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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. 

 

 

whitestix
alexberger
935 posts
 

I have a technical question for tube gurus @lynn_olson @donsachs @atmasphere

I put the intrastage transformer Hashimoto A-107 between the 6sn7 input and 6f6 driver tubes in my SET amplifier. The load resistor connected to the 6f6 grid was 520KOhm. I measured a square wave output and it had a big overshoot. I changed the load resistor to 120KOhm, and the overshoot decreased by amplitude and attenuation time. But still, there is a notable overshoot. I measured a frequency response and there is a hump +1.7dB at 35KHz. There is -3db at 19Hz and  47KHz.

Should I decrease the load resistor more to remove overshoot completely?

If yes, in which value range should be this resistor? For example, if I take a resistor less than 50K it can increase distortions.

http://www.tube-amps.net/HP_A107.htm

 

lynn_olson's avatar
lynn_olson
361 posts
 

I went through that with the Amity amplifier in the late Nineties. Nice square waves vs sonics. The amp sounded best with NO grid resistor, and the overshoot was a non-factor. Your amp may be different, of course. Keep in mind that music sources will never excite the overshoot, since it’s all in the far ultrasonic, and most studio mikes are all gone by 25 kHz.

The tube doesn't need that 510K resistor. The DC impedance of the IT is a few kohms at most, and that stabilizes the tube at low frequencies. If the driver is really unstable, maybe the 510K resistor helps, but that's usually the function of a grid stopper resistor, something quite different.

donsachs's avatar
donsachs
299 posts
 

@tinear123 Hi.. write Cloud Sessions at Spatial audio lab and he will know what is in the crossover.  I am sure they can build something for you.

donsachs's avatar
donsachs
299 posts
 

I just cannot resist.  This is a poor photo of the square wave on the Blackbird interstage transformer between the 6V6 and the 300b at 1 KHz and about 30% power (7 or 8 watts).  There are NO grid resistors or grid stoppers or networks of any kind.  The primary of the transformer is wired directly to the driver plates and the secondary is wired directed to the 300b grids.  The KT88 looks as good or better....  Lynn and I were quite impressed.  Dave Geren at Cinemag knows what he is doing.....   This is an all tube amp with no feedback anywhere.

Yeah, it sounds like it looks..... 

 

lynn_olson's avatar
lynn_olson
361 posts
 

Now’s a good time to discus the difference between overshoot in feedback vs non-feedback amplifiers. Despite similar appearance on the scope, they are caused by completely different mechanisms.

* Overshoot in a feedback amplifier is quite malign, since it indicates the onset of oscillation, something that can destroy the amplifier and the speaker it is connected to. It is caused by the amplifier running out of phase margin, possibly the result of a reactive load, but also the result of design oversights in the feedback loop.

* Overshoot in a non-feedback amplifier is quite different. Now, it might be the result of high-transconductance tubes self-oscillating, but this can prevented by grid-stopper resistors and good layout practices. Normally, though, it is merely transformer overshoot, the result of phase shift at the edge of the passband, and has nothing to do with stability. That is what we are seeing here.

Something to be considered about ultrasonic behavior in the time domain: if there is no spectral content in the frequency range of the overshoot, it will never be stimulated in the first place. It never happens.

This is the difference between overshoot in a feedback amplifier and a non-feedback amplifier: in a feedback amplifier, it is a warning sign, like the LOW OIL light in a car. You ignore it at your peril. In a non-feedback amplifier, it has nothing to do with stability, since there is no feedback loop to induce oscillation. It is simply the behavior of passive parts, in this case, the input and interstage transformers.

As you can see, Cinemag has done a very nice job here. (Same photo as above, just tidied up a little.)

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