300b lovers

@tinear123  No.  Production starts in late November when I go to Salt Lake and teach the guys at Spatial the builds.  I would expect a review by late spring and perhaps an audio show in the west somewhere next summer.  That is kind of going to be the schedule I think.   There will be a review pair that could potentially end up at a show in the east next year, but that would be Spatial Audio Lab's call.  I really wish that folks could hear what I am listening to in my living room.  It would be fun to have any of you over.  There will be a complete setup in Salt Lake City area by January and I am sure audition arrangements could be made for anyone who wants to hear it along with a top end Spatial Audio Lab speaker.  Of course this doesn't help any of you out East....     

It is definitely the plan to have a review setup of both preamp and amps and have it reviewed by legitimate reviewers next year as early as possible.  Perhaps sometime next year we can have them appear at a show in the east somewhere.

And I am very much looking forward to fellow enthusiasts hearing the Raven preamp and Blackbird power amp. I’ve been a voice in the wilderness for about twenty-five years ... neither a member of the SET fraternity (well, maybe on the edge of it) nor mainstream Audio Research/Jadis/Conrad-Johnson push-pull pentode big-watt amplifiers dominating the hifi shows. A handful of people built the Karna amps, but many abandoned the difficult project halfway through.

Don was one of the very few who persevered through two years of building prototypes that were far off the beaten path of mainstream tube gear. He’s had plenty of hands-on experience with the fiendishly difficult Citation II, the most complex amplifier of the Golden Age, and his own designs, the Valhalla (6L6) and Kootenai (KT88).

Of all the people I know in the industry, Don is the most qualified to honestly tell me what is unrealistic and pie-in-the-sky, and what is practical and a good solution. He’s been there and done that. Oh, and he has good taste, too, which isn’t that common in the industry.

You might think I’m being snarky about the "good taste" but I am perfectly serious. The industry has plenty of competent engineers, and whole hifi shows filled with high-powered marketers, but good taste? It’s not all that common, and I’ve been in the industry since 1973.

Well... Lynn has lots of wild ideas, but the thing is that they are very well thought out, and they are based on years of technical experience with Tektronix.  I learned many years ago in academia, and as part of my main career in forest ecology research, to listen to really smart people with wild ideas.  Many of them were simply ahead of the mainstream thinking.  The mainstream often ends up there.....eventually.  

Lynn suggests improvements and where practical, I try them.  We finally ended up with a larger chassis that could accommodate all the things we wished to try.  They are still under 19 inches so they fit any rack, since 19 inch is the traditional rack width.  To my ear, it worked beautifully....  The final touch was the addition of old school gas VR tubes to further isolate the input tube supply from the drivers, and of course the move to KT66/6L6 or KT88 drivers.  I think we are done finally.

I agree. This is a stable topology, taking full advantage of specialty transformers designed by two of the world’s top designers, and using vacuum tubes that are in current production as well as ample NOS stocks.

As mentioned earlier, it’s a very simple signal path, with only transformers and vacuum tubes, and fully balanced from input to output. Zero feedback, with the audio signal only propagating in the forward direction.

Another walk down Memory Lane. This time, we’ll go into the late Forties, when the Williamson burst on the scene. This English design wiped out all other designs in the USA until about 1955 or so, with the exception of the McIntosh and a few others.

How does it work? There’s an input tube, typically a triode like the 6SN7, direct-coupled to a split-load inverter, also called a "concertina" stage. This always has identical plate and cathode resistors, and gain a bit lower than unity. The plate output drives the upper half of the push-pull amplifier, while the cathode drives the lower half. Despite appearances, the voltages on top and bottom are equal and opposite ... provided the total loads match, as well.

The inverter is then cap-coupled to a separate push-pull driver stage, which is sometimes also set up as a differential stage, depending on the resistance presented to the common cathodes. High impedances move it towards a differential stage, with the limit being modern constant-current sources. 6SN7’s were typically used here, with later designs replacing them with 12AU7’s (which typically have more distortion).

The drivers are then RC cap-coupled to the output tubes in the usual way. The drawback of a classical Williamson are the two stages of cap coupling, which can introduce low-frequency instability unless the output transformer has extremely wide bandwidth. The Partridge transformer specified for the original design had one of the widest bandwidths of any output transformer ever made ... but lesser transformers introduced stability problems, sometimes "motorboating" at low frequencies, but more commonly long recovery times from overload.

The Dynaco, introduced in the mid-Fifties, took the drastic step of deleting the driver stage and its associated RC coupling, and driving the output tubes from the RC-coupled phase inverter. Although the open-loop performance was quite poor, rolling off around 100 Hz and 7 kHz, the 20 dB of feedback nicely corrected it, since the input section used a high-gain pentode and there was plenty of "excess gain" to drive the feedback network.

The Dynaco had the advantage of being the cheapest of all to build; a combined pentode/triode, the 7199, took care of the entire front end, and all that was left were a pair of EL34 output tubes and an output transformer. In addition to Dynaco, many receivers used this approach as well. It was simple, saved money, and saved space, which was at a real premium in a low-profile AM/FM stereo receiver.

Receivers in the early Sixties (Fisher, Scott, Sherwood, Harman-Kardon, etc.) all had Bass and Treble tone controls, an AM and FM tuner with two different IF strips, an FM multiplex stereo decoder, a stereo power amp with at least 20 to 35 watts/channel, and last but not least, a stereo phono preamp. All with vacuum tubes, in a very crowded chassis, with marginal ventilation and caps of much lower quality than we have today.

We don’t see many Williamson amplifiers today. The dominant PP-pentode designs are Mullards and Dynacos, depending how price-sensitive the amplifier is. The monster tube amps with 4, 6, or 8 output tubes per channel typically throw in a dedicated cathode-follower section to drive all those grids ... sometimes one cathode follower to drive them all at once, or preferably, each output tube gets its own cathode follower. The RC coupling is then moved to the input side of the cathode follower, and the CF directly drives the grids of the output tube(s). This easily provides independent biasing of each of the output tubes, which is important when that many tubes are used.