What is the magic behind SET

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.

Eric

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.