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.
Hi @lynn_olson @donsachs What is changed in sound with the driver replacement from 6v6 to kt88? On the one hand kt88 is much more powerful and low output impedance tube. On the other hand 6v6 is more linear. Did you try kt66 that is more linear and musical vs kt88. |
The self-bias cathode resistor lets you choose anything between 6L6, KT66, 6550, and KT88. So far, the KT88 is winning, but the KT66 is very good also. We're shipping them with a matched quad of KT88's, but the user can use any matched quad in that family ... including NOS unobtainium tubes if they choose, because the operating point allows a life in excess of 10,000 hours. I was reflecting on this approach vs simply using them as output tubes. Thanks to the gain of the 300B's, they're only using 1/3 to 1/4 of their potential swing, and the 300B's completely isolate them from the speaker load. So the only "load" is the 60 pF Miller capacitance of the 300B grids, not a loudspeaker. This puts them in the most linear operating region since they are essentially unloaded triode-mode power tubes, very different than the heavily loaded Class AB ultralinear of classical Golden Age amplifiers. |
If we chose, we could have a switch that would alter the quiescent current for the driver section, so 6V6’s could be accommodated as well. All of these tubes have the same pinout, but the optimal current for the 6V6 is in the 25~30 mA range, while the 6L6 and KT88 are happiest in the 50 to 65 mA range. (By comparison, the optimal range for a 6SN7 is 8 to 10 mA per plate.) If the switch is in the wrong position, the 6L6 and KT88 would be under-biased and have increased distortion, but no harm done. If it is the wrong position with 6V6’s installed, though, they would burn up in minutes, and if one shorted, would take out the interstage transformer as well. So a visit back to the factory, all from one user mistake. You don’t want to give the user the power to damage the amplifier, just from a single switch setting. A limited range switch to trim between 6L6 and KT88 would not harm either (in the wrong position), so that’s an option. But optimizing current for the KT88 still gives plenty of current for the 6L6, and we’ve found these types aren’t all that sensitive to current settings, just so long as there is enough. A general rule-of-thumb in vacuum tube design is setting the plate dissipation somewhere between 50% and 75% of max rating, with 65% to 70% usually considered a good balance between tube life and overall performance. Unfortunately, many KT88 amps take the tubes right through their ratings, so new types like KT120’s have been created for these amps. |
Let’s do a little quick math to see how hard the driver is working, compared to an output tube. The gain of a 300B is 3.9, according to this Western Electric data sheet. The interstage transformer has a moderate gain of 1:1.2, so the net voltage gain of the output section is 4.68. In power terms, that’s a ratio of 21.9, or 13.4 dB. The driver is working at 0.2137 times the voltage swing of the output stage, into an open load that only has 60 pF of capacitance, and only draws current when the 300B grids pass into the A2 region at 80 volts swing. The driver can push the 300B grids at least 20 volts positive, and surprisingly, the 300B remains linear in this region, with no visible transition when it goes from A1 (negative bias) to A2 (positive bias). There are no charge storage effects, unlike conventional RC coupling, so the transition between A1 and A2 is seamless, and recovery is immediate on departure from the A2 region. In addition, if there are any nonlinear grid impedances from the 300B as the drive voltage goes up and down, it is swamped by the orders-of-magnitude lower plate impedance (700 ohms) of the driver section. There are further subtle benefits of transformer coupling. The 300B grids do not go into the positive-grid region at the same time: they take turns. This means both driver tubes are available to drive whichever grid needs current, not just one. And the paired drivers aren’t just paralleled, but in a Class A balanced circuit, so most distortion is cancelled. This is important with an output tube with distortion as low as the 300B; the driver should be as clean as possible, yet capable of peak-to-peak 200 volt swings. At 30 kHz (which is a 38 V/uSec slew rate). |
