The SET amp question.

I have been reminded for a period of time that since my speakers are highly sensitive (110 db), why don't I try SET amps. I have owned tube amps that are of the push pull design but know very little about SET amps.

Can someone explain how they are different in design from other tube amps and which brands are recommended & proven reliable at the entry level?
The short answer-SET's amplify the whole wave form with one tube and push pull splits the signal in half (positive & negative) and one tube amplifies the neg and one tube amplifies the pos and then its put back together again.I know that somebody with more knowledge will chime in and really explain things!
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Racamuti got it mostly right, but actually a single output device is not mandated in a single-ended design, though it's certainly the most common situation.

"Single-ended" implies that there is no phase-splitter - that the output device(s) amplify the entire audio signal, positive and negative. The corollary to SE is push-pull - an amp that is one cannot be the other. However, more than one output device can be used in SE topology - this is referred to as parallel single-ended. Some amps of this variety can sound extremely good with no perceptible weaknesses compared to a single output device design.

Single-ended amps are also, by definition, class A, since the output devices must be pulling current 100% of the time.

In addition to SET - single-ended triode - perhaps also consider single-ended pentode or even some single-ended transistor amps (Pass). I, and many others, I think, consider the most important element of SET to be the first two letters of the acronym - triodes may offer the most linear amplification curves, and thus the simplest, lowest/no feedback circuits, but the more complex tubes can offer a very compelling sonic presentation as well. I'm thinking of the Almarro amps (which I have heard) and Audiopax (which I have not).
I can't explain anything about it technically, but if you have not heard a SET or SEP on your hi-efficiency speakers you owe it to yourself to have a demo. The sound can be intoxicating.
At 110 dB, unless you have a huge room to fill to orchestral live levels, you have the entire universe of SE designs to try. From the 2-3 watt 2A3 based to 9-10 wpc 300bs on up to the 50 wpc Kronzillas. Lots of great amps to try in that range.
Tell us your room size, music type, volume level and budget and we can make recommendations.
Unlike some of the responses, I cannot lend further technical explanations, but I can share some impressions of how single ended triode (yes, the "T" is in place in my experiences I'm sharing here) distinguish themselves in presenting music. I have been listening with a pair of 300B SET amps for about nine years now. I've strayed but always seem to come back to them because they seem to suit the music I prefer and the way I prefer to hear it. These impressions are my own and YMMV. SET seem to excel to extremes in breathing life into vocals and more sparse and simple instrumental arrangements. When the music starts to get more complex and layered and dynamic, SET can be bested by other forms of amplification, though it does OK for the most part. If you listen to a whole lot of music of that latter type, you might be disappointed overall. You most certainly will not be disappointed if your tastes run to the former (more simple, sparse arrangements). IMO, No other form of amplification seems to breath life and presence into music the way SET does at its best. The term "Living Presence" comes to mind. They excel at soundstaging - creating an illusion of a three dimensional space. Your sensitive speakers may indeed be well suited, but sensitivity is only part of the story. A flat impedance curve will also go a long way towards making the combination work out well. SET amps generally will not have much authority in the bass, nor as much sparkle the highs, but MY OH MY they do midrange, where 90% of the music is at, like nobody's business! A cheap introduction to the power of SET can be had in spades with the Wright 3.5 amps that use 2A3 tubes. I've seen them frequently on the used market for $800-900 for a pair of monoblocks. They are all about midrange, but that midrange is like crack to a junkie if you appreciate it. They are SET at the extreme end of the spectrum. Again, if you are a hard rocker or into heavy orchestral music or get the probably should look elsewhere. That's just my opinion. I keep coming back to SET myself, but I would not call it a versatile performer. It is an eccentric...more of a one-trick pony than a crowd pleaser for the masses. Another intoxicating alternative which has more potential for a more linear presentation, and is more versatile at presenting a wider range of musical preferences would be OTL amps, such as the excellent offerings by Atmasphere.

One further note on SET - I think pairing the amp off with the right speakers is so much more critical with SET because the wrong combination is just going to be a waste of money and effort, and can be very frustrating in that regard. You may want to seek out some opinions from folks who have paired off the same speakers you have with SET amps or SE amps to focus in on the better potential combinations.

The only thing I would add is that different tubes have different sounds, over and above how they are applied. To me, 300B's give the classic mid range brilliance, but are less good at freqency extremes. The 845, my favourite, is more neutral and more powerful, will tend to give a few more watts. My 845 puts out 22watts and will drive my speakers at 89db sensitivity, with ease. They do still lack the base grip of SS designs, I will happily sacrifice that.
At 110 db/w, you would have the luxury of being able to use just about ANY kind of SET. I would certainly consider 45s and 2a3s, because the higher power of 300bs and 845s and 211s would probably not be needed. It is a matter of taste, but these may be, to you, preferable to the other, more powerful tubes.
I would also put the late John Hogan SET's on your short list. I've owned two (one was a 2A3 & 45 and the other that I still own is a 300B), both bought without hearing first, and can only be described as AMAZING! I used both with my 99DB and 101DB Klipsch speakers.

As has been said, it's a magical midrange that just has to heard.
Everyone has done a good job explaining the differences. As for a recommendation of brands, Don Garbers "Fi X" is a great amp to try out. I think with your speakers, it will be a great match.
As Larryi stated, 845's can give the benefits of SET plus substantial power (in a relative sense). I highly recommend the Art Audio Carissa SET amp at 12 big watts. It convinced me that all this is not hype but an exciting way to go.
I was I had the luxury to try some of the low powered SETs with high efficiency speakers - if I did, I would be very eager to try the Music Reference EM7 since I have found their RM10, RM9, and RMSEs to be exceptionally good PP amps. I think there are now some reviews out there, but you can be sure any amp by Roger Modjeski will sound very good, measure well, and be reliable. I think the 2.5 watt version of the EM7 is $750 (there is also a 5 watt version for less than $1,000). The Wright 3.5 would also interest me based on feedback from owners of his amps and this one in particular - don't know the price, but his stuff is generally very high value.
The earlier post regarding the Art Audio Carissa is incorrect.

Output is 16WPC not 12WPC.
I have had 45, 2A3, PX-25, 300B, 211, 845, and 805 SET amps.

As others have noted, with your speakers, your options are completely open - and IMO the 45 cannot be beat all-around. Either the Yamamoto or the Fi (the X is good but not as good as his stereo or monos; the X is the budget version).

The 45 tube will amaze you with its rock-solid bass, perfectly neutral tonality, amazing sound-staging, and complete freedom from any weakness at all, really.
Thanks guys, your posts have been very informative and helpful.

Swampwalker, my room size is 25 by 20 feet. Mostly listen to smooth jazz and soft rock but enjoy all other types of music. Volume level is usually low to medium but at times that can change depending on the type of music I'm in the mood for. Budget right now is about $1,500.00 on the used market.
110dB?! Which make and model are they?

Donald North
I forgot that Music Reference also makes a 45 SET that sell for $1,800 or $6,000 for the deluxe version - somehow he is able to get 5 watts from a single 45, which I believe is considerably more than most implementations; not that you need it with your speakers. I would imagine that a critical issue for you would be that the amp be VERY quiet given the sensitivty of your speakers. Another interesting alternative would be to explore some of the low watt First Watt amps from Nelson Pass - again they are all too low in power for me, but your speakers may be ideal for some of these SS single-ended designs.
Djnorth, they are the Klipsch Epic CF-4s, large floorstanding speakers, same ones Paul Klipsch himself used in his system. Manufactured from 1994 to 1996. I'm not a huge fan of Klipsch but these speakers (CF-4s) are a very worthy consideration. As one person mentioned in the forums a properly set-up of Epics can be mind blowing. Anyhow I cheated and did the non-audiophile thing by using four, one in each corner. Either set or both can be switched in or out of the system.

Well enough about what I think of these speakers, the Klipsch website says they are rated at 102 db but my manual says 110 db.

I would guess that 102 is closer to the correct rating. 110 is in the territory of extraordinarily large, horn loaded (including the woofer) systems.

102 is still quite efficient, and you could use almost any kind of SET amp. Given your price constraint, it would be a bit difficult to find a decent 211 or 845 amp. Those amps have very high voltages on the plate and require very substantial, and expensive power supplies, and special wiring to handle such voltages. Personally, I would be a bit afraid of "cheap" 211 and 845 amps.

While I personally like some of the higher powered SETs (e.g. the Wytech 211 amp), I don't think they quite deliver the exquisite detail and texture of the lower-powered SETs (2a3 and 45s are personal favorites). Of course, if one needs more power one may have to sacrifice in one area to get satisfactory performance in another. There are always tradeoffs. Someone above mentioned how SETs don't perform quite as well with denser, more complex music. To me, that is evidence of an amp being overdriven just a bit, not something characteristic of SET performance. A SET operating within reasonable power bounds is better than most other kinds of amps at very naturally laying out all the texture, detail and harmonic relationships of complex music.

A friend of mine with 106 db/w speakers has no problem with single-ended 45s, but then again, he does not crank his system up very loud (good, high efficiency systems actually perform very well at low volume even though they are capable of wailing with the best). You might get away with using a 45 or 2a3 SET. My own system is 99 db/w efficient and I use a parallel-SET 2a3 amp (two tubes per channel), and I have enough power, except perhaps for certain works with large choruses.

While SETs are typically simple in design and easy to construct, they are not necessarily "cheap," because they place difficult demands on the output transformer which cannot be cheaply made for SET use. This is more the case with high-powered SETs, again suggesting that you should look at the lower-powered variety (300B, 2a3, 45). I have not personally done much listening to pentode tube used as SET, so I don't know if those are good alternatives.

Also, don't overlook good triode pushpull amps. They may not deliver all of the BIG, spacious sound, beautiful texture, and ambient decay (hall sound) of the best SETs (differences can be quite subtle), but, they have a lot of virtues of their own (punchy dynamics, tight bass).
Well that's a pretty good size space but 102 dB is pretty darn efficient. Its not quite double the size of my room (14 x 23) but I do have 8.5' ceilings. Assumming you've got 7.5' its less than 50% larger, but lets consider it twice as large for now. I found 9wpc 300b amps fine for moderate volume singer songwriter, soft rock, acoustic jazz stuff but the amps ran out of steam with more heavily layered material at somewhat higher volumes; with 89 dB Merlins (flat 6 ohm impedance). IIRC, each 3 dB increase in sensitivity is equivalent to doubling the power. So 89-92-95-98-101 That's about 16x more effective power, so the 300bs should be just fine and maybe even the 2A3s. There are lots of 300b amps out there and the cost of decent power tubes has dropped.
Swampwalkers calculations are close, but it is actually 20x so 2a3s and 45s are even more reliably "in the ballpark." It really is a matter of taste. 2a3s tend to be more lean (less upper bass/lower midrange) than 300bs, but more open on top and detailed sounding than 300bs (to me 300bs have a kind of blub, blub, blub bass response). 45s are sort of in the middle between 2a3s and 300b, in terms of leanness, and have a MUCH tighter bass response. They sound punchier in the bass than either actually, aside from the much lower overall output.

The overall sound of any amp can be altered somewhat by the choice of output tubes and the choice of the other tubes, so there is a range of adjustability. Also, with some 45 amps, you could replace the 45s with 2a3s, and perhaps adjust the bias for a little more current; this would mean running a 2a3 conservatively which would mean longer tube life. A friend of mine takes a 2a3 amp and biases the current down a bit and runs 45s hot. He likes the sound of 45s this way, but the tubes don't last very long (he has MANY spare 45s). There are lots of possibilities.
I am using a Yamamoto A-08s with my Zu Def 2s. With the original NU 45s, the amplifier struggled with large scale orchestral and choral works. The Emission Lab solid plate 45s that I now use give it a lot more oomph.
SETs get their capabilities from 3 things: Class A, triode, and reduced distortion at low power levels in the output transformer.

Push Pull amplifiers actually *increase* distortion as the power level falls to very low levels on account of the somewhat different behavior of their output transformers. Anytime distortion is increased, the masking effect of the human ear means that detail is obscured.

IOW its all about the transformer (its no problem to build a push-pull triode class A amplifier). But- take away the output transformer (get rid of it) and the argument against P-P vanishes.

At only 102db, unless you are just fine listening at relatively low levels, a 45 will not really produce enough power to be lifelike, although 45s otherwise sound quite impressive. It happens that in general, the smaller you make an SET, the better they sound (wider bandwidth). 10 years ago the 300b was king; 5 years ago the 2A3 had taken over but the 45 was already on its ascendancy...