Actually I have rebuilt 3 or 4 of the ST70 amps. The best version is to skip the 7199 or 6GH8 types and to one install of the octal driver replacement boards. I really liked the 3 6sn7 tube version that tubes4hifi used to sell and perhaps still does. It makes the amp considerably better. Really in every way. That said, they still run out of steam under load because the power supply is only adequate and there is no room in that tiny chassis to install a better supply. They are great amps for the money. But they are still not particularly great. However, there is little else you will find at that price point to touch it.
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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All classic vintage PP amps from 50-60 have very weak PS in terms of chokes, power transformers and especially capacitors values. If you use the same power supply for SET with no feedback with a such PS will be a disaster. But many of these classic PP still sound fine! Why? Because feedback or because PP has PS noise cancelation? |
@alexberger Fine is a relative term. If you improve the power supply in a citation II the amp is greatly improved. That is why the KT88 amps I used to build have FAR better power supplies than vintage amps. If you build a vintage amp with the sort of power supply you are using in your SET that vintage amp would sound much better. |
The three-tube 6SN7 circuit board for the Dyna ST70 converts it to a Mullard circuit, with lower distortion and stronger drivers. Since nearly all the ST70 circuitry is on the single circuit board (for both channels), swapping that board basically gives you a new amplifier ... while retaining the power supply, chassis, and transformers. Lots of ST70 variants, since so many were made and are still kicking around. And the output transformers are pretty good. Of course, if you are replacing the power transformer and upgrading the power supply, you might as well build on a new chassis, and have an all-new amplifier. Nothing wrong with a 6SN7 Mullard circuit and modern power supplies ... that will take you into the $3000 to $10,000 quality bracket right there. |
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