Atmasphere, it's interesting you'd recommend a document that makes as many statements against so much of what you stand for as it does.
For example, UL operation clearly more linear than triode versus your statement made in the post previous to providing the link, "How it misses triode linearity is that a triode will typically have a nearly straight portion of its curve where a UL will always have a bit of a 'bow' in its curve." Figures 3a and 3c would show you have things exactly backwards.
At any rate, please correct me if I'm mistaken, but this is a commercial White Paper (IOW, as we all know, advertisement), put forth by the company that held the patent on Ultra Linear operation, and sought to make money on the invention via licensing fees paid for by those who wish to employ the topology. In no way do I mean to imply that's not a decent and honest path toward generating income. But do not lose perspective on the intent, which the literature is completely up-front with, "The information offered here will give the reader a positive picture of Ultra-Linear operation, and assist him in choosing an amplifier containing the genuine Ultra-Linear circuit." So, going in, we're not dealing with anything on the level of a peer-reviewed industry position paper.
Thus, that UL looks as attractive as it does against both triode and tetrode/pentode should not surprise. A discerning reader should feel the obligation to not accept things strictly as presented in the paper. Indeed, there are issues that immediately jump out at us:
1) The wpc data listed in Table 1 lie far out of the realm of what 60 years of experience with UL have proved possible, and provide insufficient background on how such numbers were generated
2) Returning to Figure 3a reveals an overall sloppiness of presentation; the triode schematic shows the screen grids (which obviously do not exist on true triode tubes, though the text of Figure 3b does reference "the same tetrode tube"), connected to the plates (yes, triode operation for a tetrode/pentode vacuum tube), with the accompanying curve traces labeled (true triode) 2A3/6B4 output tubes
3) Forgetting #2 for a moment, the text for Figure 3a, 3b, and 3c mentions tetrodes exclusively. No can say if that was intentional or an oversight; again, sloppy presentation. But obviously, if the inherent deficiencies of tetrodes did not exist, there would be no need to develop the pentode
Through such advertisements and the pressure to keep up with the competition, as I previously mentioned, designers were literally forced in to producing UL amplifiers regardless of whether they felt it the right decision or not. Obviously, the latter part of that implies many engineers (and I'm incredibly lucky and thankful to have met a bunch over the past several years) understood things were not as convincing as we were led to believe.
Regardless of truth or fiction the UL "standard" for hifi tube amplifier operation represented, triode (and tetrode/pentode) operation fell completely out of favor. In the age where the race for watts was preeminent, this hit triode all the more hard than tetrode/pentode, which could at least stand on the typical data of higher wattage and survive outside of the world of hifi. After all, what good would an obviously antiquated amplifier design that makes less than half the power and generally double or triple the distortion be to anyone?
Full credit and abundant praise must be given to the persistent courage of the Japanese in regard to the triode tube amplification renaissance. Many of us, AtmaSphere included, reap the benefits. Perhaps, tube amplifier designers will also reconsider tetrode/pentode operation in the same vein. Again, I am an obvious fan of UL, but the path to the ultimate can only come out of the position of curiosity, honesty, and impartiality.
