A good place to read about SET is the SET forum on Audio Asylum. Very large archive of information to learn about SET amps.
To me the biggest SET magic is the immediacy factor. A SET system can sound bright, soft or somewhere inbetween, but the music always seems to be more intimate or in contact with the emotion. That gives a live feeling that other system often found lacking. Whether this factor is true to the recording is debatable, but I don't really care that much about accuracy. I am just here to enjoy the music.
Some of the technical aspects of SET amps that many people appreciate, are pure class A operation, simple signal path, very linear amplification devices(triodes), and almost totally 2nd order harmonic distortion profile.
This typically leads to a very pleasant, coherent, and lifelike presentation, as long as the speakers are a good match. It is very difficult to match the purity of this type of amp, with anything else.
As with most things, the cheaper SET amps will suffer compared to the higher priced ones, primarily because of the cost of good output transformers is quite high. The output transformer characteristics will significantly influence the sound that you get from the amp, and cheap ones give the "rolloff and bloat" that many people associate with SET amps, because they have not heard the better ones. If you are serious, make sure that the SET amp you get has very good output transformers, as well as having a good circuit design.
Regarding tube choices, 300B is very popular, but I prefer the lower power choices, especially the 45. It limits the choices of speakers to quite a narrow range, but if you can get the right speakers, the sound is quite amazing. Alot of people are finding the 845 to be a great sounding tube in a SET amp, and this gives much more power and opens up the range of speaker choices.
Both of the comments above are spot on. I was actually at a dealer looking for an OTL amp to get the kind of immediacy and vibrancy that my pushpull triode amps do not quite deliver. I found that a SET can actually deliver that kind of immediacy without the slightly rough and crude sound of some OTLs. I ended up with an OTL that uses parallel 2a3s. The 8 watts they deliver are enough for the 99 db/w speakers that these amps have to drive.
TWL is absolutely correct about transformers. My SET amp is made by Audionote. I've heard various models of their amps, and the ones with better core material and silver wire in the primary and secondary windings sound much better than models with lesser trannies. I know it is a matter of preference, but I also prefer triodes other than the 300B, which, to my taste, sound a bit bloated in the midbass. I personally like 45s; 2a3s can be a bit lean and analytical sounding, 45s sound like they fall between the 300B and 2a3s.
I too am a new SET covert. Have been using a high powered SS amp with a tube pre for awhile. It was suprising for me to find out how easily my cheap 300B amp paired with lowthers & horn speakers outclassed my $$$ system. Very natural sounding. One thing to remember is to partner with efficient speakers as otherwise you would loose the dynamics & magic. As suggested above by others you need to hear it to know how it sounds like. Dificult to describe in words.
Have heard quite a few mega buck SS & tube systems. Was never quite impressed as i was by the 300B's with horns. Very different presentation.Give it a listen & see if that's the sound you are after.
In addition to the above one of the qualities I enjoy most from SET amps is something called "flow" or "continuity." The music has a more natural and continuous flow to it that you don't notice is not there in most components until you hear it there in a SET amp. This quality makes the music more enjoyable and also easier to relax into the performance. One of the elements that leads to this impression is the decay of notes is more natural in SET (and some tube) amps. I currently have an 845 SET that is simply wonderful.
I only wish these amps were not so hot and heavy!
For ANY tube amp, not just SET, the audio output transformer is the story. All that other stuff, resistors, capacitors, tubes, etc. is just auxillary equipment to power up the transformer.
I only exagerate slightly.
TWL really hit on it, but the biggest issue is the simplicity of the signal path. The less junk in the signal path, the less the signal is altered. That is over simplified, but it explains the design philosophy.
I'm finding this to be a fascinating thread.
My system is comprised of a tube preamp matched to a solid state amp, but I've been very curious about possibly getting a SET for use as a second amp.
I'm just not sure which tube design to go with. Mostly likely I'll try an amp that will deliver the full bloom story, but I'm not sure if it will be a good match for my Blue Circle preamp which already has this slight characteristic.
Tom, your post really got the rusty cogs turning in my head. I blame you! Heheh.
SET can be a mind numbing brain melting experience. It can make everything else seem "wrong" it is so emotionly, vivdly right. Then after not hearing it for a few days sanity returns but you always carry this religious experience in your soul. It can turn your world upside down musically, suddenly the world is flat....don't go there, I can't live with tubes, I can't live with tubes...there, I feel better.
i'm a SS convert to SETs, which on my wilson WP6s should've been a non-starter, but not all SETs are created equally; some can drive real-world loads (which is essential IMO, as those high-sensitivity speakers generally suck IMO), and IMO one should start there; if its a competent design, it can be improved upon (like any amp) w/ proper part substitutions, and then you can go about finding speakers that match well (yes, amp before speakers).
trying to explain why a SET is better than other types of amps is like trying to explain why my wife is better than all those other broads i knew. some things i can specifically define, but the general sentiment is beyond articulation.
i agree w/ above posters: in my SET (Wavac 572), the combo of the superb tango transformers along w/ a capacitor-free signal path makes for a great sound (despite the shortcuts taken in other parts w/in the amp, which can easily be corrected w/ some brains an a soldering iron).
as lew conrad once said, there is no perfect part.
Here is my journey:
I started out with Cary 300B SEI 5-6 years ago, but my speakers were not good match for them. I ended up going to an all Levinson Avalon system for a few years. I was never quite satisfied with that system. It did all the technical things right but I could not enjoy it for various reasons.
Anyway, I ended up switching to a home theater only system.
A year or so ago, I decided to try out SET again in the bedroom ssytem, but first thing I did was to find some efficient speakers. Coincident Triumph UHS were the only speakers that fit in the bedroom and 92-94dB efficent/high-impedance. (My wife would never allow horn speakers in the master bedroom) Then I tried out an ASL Orchid 2A3 integrated. It sounded great but it always runs out of power with rock/symphonic music. 3W/ch is not quite enough. So I switched over to a Dared 845 SET integrated. Rolled a few tubes. Funny, this is the most musically involved system I had in the last 10 years and it did not cost $40k.
My suggestion for SET game is:
1) Find the speakers that you like. Flat impedance curves > 4 Ohms is most important. Efficency will mater if you have a mid to large room.
2) Match the amp output power to speaker accordingly. The power output requirement will determine the type of tubes you can live with.
It has the magic in front and back and left and right. That is whats behind the SET.
Some designs utilize transformers to couple the driver tube to the output tube which eliminates any capacitors in the signal path. With very, very good transformers this is about as close to "straight wire with gain" as one can get it would seem.
Indeed, my Moth Audio Si2A3 is just like that. Input wire to driver tube to output tube to output transformer. Nothing else in the signal path and its very sweet.
C123666, are these the interstage coupled amps you referred to in another thread? Could you guys list a few? I'm very interested. Thanks!
Wavac is one company that uses DC coupling which also eliminates capacitors in the path. You'll have to search; there are not very many at all. Sun Audio has an interstage coupled model, methinks. Border Patrol offers a beautiful interstage coupled four chassis solution for about 10 large (plus shipping, duty, sales tax); their products rarely come up for sale used. There must be a reason.
Regarding loudspeaker matching, 805 and 845 output tube designs are available with 40-50 watts/channel. I recently bought a pair of Ming Da MC805-A monoblocks from Coem Audio in HK for under $1400 shipped and have been most impressed. Their 50 w/channel produces formidable and articulate bass from my 91 dB @ 8 ohm 3 ways with 10 inch woofers. No manual/customer support were potential downsides and you must be competent at opening up the bottom and biasing output tubes with 1000 volts nearby. ASL has a model with outside bias meters, occasionally for sale used in the $1500-$2000 range. The upside is SET sound with preserved dynamics at quite loud volumes out of many full size "conventional" multidriver dynamic loudspeakers. It doesn't hurt that NOS 805 tubes can be had for $60-$100. Surely not the "magic" of transformer-coupled 300B amps, but just as surely the SET "family" sound in a far more affordable/versatile package.