Ultralinear vs. Triode vs. SET

Ralph makes an excellent point in the inherent superiority of an amplifier employing triode tubes to one operating tetrode/pentode tubes in triode.

However, Ralph's statement, "The result is an output section that that has nearly triode linearity, and nearly pentode efficiency" undersells pentode operation, while overselling ultralinear.

Things seem to fall right inline with simple arithmetic interpolation - connecting the dots. Two tubes in pentode push-pull operation can produce 60 - 75 watts, the classic 30 - 40 watts in UL that we all have grown accustomed to, and 15 - 25 watts in triode (not muddying the waters here with Class A, Class A/B, fixed bias, cathode bias, etc.). Without providing analogous data for linearity, the fact that an abundance of current day high-end audio amplifiers provide the means to switch between UL (often mistakenly calling it pentode) and triode operation makes the point that again, the distance between the ultralinear and triode is not insignificant. As it always seems in life, we just don't get something for nothing - the old "there's no free lunch" adage.

Continuing on the pentode - ultralinear - triode vein, what almost always turns out to be the case when one is talking about pentode in this day and age, they mean ultralinear. That's why there are so very few two tube per channel high-end audio amplifiers putting out 60 - 75 watts today. Thus, very few of us have actual experience with them.

Although the tube linearity of ultralinear operation may slot in between triode and pentode, there definitely is, to my ears, something else at work. Ralph has long argued the destructive sonic effects of feedback, and points out that ultralinear operation introduces feedback into the equation. That might explain why I hear the argument of the sonic superiority of ultralinear over pentode oten fall apart. For those who have heard a true pentode amplifier, it can actually be a revelation; there is an openness, clarity, and immediacy that jumps out at you. Maybe that is why some of the older designs (some of the famous Scott amps) which employed true pentode operation continue to be respected, sought after, and even prized despite all of the extraneous circuitry onboard that the past generation or two of high-end audio have stripped off.

I'm not advocating pentode as the be all and end all, but it's more than obvious that the pendulum has swung far too much in the other direction. I believe the hobby would be better served if there were more pentode amplifiers available to us. In no way do I think there is anything insidious at work, just a kind of "because that's the way grandma did it" scenario. It's one of those things where the argument is accepted as truth without another word because it's been such a long time since there was any challenge to it. Obviously, I'm standing up and challenging it here...

My problem with what I consider the better (45, 2A3, 300B, etc.) SET amplification is that there simply is not enough power to truly do justice to the music. You can get sound, you can get volume. You can even a lot of volume. However, in my opinion, if you really want to power a real world, full range loudspeaker the way they were meant to be, these amplifiers are not a very good way of doing so.

Many feel the higher power SET candidates such as the 211 and 845 are the answer to that. In my experience, you definitely do lose that magic with these tubes. Though we're still dealing with SET, it really is a different kind of sound.

As has been mentioned, push pull triode amplifiers are another (often excellent )option. Though again, you do take a step back from that sought after SET quality.

So far not mentioned here is the parallel SET (PSE) topology. In my experience, you lose little, if any, of that SET purity, while effectively increasing amplifier power in the expected ratio (two tubes provide double the power of one). Still, I would argue, not to high enough levels to adequately power a full range loudspeaker, but for at least a lot of audiophiles, you're close enough to have that conversation.

Another route for someone who is drawn to the 45, 2A3, 300B type SET would be powered subwoofers, a loudspeaker that features onboard amplification of the low frequency drivers, or rolling one's own with separate amplifiers, an external crossover device, and loudspeakers that allow such an implementation.

Paul, I will preface my answer with the statement that due to the breadth of the hobby, one would think there are more pure pentode push-pull amplifiers available than I know of. But, the only two I can say with certainty are the legendary Quad II, introduced in the 1950s/reissued in the past decade, and the Audio Note (probably the most SET devoted tube amplifier manufacturer) L4 EL34 kit. For differing reasons, however, neither make much power. Obviously, we can all rattle off a list of current UL or triode tube amplifiers as long as a gorilla's arm.

The famous Scott 299 and later variants of the more ubiquitous 222 employed pentode operation. The evolution of the latter, in shifting from ultralinear to pentode, infers some definitive rationale.

Interestingly, the Quad II features the cathode cross-coupling Ralph touched on. Coming from a man who threw around such compliments like manhole covers, Bud Fried's oft repeated statement, "Peter Walker had a first rate mind" is indeed high praise. I'm not sure if their original design, the postwar Quad I did or did not; I don't know much about that product. My Jadis DA30 and DA60 (but not my JOR) do as well, though one could argue it's a slight bit different.

Larryi, the issue your friend described regarding PSE design is the common criticism of the topology. Still, my feelings mirror yours regarding your amp, I like the end result - the sound.

Being the former importer, I was fortunate enough to extensively compare three versions of a commercial 300B based amp: an 8 watt SET, a 15 watt PSE, and a 20 watt PP. Echoing my previous post, the SET simply did not have enough power to partner with a full range loudspeaker. The PSE gave up a little in the way of why people become smitten with SET, and though I still felt it was power shy, some disagreed. The PP was arguably adequate in terms of power, but a lot of the magic was sacrificed. Still, as I mentioned, the triode push-pull option is often an excellent one. In fact, of the line I represented, I felt their lowly, overlooked, affordable 11 wpc PP 2A3 was one of their best products regardless of price or status.

One huge point which needs to go along with any discussion of amplifiers is that a system truly represents a marriage between amplifier and loudspeaker. Of the different types of loudspeakers I have - typical ported dynamic, planar, front-loaded horn, backloaded horn, and true TL (I cannot speak to ribbons such as the Apogee), some of the partnering amplifiers I have/had (SET, PSE, OTL, PP triode, UL, or pentode, and SS) can drive them well, some can't. The ability to put power into a loudspeaker often transcends WPC or the loudspeaker's published sensitivity and impedance specifications.