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 @atmasphere I just found schematics with a cathode follower 12BH7 300B driver. Is it similar to the schematics you were talking about? https://www.diyaudio.com/community/threads/300b-schematic-recommendations.340133/page-2 Just don't need pentode input and global feedback thought input tube cathode. |
@alexberger Yes, that's how its done. Note that the EF86 input tube is wired in triode mode. Its quite linear that way; my Neumann U67 microphones use the same tube in the same manner sans feedback. |
Interesting SET that harks back to the original Western Electric 91A, which also used feedback to get the desired performance. I have mixed feelings about cathode follower drive: the output impedance of the CF is low (probably 100 ohms or so), but the peak current available is no different than anode drive. Considering the load is dominated by the Miller capacitance of the 300B (about 80 pF), the CF will definitely extend the small-signal bandwidth, but will have no effect on the large-signal bandwidth (also known as slew rate) which is determined by the (linear) current available to charge a capacitive load. The small-signal benefits would be useful in a high-gain feedback topology as shown above, since it would improve phase margin at high frequencies, which in turn improves distortion at high frequencies. And the circuit shown above is definitely high gain compared to typical zero-feedback SET amplifiers; performance would be dominated by the feedback loop, not the 300B. (Note for those puzzled by the schematic: there are two ground lines, not one, with the upper ground line at the top of the schematic. It winds its way down to the lower ground line, which ultimately connects to chassis ground at the RCA input at the lower left side of the schematic.) |
Sorry, have been absent from this thread for quite some time. Not spending too much time on here. I will be in Dallas at the SW Audio Fest next weekend in the Spatial Audio room and the Raven and Blackbirds will be driving their best open baffle speaker. So if anyone wants to chat..... @theclipper There are 3 or 4 sets of the final production Blackbirds in the world. One is probably still in the boxes (go figure) as the person was building a new house or some such. There is one guy who loves his and posted on the spatial audio circle I believe. There are a few sets of the prototype ones in the smaller chassis with 6V6 drivers floating around and people like them. But no one is really active on chat boards. C'est la vie. A review pair will make its way into someone's hands this year most likely. The Raven preamp has had more people posting because it costs less and there are around 30 of the production ones out in the real world.
As for my history with tube amps... I helped my older brother with his Dynaco kits when I was about 8 or 9. Then I got on to the SS bandwagon with everyone else of my generation, but I never forgot how that Dynaco gear sounded on my brother's AR speakers with AR XA turntable. I built various SS kits in my youth... anyone remember SW Technical Products? Then Hafler kits, preamp and amps and learned to mod them. Then had kids..... I finally bought another tube amp in about 2004 or so, a modded Jolida integrated and it sounded far better than my Krell integrated that I had at the time. Then I started restoring vintage tube gear, met Jim McShane, became his main tech and restored probably 500 pieces of vintage tube gear including probably 75 citation II amps and 40 or so citation I preamps. Plus tons of Macs, Sherwoods, Marantz, Fisher, etc... Learned a lot about circuits and started making my own gear. I think @atmasphere has fooled around with MANY vintage tube pieces as well as I gather. You learn a lot.... And here we are today...... So many tubes, so many circuits, so little time. Always room for personal taste and disagreements. That's wonderul as long as all folks are polite, and this thread is testament to that! As for the driver tube on the Blackbirds... It has a huge effect on the sonics of the amp. I didn't try an EL34 because over the years I have heard many EL34 amps and never liked that tube compared to a good 6V6, 6L6, or KT66, KT77, KT88. We all have our taste. I tried a number of those and the KT88 was best by a good margin to my ear. I have some rather esoteric DHTs as drivers in my own Blackbirds, but you have to find matched pairs and they are not in modern production, which is what you need for commercial production of tube gear. I happen to have good sources and good stocks of these tubes, but again you want the customer to be able to buy tubes that are in modern production from more than one source. There are exceptions to that rule of course, but you better have a good stock of tubes available if not using current production. So the Blackbird uses tubes you can easily buy without me (or Spatial Audio) being involved. I am not into the "find a vintage tube, use it, buy a 1000 of them and then mark them up 100%" model of business. The exception is that we use the VR tubes, but those are everywhere and cheap. Flea bay has millions and they last a long time. Same with damper diode tubes. |
@lynn_olson I think you have the highlighted bits wrong. A CF driver can deal with a lot of capacitance in the grid of the output tube. We use a single 6SN7 section to drive 14 such grids (in our MA-1 amplifier) and it does it with no worries even in class A2 (or AB2, if the amp is subjected to a low impedance load) where grid current is present, with good linearity. The peak current available is higher because the coupling is more efficient and the output impedance of the CF so much lower. When AC coupling (anode drive) it is very difficult to get the driver to be able to handle grid current in the output section (transformers are good at this though)! The large signal advantage is several: no blocking distortion at overload since there’s no coupling cap (so overload recovery is instantaneous) and the Voltage amplifier sees a very high impedance load so it has a much easier time doing its job (so it can be lower distortion). This allows for the coupling cap used between the Voltage amplifier and driver to be a small value, which is advantageous because there’s less inductance associated with the coupling cap, so it can sound better and also offers better layout options. This aspect helps with HF bandwidth but also helps if you want LF bandwidth since the coupling cap value is so small. In our MA-1 we have full power to 2Hz using a 0.1uF capacitor. Another advantage is as a fixed bias scheme, its extremely stable since the impedance controlling the power tube’s grid is so low. Put another way its more reliable. My advice, since its obvious you’ve not tried it, is to do so. If for an SET I would limit the LF timing constant of the Voltage amplifier’s coupling cap since SETs have such terrible problems with elliptical load lines at low frequencies. |
