondary (output). It doesn’t matter which end. The problems start with an interstage, where the secondary is driving a grid, which has a near-infinite impedance that is somewhat unpredictable, and a primary connected to a plate. The nicest sounding tubes tend to be the old octals, or even the true antiques, the five-pin tubes from the Thirties.

They all have pretty high output impedances, 7.7 k or higher. This is a really high impedance for a transformer. The lack of bandwidth wasn’t a problem back in the day, since AM radio bandwidth was never higher than 8 kHz, optical movie soundtracks the same, and shellac 78’s were mostly noise above 6 kHz. And program sources didn’t go lower than 50 Hz. Modern bandwidths of 30 Hz to 15 kHz didn’t arrive until the mid-Fifties, with magnetic tape, modern LP’s, and FM radio. By then, transformers were used for line level applications in studios, and for output transformers in power amps. This was the all-analog vacuum tube era, of course.

Interstage transformers are a very special use case. We are reviving a 1920’s and 1930’s technology to modern high-bandwidth applications, but there are still limitations, mostly the result of using high impedances. I did use what I call "Interstage 1" in the Karna amplifier, but that was really putting the transformer right to the edge of what can be done. Adding phase splitting to its task list means I will likely see phase spread at the top of the band due stray capacitances not matching between sets of windings. This is a solved problem for studio line-level transformers, but asking interstage transformers to do this results in a not-very-good interstage transformer.

And frankly, for what benefit? It isn’t like a SE input tube is all that awesome. In phono preamps, sure, SE circuits make things easier, what with RIAA compensation combined with noise considerations. But for the input stage of a power amplifier? Where’s the benefit, except for tradition?

So I restrict phase splitting to the easiest location, the input, where it isn’t doing much else. This is proven studio technology that’s been around since the 1930’s, and well-refined by the 1950’s. The interstage, a far more difficult task, is confined to the driver/output interface, and is fully balanced on both ends.

Yes, there are interstage transformers on the market that are SE to PP. I would not use them. It is very difficult to get HF symmetry on the secondaries, but modern transformer designers can do a lot that wasn’t possible even ten years ago. I wouldn’t be surprised to see a 300B SET driver interfaced to PP 300B outputs through such a transformer. I won’t be the one designing it, though.

A power amp with three 300B’s would have a certain visual appeal, and you could weave a fun story around it to match the visuals. At the hifi shows, you could hang pictures of famous trimotor airplanes, like the Ford Trimotor.