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. 




I’m not surprised. The great enemy of transformers are high impedance, which decreases bandwidth on both ends of the spectrum, and increased insulation resistance, which takes up room on the winding stack. Both act to increase the size of the core, the winding stack, and the entire transformer.

When transformers get bigger, inter-winding capacitance goes up, and HF bandwidth decreases. Yes, little transformers are better, at least at high frequencies. To get the lows, though, you need more inductance, and that makes it bigger and heavier.

The technique to increase HF bandwidth are more complex interleaving schemes between primary and secondary, but this can cause ultrasonic resonances and poor square wave performance. The art of interleaving has fortunately been somewhat simplified by computer modeling, but there’s still plenty of art involved. You want a skilled builder with plenty of experience with audio transformers.

There ain’t no free lunch with transformers. They solve problems, but you really have to keep impedances low, and work closely with the transformer designer. They’ll tell you what they want, and you tell them what you want, and you work together to meet your goals. In my case, I wanted matched capacitances on the primaries, good phase match at 20 kHz, and the ability to tolerate a certain amount of imbalance current. With SE, of course, it’s all imbalance current, so a (very) large gapped core is required.

A problem with using old transformers is corona discharge where the enamel on the wire has thinned or cracked. Once a point of breakdown occurs, those windings are shorted to each other, and the problem cannot be solved unless the transformer is disassembled and completely rewound using the same interleaving scheme. New transformers are commonly tested with HIPOT, a test fixture that generates 5 kV between the windings and the core.

The evolution from the original Karna amplifier, built by Gary Pimm in 2002 and designed by yours truly, to where it is now, is a desire for rationality combined with reliable performance. The original was a proof-of-concept, the kind of thing I’ve been doing since Shadow Vector in 1975. I’m not so good at manufacturing engineering, which is where my collaborators come in ... Gary Pimm, Thom Mackris, and now, Don Sachs. These guys keep me on the straight and narrow and argue me away from my wilder flights of fancy.

The original Karna was based on a desire to get rid of all capacitors in the signal path, since I truly despise these things. But realistically, transformers are mostly problem solvers, so it’s better design practice to give them a problem to solve.

1) The input transformer accurately phase splits and offers ground isolation between preamp and power amp, if desired.

2) The interstage offers Class A1/A2 operation for the output section, a favorable load to the driver (compared to RC coupling), and balanced Class A drive for lowest distortion into a reactive load (the Miller capacitance of the 300B grids).

3) The output transformer multiplies current by about 28 times, so the peak currents the 300B offers (in the 200 to 300 mA range) now become many amps at the 4 and 8 ohm taps.

All three solve problems. The first interstage in the original Karna was mostly there for spite, to get rid of coupling caps once and for all, just to prove a point. But that required a low impedance tube, and I didn’t want to use a 6DJ8, which is woefully unsuited to power amplifier use. That left a family of industrial tubes that are reasonably linear (much more linear than a 6DJ8), the 5687/7044/7119 family. They worked reasonably well but I wasn’t entirely happy with the tone quality, but I was stuck, since the high plate impedance of the 6SN7 made it a no-go.

The founder of Tribute Transformers saw this as a challenge ... 15 K plate-to-plate ... and built four interstages with an 80 kHz bandwidth and perfect square waves. They’re in my Karna amps and Gary Dahl’s Amity amps now, and no, they weren’t free, both of us paid in full for them. So yes, it is possible to build an amp with two interstages and no coupling caps at all. But ... using a truly exotic interstage, a genuinely one-of-a-kind design from Tribute.

I wasn’t going to lay that burden on Don. We started with the greatly simplified Symmetric Reichert and improved it step-by-step, edging back to the original Karna, but certainly not using a four-box design with a quite exotic grounding system, aviation-grade Amphenol connectors, and four massive power supplies. You really have to draw the line somewhere.

Don had a lot more experience on the practical side, as well as real-world production experience with top-quality, and most of all, reliable power supplies in his well-regarded KT88 amp. So off we went.

I have never had a wooly or bloated 300b amplifier owned about 30 of the things not one had any bloat. All had a very wide frequency range when powering proper loudspeakers. Now if you don't use loudspeakers that are correct for SET or 300b amps they may have issues think that's where most of the bloated 300b comments come from. Since I see many SET or PP 300b etc users running speakers that are not best used with it.

It’s been fun writing about the zigs and zags that took Don, myself, and the team at Spatial to where we are now. More to come? Who knows? The preamp (with remote control thanks to Khozmo) and power amps are off to a good start.

Phono preamp? Ooof. Now that would be a challenge, not one I’d like to take. DACs? Definitely not. I leave those things to the pros. Speakers? Spatial already has a crackerjack in-house team, plus I have no clue how these dipole things work. Maybe they could teach me.

People ask me about 100-watt or even 200-watt tube amps. Designing around banks of KT90’s in massive parallelism, mostly likely in Class AB, is pretty much the polar opposite of the current project. Seriously, if you need Big Watts, just go Class D (with Bruno Putzey tech) or buy the Manley professional products. Both approaches are reliable and get the job done.

To Lynn's comments about the need for big power for your speakers, that Class D amps get the job done, I would humbly add that an excellent tube preamp such as Don makes in the path adds a measure of bloom and euphonic sound that so many of us desire.  I prefer to own speakers that mate well with tube amplification, but that is just me.