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 @donsachs and @lynn_olson , Here is square wave 1KHz on my amp output: https://photos.app.goo.gl/jcdjn88Q71WaEvoR9 Output transformer and driver IT have much wider bandwidth compared to input to driver IT. During the measurement the 6sn7 current was 7.2mA. I just increased it to 8.75mA. Load resistor is 56KOhm. |
Try removing the 56K load resistor entirely and give it a listen. You might like it better. Remember, with conventional signal sources, there is almost no ultrasonic content, so the ringing in the transformer is never stimulated. And getting rid the load resistor has no effect on circuit stability or DC stability, since the grid sees the DCR of the transformer, which is a few K at most. All the grid resistor does is make square waves look pretty. It has no other function. Now, in a cap-coupled circuit, it is absolutely necessary, since grid current, as small as it is, has to go somewhere. With transformers, it just goes through the secondary, and the current is so minuscule there is no effect on the core. |
Its been my experience that allowing an audio coupling transformer to ring will result in brightness, since any signal presented to the transformer can cause ringing (this is easy to demonstrate- try it!). I'd not be surprised if some people mess with the damping to compensate for a weakness in the circuit elsewhere; IME/IMO you're far better off finding those weaknesses, sorting them out and making sure your transformers perform as good as they are able! The overshoot, as these things go, isn't bad. You might be able to zero in on it a little bit more. The thing is, the more energy the overshoot has, the brighter/livelier the presentation so if you're going for a warmer sound this is something to avoid (you can see here how easily distortion can influence the tonality of the circuit). I would not reduce the loading resistance to the point it rounds the leading edge. A slight bit of overshoot is OK when trying to hit that critical damping value. In a zero feedback circuit you have no correction to deal with this sort of thing, so you have to sort out details like this and get them right. The reward is greater detail since distortion and detail really don't go hand in hand. You can see by doing this sort of measurement how different power tubes and different speaker loads affect how well the output transformer can perform- and why people might have contradictory observations about how the same OPT and power tube sounds, because the way the transformer behaves changes depending on the speaker load.
|
In my case, I compared the sound of my amplifier with different IT resistor load. 120K (Kiwame 2W) vs 56K (Alan Bradley 2W). |
Alex, remember, you cannot harm your amplifier if you completely remove the grid-load resistor. With a transformer, runaway from DC instability is impossible. There is always a DC path through the secondary ... as long as the secondary is intact. If you have an oscillation lurking in there, at some high frequency like 5 to 20 MHz, that's a different story, and unrelated to the value of the grid resistor. 5 to 20 MHz oscillations, even at a very low level, will absolutely make the sound bright and unpleasant. If they are -40 dB down, you will never see them on a scope ... that's no more than a trace width. You need an RF spectrum analyzer to sniff out the little monsters. They look like little spikes rising out of the noise floor (which should be very smooth). If you suspect this, you need a grid-stopper in series with the grid pin, like 100 to 500 ohms of carbon-comp resistor soldered no further than 1/2" from the grid pin. (NEVER use a wirewound for a grid-stopper.) That will kill self-oscillation. I would try a grid-stopper first before futzing around any more. The only way you can solidly rule out self-oscillation is use an RF spectrum analyzer that's good to at least 20 MHz, preferably 100 MHz. These things aren't cheap, and only have one use, chasing out RF nasties. Low-level RF oscillations are surprisingly prevalent in high-end audio equipment, with poorly designed regulators as the usual culprit. Try the grid-stopper first before anything else. After that, play around with various value of grid resistor, including nothing at all. It should not be sounding bright, unless something is wrong. |
