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 300B ... all of them ... quite happily accept at least 20 volts of positive grid drive. This is not secondhand info gleaned off the Internet, I’ve seen it for myself on a Tektronix scope screen back in the Nineties. I was frankly surprised, because there wasn’t even a trace of a kink or a glitch as it went from negative to positive grid drive. I was expected more drama from the Big Bad Positive Grid Drive, but nothing, no drama, and no signs of grid or plate overheating, either. The 300B is a pretty tough tube, even the bargain-basement Chinese tubes of the day, back in the Nineties. The only reason I stopped at 20 volts is I lost my nerve at that point, and rolled back the gain. I actually have no idea how much power can be pushed into the 300B grid.

That’s when I realized why this amp, or rather the Amity precursor, sounded like a 60-watt tube amp, or a 150-watt transistor amp (I compared it to a Crown Macro Reference and it played just as loud). It just acts like a compressor when things get hot and heavy, and the separate B+ supply for the driver sails right through the heaviest output stage overload. Best of all, the interstage transformer recovers instantly from overload, nothing like RC coupling which requires the cap to re-charge once grid current flows.

An interstage transformer is certainly a benefit for a single-ended amp, thanks to the efficient power transfer and smooth entry into the A2 region, but the benefit is much greater for Class A push-pull. You see, in "normal" or Golden Age amplifiers, only one driver plate is available when the power tube grid goes into A2 and current starts to flow, or if heavy current is needed to overcome Miller capacitance.

The power tube grids never demand A2 at the same time; they take turns. This is important when a push-pull driver, coupled to a balanced interstage, needs to deliver A2 current into a 300B grid. Thanks to summing in the interstage, both sides of the driver circuit are available to push current at every moment, and not only that, because it is a symmetric circuit that always remains in Class A, it is far more linear than a single-ended driver. It really is a small, very linear power amplifier in its own right, so it shrugs off the demands of the DHT grid.

The key principle in a non-feedback amplifier is the lowest possible inherent distortion, coupled with immediate recovery from overload. A balanced, low-distortion driver that uses a well-balanced interstage transformer offers the greatest voltage swing, greatest linearity, and greatest immunity to a reactive load. And the requirements aren’t trivial: one grid needs to swing up 100 volts while the other grid swings down 100 volts, and this needs to happen at 30 kHz with no slewing, transformer saturation, or power supply sag.

This is far beyond the drive requirements of any pentode (35 volts typical), and double anything seen in a SE amp, unless it uses an 845 transmitter tube. (If you’re wondering why so many 300B amps sound mushy and dull, there’s your answer. It isn’t the 300B. It’s the driver falling short.)

That’s also why I discarded the SE driver -> PP output topology. I built an amplifier that used this approach, but the demands on the interstage transformer were too severe, and it never had the clarity or the sense of unlimited headroom of the balanced driver approach. That’s also when I switched from 5687/7044/7119 driver tubes to push-pull 45’s ... actual power tubes with 25~32 mA of current going through each one. Since the 6V6 was specifically designed to replace the 45 (they bias up pretty much the same) in an era when feedback was not universal, the performance of the 6V6 had to be good enough to replace the 45 in hundreds of thousands of radios in the mid-Thirties. The guitarists adopted it and the rest is history.

Which I guess leaves the question why does the input stage have the topology it does. The quick-and-easy approach would be a Mullard-style long-tailed pair or differential stage, or maybe borrow from Williamson or Dynaco and direct couple a half-6SN7 input tube to a "concertina" or split-load inverter. It would certainly be cheaper, and is the approach of just about every Golden Age amplifier.

This is the intuition part. There is something wrong with the sound of Golden Age amplifiers ... hard to describe, and it’s not there in SE amps. Something to do with diminished low-level detail, subtly flattened tonality, and a lack of air and "shimmer". The folks at Sound Practices were confident that this "PP" coloration was inherent in push-pull itself, and that’s where I parted company with the common wisdom.

I became convinced the problem was the phase-splitter tube. For one thing, the three approaches to vacuum tube phase splitting (split-load inverter, long-tail pair, floating paraphase) sound quite different, and they all have varying levels of that "push-pull sound". So why not take a passive approach? Studio transformers have been around a long time, and if they are good enough, retain phase integrity through 20 kHz. Then the rest of the amplifier can simply be fully balanced, with none of the circuitry devoted to phase splitting, just amplification. Do one thing, and do it well.

Sure enough, even in the first version of the Amity in 1996, the coloration was gone. It didn’t sound push-pull, and it didn’t sound SE, either. It sounded like itself, and not like anything else. The rest of triode community went their own way, off in SE-land, and I did a lot of historical research for Glass Audio and Vacuum Tube Valley, while thinking of the next steps beyond the 2-stage Amity amplifier.

I’m sure this will sound wonderful - but I can’t stop and think about why not simply run 845 SE, what are the sonic factors driving the adoption and development of 300b push-pull?

I do understand the desire to avoid working with high voltages however.

The 845 is not happy at 500 volts. It is a (low-power) transmitter tube, and is designed to work from 800 volts (minimum) to well over 1000 volts.

Once you go over 500 volts, construction, and the parts required, are a whole different world. It requires ham-radio transmitter technique. Parts are air-spaced, wires DO NOT lay on each other, circuit boards are out of the question, and special-order high-voltage power and output transformers are required. Electrolytic caps have a hard upper limit of 550 volts, and 1 kV film caps are industrial parts, not audiophile specials. In short everything is different. Consult a 1950’s American Radio Relay League (ARRL) handbook to see what safe construction technique looks like. It is nothing like audiophile practice.

Sure, the builder can ignore safe construction technique and build it the regular way, but that’s a very serious safety and fire hazard. You do NOT want an amplifier exploding and then catching on fire. Transmitter technique takes us out of consumer electronics and into the realm of professional high-voltage equipment ... interlocked chassis doors, special start-up techniques, status lights, etc.

Yes, I see audio equipment at shows with hard-core Eimac transmitter tubes that light up the room. I would never allow anything like that in my house, unless it was in an outbuilding. The companies that build these high-voltage amplifiers have no track record of building ham or pro radio gear ... they’re just winging it, despite the curved glasswork and the pretty CNC chassis.

By contrast, the 300B lives in KT88 territory, with similar voltages and operating currents. Standard hifi building technique, but still not a plaything. The voltages in the B+ caps are quite lethal, so no poking fingers where they don’t belong.

Best thread on Audiogon in a long, long, long time.  Great information, and has me wishing I could attend the Seattle show.