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. 





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.

Don, Lynn

Like many have said, there is a lot here that is technically beyond me...so, I'd like to see if I've understood the essence of what you are saying..which I believe is....

That by eliminating all harmonic distortion above 3rd...and by driving 2nd and 3rd harmonics to extremely low levels you have achieved an amplifier that is not "tubey" in the classical sense but absolutely open and clear that gets out of the way resulting in truthful musicality.

Obviously, this is an extreme simplification of what you've achieved and how you've achieve it...but is this a correct interpretation or have I over simplified or missed some key discoveries?


To simplify.. All tubes have distortion. The higher orders are what are like fingernails on a blackboard to your ear. DHTs have lower levels of higher order distortion than other types of tubes, and good indirectly heated octals like the 6SN7 and 6V6 are reasonable regarding distortion in the higher harmonics. When I rebuilt a ton of vintage tube gear I instantly could hear that pieces using small signal tubes, particularly the 12au7 were not very good. If you see a 12au7 in a piece of gear other than a tuner, go buy something else. I always preferred amps with octal tubes as drivers. Anyway, to keep upper harmonic distortion down we choose a great DHT, the 300b for the output tube. Traditional feedback approaches can keep higher order distortion down, but phase split circuits and the usual coupling methods, and the feedback itself rob the amp of clarity. Push pull amps have lower levels of distortion. Class A is the way to go. If you can build an amp with no feedback at all and solve all the problems it will sound cleaner and clearer. The driver stage has to be able to really push a 300b. There are numerous forms of coupling, but interstage transformers (if really well made and designed for the circuit) solve a lot of problems. Recovery around clipping is really important for the driver stage and the transformer coupling aids that considerably over traditional RC coupling. LC coupling sounds very good for an input stage where an interstage transformer would have to deal with the higher impedance of the input tube. Power supply topology and design is critical and the output stage should be isolated from the input and driver stages. Of course the quality of the output transformers is a key as well as the power transformer, which should have good regulation. I am sure I am leaving some things out, but those are some key points of the design.


I did check catalog of one European manufacturer for SE to PP interstage transformers. Within the limited offerings, it does seem that a tube with low Rp is needed in the SE stage - if not a DHT, ones like 6EM7 (the half with 750ohms Rp) may work. The gain will be less, but can still work with an active preamp. 


Ultimately, I agree that it all depends on the design goal - I wonder whether having some 2nd order harmonic would end up being a good thing for someone who prefers sound of SETs. But there is a lot factors to balance in such a design that uses SE in the first stage.