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

@whitestix 

When we are treated to a master class in tube design philosophy, freely given, participants clearly appreciate the generous sharing of information by the masters: Don, Lynn and Ralph.  It has been a real education for me

Very well put.👍

This thread you initiated has miraculously avoided the all too prevalent trolling and disruptive deterioration. Bravo!!!

Charles

Ralph brings up a very good point about feedback: the underlying theory assumes a distortionless summing point. (The summing point is the comparator input between signal input and the sampled output.) Any distortion introduced at this point of the circuit will be amplified without correction, and there is a real possibility of introducing new, higher-order terms that are not present in the forward path of the physical amplifier. Norman Crowhurst mentions this in passing in his Audio magazine articles in the late Fifties.

Don and I go to some trouble to avoid even local feedback in the Raven and Blackbird ... all cathodes are bypassed in every stage, we do not use the Ultralinear connection in the triode-connected KT88, nor global feedback. Any distortion that is present is the result of first-order effects in the tubes themselves, not anything else. It also mandates selecting tubes that have a minimum of high-order distortion terms in the intended operating range.

This overlaps with the philosophy of some SET amplifier designers, but we prefer to use complementary pairs to cancel most of the distortion without recourse to feedback (local or global). The transformers, if correctly balanced, cancel out most of the distortion before passing the signal to the following stage, thus, each grid-pair is fed a cleaner signal.

This comes in particularly useful for the 300B grids, which can be a difficult nonlinear load for the preceding stage. Unlike RC coupling, when one grid demands current, both phases of the driver pair are available to deliver current to the 300B grid that needs it. This enables seamless and jump-free transition to A2 (positive grid drive) when the signal demands it.

In essence, the signal flow is: Complementary Pair of 6SN7 sections -> Signal is re-summed in the transformer -> Complementary Pair of triode-connected KT88 drivers -> Signal is re-summed in the transformer -> Complementary Pair of 300B outputs -> Signal is re-summed in the transformer -> Loudspeaker Output.

To minimize interactions between output section and the rest of the amplifier, there are separate and isolated high-voltage regulators for input + driver section and the output section. The only signal flow, either from the audio path or the power supplies, is from input to output. There are no secondary paths, at least to the 130 dB limit of filter isolation offered by the power supplies.

A few SET amplifiers have a roughly similar signal flow, but only offer a few dB of RC filter stage-to-stage isolation between sections, so there is a signal backflow pathway between sections. Isolated power supplies resolves that problem. In a SET amplifier, 2nd-harmonic distortion builds up from stage to stage, but there can also be unpredictable level-dependent cancellation effects since signal polarity is inverted with each stage.

If all these transformers disgust you, or you want to use local or global feedback, you can re-visit the Acrosound schematic from the late Fifties. Three stages, all differential, each RC-coupled to the next. It can be easily updated with modern current sources on the cathodes, giving precise differential action and almost no even-order distortion. The Acrosound circuit also offers some interesting options with cross-feed feedback between sections.

If you are bold, you can DC-couple some of the stages, but you will need good servo circuits to maintain DC balance as the tubes warm up and then gradually age. The servo circuits also need to be failure-proof, since a runaway servo circuit can destroy the amplifier.

Acrosound UL-II schematic. Note the input 12AX7 is direct-coupled to the 12AU7 driver stage, so DC drift from internal 12AX7 mismatch could be a problem.

Technical analysis: The input signal goes to the top half of the 12AX7. The bottom half is connected to the feedback signal, which comes from a dedicated winding on the output transformer. The feedback summation occurs across the paired cathodes of the 12AX7. The plates of the 12AX7 are direct-connected to the grids of the 12AU7 driver.

There is a HF phase-adjust circuit from the 100 pF cap in series with the 47K resistor. This improves the stability of the feedback system. The 12AU7 driver has a 5K 5W current-balancing pot in the cathode circuit, most likely to correct for any DC imbalance of the 12AX7. I suspect the 300p marking of the bypass caps on the 12AU7 is in error. This should be 2 to 10uF, whichever gives the best square-wave response.

Similarly, there is a DC balance pot in the Bias 1 grid circuit of the EL34 output tubes. The 4.7 ohm series resistor in the cathode of each EL34 is there to meter current, which is adjusted by the 5K pot. Although not drawn, there would be test points for each cathode to ensure the correct bias point.

Although not marked as such, this is a fixed-bias output section set up for Class AB (high power) operation. Correct bias setting for each EL34 is essential for long tube life.