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.
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- 870 posts total
@jaytor You’ll want as much gain out of the voltage amplifier as you can get, since the more gain also yields better Common Mode Rejection Ratio. But you’ll need a good Constant Current Source to optimize the gain stage. The D3A, triode strapped, will give you a good mu value to work with, although you could get that with a single 12AT7 and be in the same ballpark. A 12AT7 will allow for plenty of bandwidth. If you use a driver transformer as a plate load, the issue you’ll be up against is imbalance of plate current between the tube sections. The better your CCS the less you’ll have this problem, and matching is a very good idea. The more current that isn’t cancelled in the magnetic core you can also expect greater distortion. So you can see getting the plates to have equal plate voltage is important. To this end, using a balance pot in the cathode circuit to balance the plate currents, while effective, has the effect of also reducing the differential effect, increasing distortion and reducing gain and bandwidth. So matched tube sections and a really effective CCS are paramount. One thing to consider is its really impossible to get a perfect center tap in a transformer. Its always going to be a little off. Since its often difficult to get really well matched tube sections (they should be matched on a curve tracer for best results), I prefer to use RC coupling to a cathode follower, which is in turn direct coupled to the grid of the power tube. The power tube would thus obtain bias from the driver tube. If the driver tube gets weak, the power tube will conduct less also, preventing damage. I explained this topology earlier in this thread. If you are considering using a differential amplifier as the input voltage amplifier, you really should consider a bi-polar power supply of equal B+ and B-. This will improve the differential effect and the effectiveness of the CCS. If you have such a power supply then you have a good way of setting up that cathode follower I mentioned. If you go this route, care must be taken to make sure the plate of the driver tube is well bypassed so even at full output there is no noise, no artifact caused by the signal on that plate. This will really help the amp assume greater authority. |
@charles1dad - Thank you. @atmasphere - One of the reasons I was thinking of using single section tubes was to improve matching. I've noticed this section matching problem in LTPs I've been playing with, particularly with some of the soviet tubes (e.g. 6N6Ps) where the quality control seems to be lacking. Are there any tubes that, in a PP IT loaded design, would be reasonably linear and provide enough drive for a single 300B without a cathode follower stage? I'm confused why a bipolar supply would help. I'm assuming I'd need to have some kind of negative supply for the fixed bias circuit which I would also use for the LTP CCS connection, but I would expect this to be around negative 100V. Alternatively, I could use a balanced input transformer with the secondary center-tap tied to a positive voltage to allow enough voltage across the CCS tied to ground. |
@jaytor Sure! We are assuming that you plan to use a high efficiency speaker so you’re not also looking for a lot of gain. But I have to get some clarification- do you want the ability to have enough drive for the power tube, along with enough voltage gain at the same time? They are not always the same. Tubes that can swing the voltage needed for the 300b include the 12AT7, 12AU7, 6CG7, 6SN7 and so on. I would stay away from the 6DJ8 and similar frame grid tubes; while they are very linear and can handle the current you need, they aren’t very good with microphonics as they are not meant for audio. A bipolar supply supply is helpful because there is always a bit of differential performance left on the table by any CCS. The more negative the supply, the higher the impedance of the CCS can be (which is a good thing). You can, with modern semiconductors, easily built a good CCS that will work on -100V, but If you are already building a B+ supply, its really not all that hard to build a B- supply from the same transformer. I like to use a separate power transformer for the driver and voltage amplifier, so as to minimize noise appearing in the power supply of the output section from messing with the rest of the amplifier. This helps reduce IMD.
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Here’s the latest: Don is training the folks in Salt Lake City as we near production. The Raven preamp is the same preamp you heard at the show, while the Blackbird now has KT88 drivers, at twice the power of the 6V6 drivers, with third-generation Cinemag interstage transformers and Monolith output transformers, If you’re wondering who Monolith is, you heard them in the Songer room on the Whammerdyne amplifier. Merry Christmas, Happy Hannukah, and a Happy New Year, to all! |
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