Ultralinear vs. Triode vs. SET

I currently have a Rogue Cronus that I have been quite satisfied with, but I am intrigued with the possibility of a SET amplifier. From both a technical and sonic perspective, what are the differences between a tube amp with switchable ultralinear and triode mode vs. a true SET amplifier?
SETs are where it's at! With the right speaker SETs are the reference. My favorite is a 45 SET. Playing through my Cardersound Madisons with the planet 10 fostex is very nice.You'll notice much more detail,staging will be accurate with each recording just on and on. How many times have you seen an add where the seller is going to SETs? Try one you'll hear the difference. I've had a lot of people try tube amps and were floored then they go on to a SET. Well that's the end of the story.Basically if you like a tube amp you'll love a SET.
There are significant limitations to SET's. Glorious midrange, the best, really, but many people miss the deep bass, high frequency extension and dynamic range that other amps do better, or do not wish to be limited to highly sensitive speakers. SET is not the way to go if your primary listening is rock or classical orchestra.

You should search the many posts about these differences in the archives and various audiophile forums, but you need to listen to them to understand the differences and get what sounds best to you. I generally prefer push-pull triode as a kind of compromise between the extremes of ultralinear/pentode and SET, but not always.

On the spectrum from triode to ultralinear to pentode, power increases, so bass gets tighter and the soundstage moves forward, but the sound can become harsher and you lose some ease and air.
Well...I think it's a bit more complicated than Philefreak indicates, but I'll say first that generally, I LOVE the sounds of SET amps.

SET amps definitely have limitations...in power and damping factor, mainly. Most SETs are low in power--that's where the term 'flea-powered' comes from, and I LOVE it! That means that most are limited to single-digit number of Watts. Larger tubes such as the 845, the 805, and the GM70 (and a few more, of course) are required to produce more than the 8 Watts or so that a 300B output tube produces.

BUT...BUT...BUT...good SET amps reproduce music that sounds simply more like music rather than reproduced music. I've owned 4 stereo or pairs of mono SET amps, and I'll never go back. Some say that SETs can't sound great driving lo-impedance or lo-sensitivity speakers. They need to come to my house, where I'm driving the top-8 octaves of 89dB-(in)sensitive, 4-Ohm-rated Audio Physic Avanti speakers with TWELVE-Watt SET amps, and the combination sounds not simply better than it has any right to but unqualifiably excellent.

I'm not a golden-eared audiofile in that I cannot recognize the dozens of tiny differences among pieces of audio equipment that the GEAs can hear and describe. My approach is longer-termed and I deal with overall sound character...a macro approach, one might say.

You might benefit from reading Art Dudley's 2004 review of the Antique Sound Lab Explorer 805 50-watt SET amps.

... http://www.stereophile.com/content/antique-sound-lab-explorer-805-dt-monoblock-power-amplifier ...

I read that in 2004, and having the same Quad 989, lo-sensitivity speakers Art had, I bought a pair of ASL805s sound unheard. I LOVED them and, after going thru even-less-sensitive speakers, and high-power push-pull-triode amps, and hi-sensitivity speakers, I'm back to SETs and have just ordered my 2nd pair of ASL 805 amps.

The music just sounds better. ... :-)
If your Cronus already provides enough power for you (55 watts?) and you wish to try SET, get Audio Mirror 45 watt SET monoblocks. Nearly the same power as your Cronus, but SET! Glorious amps.
" From both a technical and sonic perspective, what are the differences between a tube amp with switchable ultralinear and triode mode vs. a true SET amplifier?"

A tube amp with a UL/Triode switch will most likely be using a pentode. The switch makes it operate "like" a triode but it is what it is and some may like the sound of a KT88 or EL34 wired that way but it's not a true triode. A true "SET" amp will probably be using a 45,2a3 or 300b tube each having their own flavor/character and power output.

That's kinda general but the best I could do per your question and keep it simple.

I do think anyone who loves music should at least give a listen to a "true" SET mated once in their life. Hopefully it will be mated to a speaker it likes.
Ooops, was rushing at the end of my post. Last paragraph, delete "mated" after SET.
Very simply: a pentode based amplifier will be more versatile with speakers. A SET is very finicky and will like some speakers and music more than others. Despite the poor measured performance of the SET, it somehow manages to sound more realistic and more believable than any other amplifier type ... provided it is matched with the right speakers and the right type of music.
Thanks for all the great replies so far. A couple of additional questions:
1. Would a single ended pentode amp typically have the same sonic benefits as a SET amp?
2. Would a push pull amp with no negative feedback potentially have similar sonics to a SET amp?
HI All,
For me, the prime drawbacks of SET amp is their low power and restricted frequency response/dynamics. I won't argue against their mid range. It's great.

My problem is I have never heard a high efficiency speaker that I would die for. Yet, these are the only speaker types that a set amp can drive. This plus they basically sound like active tone controls.

No thank you.

Maybe take a listen to a tube amp where the tubes are direct heated. I think Coincident makes one like this. More of a SET sound with frequency extension.

Happy Listening.
Hi Droz,
I was in your shoes about 14 months ago. I prefer tubes over SS amps and had been using push-pull(PP)amps with mcuh satisfaction. These amps were pentode/triode switchable and were El34 or KT88/6550 based. Well after reading reviews and articles concerning SET amps my curiosity got the best of me and I purchased the CoincidentMK2 300b monoblocks.I`m absolutely hooked on these amps they`re just fantastic. As other posters have said above a good quaility-well designed SET amplifier just sounds so naturally real and live. The tactile and palpable presence is the best I`ve heard compared to SS amps and even very fine sounding PP amps. The sound is very organic, fast and dynamic, yet it retains such a relaxed musical flow.
I disagree with those who say you give up upper and lower extension for sake of a great midrange NO YOU DON`T! This is a misinformed stereotype Droz. The key to sucess is matching to a appropriate speaker(high effiency and favorable load-ohms). You do not have to settle for rolled off highs or soft/flabby bass. You will have many choices in different DHT tube types and also various speakers. Just be patience in your search and read many reviews of SET amps to increase your knowledge base. The world of SET can be wonderful with the proper approach. I hear nuance harmonic overtones and micro detail that were hidden with my PP amps.
All great information, but not all of it true for all SETs. I love my speakers (87dB) and did not wish to give them up, but did wish to try a true SET. Thankfully, Audio Mirror 45 Watt SET amps are true SETs, yet very powerful, and not lacking frequency extension in any way. If you love your speakers and wish not to change them, you owe it to yourself to try a true, powerful SET. I believe there are a few other manufacturers available with true powerful SETs as well. Swapping the stock driver and input tubes with NOS took my system to a whole new level.

1. Pretty much. It's single ended. It will have it's own flavor depending on the tube used. One of my amps is the little Glow Amp that uses 1 EL84 per channel. Nice sweet sound.

2. It would be close. A push pull 300b amp would have a similar flavor to a single ended 300b. I hear a purer tone and better decay with SE.

What do you value more, loud cresendos or smooth decay?
If you have not done so already, search the site for high effiency speakers this topic has been well covered on audiogon. The old threads contain much information and many good suggestions, there`s no shortage of very good speakers of this type.In contrast to Sparky`s statement above, in my experience the high effiency speakers sound more real/natural and less HiFi in many cases.
Push-pull amplifiers make more power, but often do so with increased 5th harmonic distortion, which causes them to sound brighter. This is not always the case though, if the amplifier is fully balanced from input to output the only distortion component will be the 3rd harmonic, which is considered one of the lower ordered harmonics and is more innocuous.

A pentode amp will make the most power for the tubes involved, but will also be the least linear, and so will need negative feedback to linearize it. There is a price paid here- you get most forms of distortion reduced, but with an increase in the odd ordered harmonics, which the ear finds unpleasant.

Ultra-Linear is a method of using the output transformer as a very local feedback loop to linearize the output section. The result is an output section that that has nearly triode linearity, and nearly pentode efficiency. This is a very nice way to set up a push-pull amplifier, but if the amp were actually using *real* triodes, it would sound even better as it would be more linear.

Output transformers are such that the bigger you make them, the harder it is to get bandwidth. This is particularly true of SETs, and is a primary (no pun intended) limitation in the higher-powered examples. In most cases, the 7W of a 300b is about as big as you can get and expect bandwidth that might be considered 'hifi'. This is also why the smaller SETs, the 2A3s, 45s and the like have the reputation for sounding better- they have greater bandwidth. Now you can fudge things a little- optimize for bass or highs (and since most high efficiency speakers have troubles making deep bass, the tendency is to go for the highs rather than the bass) since you aren't going to get both.

Push-pull overcame a lot of these issues to a certain degree, with its own set of trade-offs. For example, an SET will have nearly unmeasurable distortion as the signal level heads towards zero, whereas a push-pull amplifier will usually see increased distortion (this is commonly attributed to an additional circuit in P-P amps called a phase splitter; its worthy of note that not *all* P-P amps have to have this as a separate circuit). This is where SETs get the magical midrange that they are known for- when distortion is reduced, detail is revealed.

You still have the issue of bandwidth with P-P, but in general you can have about 10X more power available compared to an SET with equivalent bandwidth.

The DC that the power tube draws through the transformer of an SET can cause saturation (distortion) issues with the transformer. This is a major design consideration of the OPT for an SET. You don't have the DC saturation issue with P-P, as the DC currents used by the power tubes cancel in the core of the transformer. On top of that, even-ordered harmonics are also canceled in the load, making for an amp that should be more transparent at greater power outputs.

The distortion of an SET is mostly lower orders until the output power becomes significant. At that point the higher orders become significant (although measurably insignificant) to the human ear. The 5th, 7th and 9th harmonics are used by the human ear to determine sound pressure, so when they are distorted (even so slightly that it is difficult to measure) the effect is that of increased loudness. Since more power is usually used on transients, what you have is the loudness cues occurring on the transients. The result is that SETs will *seem* to be very dynamic for the amount of power that they have, and often people will say its all the power that they need- they don't want to turn it up any higher.

So now you can see that in conversations about such, that if you substitute the word 'distortion' for that of 'dynamics', the meaning of the conversation will not be altered! It happens that if you can eliminate the odd ordered distortions, the amplifier will have no loudness cues, and the immediate effect will be that of turn the volume up higher, as it does not sound 'loud'.

Now IMO and IME, 'loud' is something that a stereo best not do, **regardless of the actual volume**. That can be a bit of a trick, but is well worthwhile.
I can't qualify my preferences like Atmasphere, but like many, I find the limitations of SET's too restricting. My adventures in audio began with solid state, but as soon as I realized that tube amplification was the common thread shared by the systems I liked, I've been devotee, and spent time with several examples of your subject designs. All choices involve some compromise, which then boils down to your personal musical values. The best compromise, for me, has been a well executed 300b push-pull design of moderate power. My tastes in music are pretty wide ranging and this design has allowed me to enjoy the entire range without extreme limitations being placed on my choice of speakers.
I currently own an Atma-sphere OTL (S30), 4 SET amps (45,2A3,300B,GM70), a SEP amp (single ended pentode using 6CL6 tubes), a reburbed Dynaco ST-70 and a Class D amp. I have owned a myriad of other SS, PP and SET amps over the years.

With appropriate speakers, Single ended and OTL topologies bring me far closer to the music than any other type of amplification. The clarity and presence of these type of amps can be stunning. I'll leave the technical explanations to more qualified folks like Atmasphere. To my ears, the distinction between good SE/OTL and other amps is not subtle. It's readily noticeable.

However, saying that, there are a few important considerations when using these amps. First, you must match your speakers accordingly. For the most part, SET's will not perform optimally with inefficient speakers. If you love large planars and Electrostatics, then you are probably going to need to look elsewhere. Also, your listening tastes come into play. A 45 or 2A3 SET is wonderful with intimate vocals, small scale accoustics and jazz. Realistically, not enough muscle for Bruckner or Mahler symphonies. If you love large scale orchestral music, you probably want to look for a high powered SET (211,845,GM70) or OTL's.

If your speakers match well with either SET or OTL, I really think you owe it to yourself to investigate these amps. It can be a major ear opener.
As predictable many opinions and experiences expressed here, as each type of amp design has some level of compromise(SET,PP,OTL,etc.) The 300b PP route is ok but won`t offer anymore speaker flexibility than say a 845 or 211 tube used single ended. There are`nt extreme limitations of speaker choices either( even if you choose a 300b SET). With a PP 300b you will gain power, but now you`ve introduce phase splitters to the circuit. The SET approach will generally sound purer and even more natural(very simpler circuits tend to sound more natural) Since the original post was an inquiry of SET AMPS i sincerely hope you have the chance to hear them in a well thought out system and judge with your own ears. Just for the record I can play any type of music I want with my 8 watt Coincident Frankenstein. Don`t think you have to limit your music selection at all.
Best Wishes
For my taste, a speaker well matched with an OTL is the very best type of amplification overall, especially if you include compatability with a broader range of speakers than is possible with most SETs; and much better frequency extension with very resolved midrange. If you are intrigued with SET, a good time to also consider OTL (Atma-sphere, Joule, Transcendent, and soon Music Reference - there are others).

Charles1dad made the solid statement that you would get many "opinions and experiences expressed here" and then expressed his. As I read the original post, it is not an inquiry exclusive to SET amps but a comparison between SET and other designs. While I guess the word extreme is subjective, we disagree that SET's, in general, aren't much more demanding and restricting of the speakers they're matched with.
Sparky: "My problem is I have never heard a high efficiency speaker that I would die for. Yet, these are the only speaker types that a set amp can drive."

WRONG WRONG WRONG. SETs can drive any kind of speaker, some better than others. You just haven't experienced SETs driving relatively-lo-sensitity as I have. My 50- and 12-Watt SETs drove and drive my Quad 989 and Audio Physic Avanti speakers excellently.
Hi phaelon,
No real disagreement actually, Droz said he wanted comparisions with his switchable PP tube amp and a "true SET"amp. If he wants to expand his options, a 300b PP amp would be a good way to go i.e. the beauty of DHT tube vs a pentode indirectly heated tube. Many opinions is what I`d expect on a open forum that`s all. Regarding speakers , yes theh`re some restrictions but Droz has a lot more choices available then some would lead him to believe. Zu, Tonian Labs, Coincident,Horning,Audio Note, Living Voice, you get the idea as the`re many more.On the other hand if he was basing a system around an inefficent speaker he would then have to deal with limiting the pool of amplifiers( always some compromise is`nt it). Phaelon you have the NAF amp from Italy correct? I`ve never heard one but love the look of them.

Take Care
"I`ve never heard one but love the look of them."

Yeah Charles, that's the nice thing about the Italian stuff; if you don't like the sound, you can always put it on the mantle and call it art. :-)
HI Jefferybehr,

How can my statement be wrong? The speakers you mention may impress you. But me? How can you put those words into my mouth? I resent you trying.

And how do you know that excellent to you may be junk to me. Do not try to push me in that direction. I know what I like. And, I don't like SET's. Sorry. I don't even think they can be called hi fi.

Atmasphere, thank you very much for that clear and informative synopsis. Great stuff.
Some really excellent posts above!

I'd be interested in further comments that anyone might have on how much of the 45/2A3/300B "magic" tends to be sacrificed in well executed designs that use higher powered transmitting triodes, such as the 211 and 845. For push-pull as well as SET configurations, if possible.

-- Al

In well executed designs, I think the SET "magic" is there for the high powered SET's as well. My favorite high powered SET tube is the GM70. I like my GM70 amp just as much as my 45,2A3 and 300B amps. In fact, If I could have only one SET amp I would probably take the GM70 due to its amazing presence and versatility over multiple music genres.
I agree, the SET experience most certainly is`nt HiFi it`s music presented naturally.
" is`nt HiFi it`s music presented naturally."

LOL - Oh no! You're not really going to bring that one back, are you Charles?
Sorry Phaelon, I could`nt help my self.
HI 1dad,
Clearly, you and I have very different ideas about what the term "hi fi" means. To me, hi fi sound is the holy grail. It is reality. Any deviation from this objective can be described as distortion, coloration, signatures, etc. IOW, anything that deviates from reality. Hi fi is a real term not subject to individual interpretation. It is what we as audiophiles should aim at. You clearly don't by your own words.

Your definition is so loose that anything you say looses meaning, has no anchors, and is useless. You are definitely a SET type. Stay away from me!! I don't want to catch your brain wanderings.

Sparky relax! it`s just a hobby we both enjoy but with different taste and perspective. In my audio evolution over the past 22 years I`ve gone from SS amps( Symphonic Line was my best in this catagory) then various PP tube amps(which I liked more than the SS amps). SET amps for me were a significant step above the former amps because they sounded much more like the sound I hear at the local jazz clubs(unamplified and acoustical) I visit quite often. It was a revelation for natural sonic realism. I used to play trumpet and man, my 300b amp is spooky close with the tone, power and the brass body reproduction. But that`s me , you obviously simply prefer something else and that`s perfectly fine. The orgin of this thread was based on an interest in SET amplifiers, I was happy to share my own very deep joy with this type of amp, nothing more nothing less.
A lot of comments here that fit my experience. I own, and like, SET amps. Yes, there are things that they do that really cannot be matched by other designs (realistic decay of notes, truly enveloping soundstage).

But, the difference between the SET topology and pushpull is perhaps not as big a difference as that between tube types. I agree with the poster who noted how much a 300b in SET and pushpull share common characteristics. I have heard quite a few 300b, 45 and 2a3 pushpull and SET amps. I own, and like the sound of my pushpull 45 amp, even though it doesn't do all that a good SET can do. The amps tend to sound most like the tube type, with pushpull topology sounding "tighter" and seeming to not have as much "bloom" and not delivering all of the harmonic richness of the SET.

On the power limitation/speaker compatibility issue, I would agree that, if one happens to like a speaker that demands more power, then the priority is to find a compatible amp, and if that means foregoing certain topologies, that is just one of MANY compromises one must make when building a system (to me the priority is speaker choice first and foremost, then finding a compatible amp--not the other way around). But, I find that most people overestimate how much power they really need and the range of speaker choices for SET amplification is really not as narrow as some would believe. I've heard several 845 and 211 SETs that would work with almost anything, provided the room is not too large and one does not require extremely high playback levels. If one finds that much more power is needed, I would look at OTL amps first, then solid state amps (I tend to dislike higher powered pentode tube amps).

On the subject of OTLs, it may well be the case that a good OTL would be a first choice, regardless of power requirement. I love the speed, liveliness and "directness" of OTL. Within their power limitations, SETs can also have that quality, but, it is really hard to beat an OTL in those regards. To me, the tradeoff is that the sound is a touch "raw" or "rough."

One more thing, there is a pretty wide range of capabilities of SET amps. Although the SET topology is quite simple, SET amps are actually quite costly to do correctly because of the demand placed on the output transformer. In order to handle the DC current in the primary, the transformer must be big, air gapped, etc. SET transformer design requires even more complicated balancing of tradeoffs than pushpull transformer design. The better SET amps tend to be quite expensive.
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.

I do agree with a lot of what you said above. The transmitter tube SETs (845, 211) do not have as much of the favorable qualities of the low-powered triode tubes--like almost everything else, it is a matter of picking tradeoffs--but I don't find them completely wanting.

My primary amp is a parallel SET (two 2a3s per channel). I like this amp a lot, particularly when the 2a3s are EML meshplates. A friend, who designs amplifiers, said that he personally could not get that topology to work; one tube in the pair inevitably ended up hogging the current and doing most of the work even when the tubes started out as a matched pair. I have also heard other claims that some "purity" is lost and the sound becomes a bit muddled when more than one output device is used (the same claim is made for pushpull and even transistor amps). Also, while output is higher in parallel (I would guess primary impedance is also lower) we are still talking about pretty low output. My amp is rated at something like 6 watts from the parallel 2a3s. Am I losing something because it is a parallel SET? I don't know, but I like the amp.

I have heard a number of nice sounding pushpull pentode amps that put out 40-60 watts using two tubes per channel. For the money, small pushpull amps running EL84s sound pretty good to me (punchy and fun sounding). What has been most disappointing to me are the really big, expensive pushpull amps with many output tubes, particularly if those output tubes are 6550s. These sound brittle and dry to me (MUCH more so than decent solid state amps). Those that can be operated in triode (screen tied to the plate) do tend to sound better that way, but, I still don't really like them that much.
What two-tube per channel P/P true pentode amps are there in current production?

Off the top of my head, there is the Audiospace AS-6m (KT88, ultralinear/triode switchable), Primal Luna Prologue 5 integrated amp (KT88 ultralinear), Cayin A-88 integrated amp (KT88 ultralinear/triode switchable), at least one Ayon integrated amp(KT88?), Rogue Atlas (EL-34 ultralinear), Quicksilver Audio Mini-Mite and Mid Mono (EL 34, KT88, KT90, 6550, etc).
If the amp is ultra-linear then its not the kind of amp Trelja was referring to. An example of that is the Citation 2 made by Harmon Kardon.

Now its a simple fact that pentodes, left to their own devices, are not very linear. So you usually have to do *something* as they won't otherwise be hifi. So you either run an ultralinear circuit, loop feedback, cross-coupled cathodes or maybe even all three.

Cathode cross-couping has not been mentioned so far, but works very similar to ultralinear operation. This is a local feedback technique that involves the secondary of the transformer and the cathodes of the power tubes. The technique was first used in the Leslie speaker cabinets for Hammond organs in the 1950s, and has been used in most ARC amplifiers made. You can get enough linearity using this technique that you don't have to use loop feedback.
Thanks Ralph, that's what I meant to ask about "pentode"
operation. By the way, can someone explain tetrode [?]
operation - I often see that word used. Is this yet another
possible tube application?
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.
"The ability to put power into a loudspeaker often transcends WPC or the loudspeaker's published sensitivity and impedance specifications." Amen.

I noticed this when I finally decided move from the 8ohm tap to the 4 ohm tap of my Music Reference RM10 MKII (Ultralinear P/P. A/B) to drive my Merlin VSMs. I thought I was on the lower limit of power (35watts) to drive the Merlins, though plenty loud! By switching to the 4ohm tap (light loading), the power was reduced by 20% (down to 27 watts), yet darn if it did not sound better, had better, more articulate bass, and better transient attack. Perhaps fewer watts, but apparently less distortion, and more current available for transients. The amp was more in sync with the speaker.
Paul, I suspect that the lower output impedance/higher damping factor of the 4 ohm tap (probably a factor of 2 different compared to the 8 ohm tap) was also a significant contributor to the differences you heard. That would be particularly so because your speakers, like a lot of others, have very wide impedance swings in the bass region, iirc.

Best regards,
-- Al
IS there any consensus (not kidding:)) that for a given circuit, fewer output tubes always sound better than more IF there is sufficient power to drive the speakers? Maybe it was Sam Tellig who once said that some of the best sounding tube amps tended to be 40-60 watt tube amps, most if not all with just a pair of tubes per side, I wonder how important that is and whether running 2 pairs per side makes it that much harder to get the "purity" of sound you get with a single pair (or just one output tube). Again, assuming the lower power rating does not present an issue for driving the speakers.
No- no consensus. Sometimes its true and sometimes it isn't. If the designer paid attention to the idea of current sharing amongst the power tubes, more tubes will not be a big deal.

If you have a transformer-coupled amp and its designed for 4 power tubes, pulling 2 out may not work right any more- it really depends on how the circuit is designed.