Ultralinear vs. Triode vs. SET

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. ... :-)
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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.