What is the magic behind SET

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.