300b lovers

A current source could replace the resistor, but in practice, the performance is very similar to a current source, so it’s rarely done even in modern amps.

Usually in a differential amplifier, the plate resistors are matched if both halves are driven. In the case circuit of the above description, only one side is driven. So if a CCS is not used, the plate resistors can't be matched; one side must have a slightly higher plate resistance to compensate for the mu (gain) of the tube of the un-driven side, so as to get equal outputs from each half.

A good CCS eliminates this problem (which is nice since in the real world you can't count on the mu of each section to be equal or matching that of the tube specs on paper). A good CCS is both inexpensive and reliable, allowing the tube to be removed from the circuit while active (hot plugged) without damage.

Further benefit can be had from placing a CCS in the output tube cathode circuit, if you control the output tube(s) bias using fixed grid bias. The cathodes are tied together and the CCS feeds them; thus improving the differential effect of the output section, which reduces distortion and makes it slightly easier to drive due to increased gain.

Of course, if you have the input tube be a differential amplifier too, it can accept a balanced or single-ended input and can have excellent performance if a CCS is used for this stage as well. But its not a good idea to direct couple both plates to the succeeding driver stage; its OK to do one but the other should be capacitively coupled so as to prevent DC offsets of the first stage of gain from causing distortion in the driver.

The original BAT VK-60 of the 1990s used a differential input direct coupled to a differential driver; to deal with the DC offsets a potentiometer in the cathode circuit of the input tube allowed the plate voltages to be equalized. I found this approach to be problematic (we had tried that back in the early 1980s; one obvious problem is that it requires the user to make this fairly critical adjustment...) and often causes more problems than it solves.

So in the quest to keep the number of coupling capacitors down but retain easy operation, we started using a differential cascode voltage amplifier. The advantage of this was that all the gain of the amplifier was in a single gain stage, consisting of three dual-section triode tubes, one for the input differential amplifier, one for the top of the cascode, being plate loads for the bottom tube sections, and finally a 2-stage Constant Current Source for the circuit, tied to a B- supply of equal potential to the B+ supply. The CCS prevented changes in the AC line voltage from affecting performance of the voltage amplifier from 107VAC to 126VAC the difference was only 17 parts per million. So you couldn't see any performance change on an oscilloscope over that range!

So that allowed for enough gain, low distortion (once the correct operating point was set up), and only one pair of matched coupling caps (of a small value, in our case only 0.1uf, further minimizing the sonic impact of the coupling caps). They drive a pair of cathode followers which are direct coupled to the output tubes. So the power tubes obtain their bias voltage from the driver; therefore the bias and DC Offset controls are in the grid circuit of the driver tube. This allows for instantaneous overload recovery and rock solid bias control of multiple high-capacitance triode grids, with low frequency response to 1 or 2Hz no problem at all.

If you use a coupling cap in the critical area of the grids of the output tubes, it must be large so as to get good bass response since the grid bias network must be of relatively low impedance to properly control the power tubes. This means that the driver tube has a difficult load to drive and the large coupling cap can cause blocking distortion and slow the overload recovery. While this really isn't much of a problem driving pentodes, using this topology to drive triodes is a bad idea IMO/IME.

I've been describing how our OTLs work but obviously this would work well with a 300b too. We've managed to get our OTLs to 0.5% THD which is pretty low distortion for a zero feedback circuit! SETs by contrast tend to be about 10% THD at clipping which might be only 7 Watts. Since the OTLs tend to be much higher power capacity, the tendency is, for any power level the SET might have, the OTL has distortion that might be 2 orders of magnitude lower or more at the same power level. This is why they tend to be so much more transparent than SETs.  I've no reason to think this cannot be applied to a 300b circuit with similar results; SETs have the distortion they do out of the topology rather than the power tube that is used. So a pair of 300bs could be used to much greater advantage!

For those that might want to see more about how our OTLs work (and how this might be a topology for a 300b amplifier), there is a DIYaudio.com thread from several years ago that has a schematic and discussion. A lot of this would work very nicely with a 300b; for example the Circlotron output can be transformer coupled of course and have all the advantages (such as zero DC saturation of the output transformer) it offers.

Hi Lynn & Don... Will you be at Capital Audio Fest with the Blackbirds?

@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.