SET vs OTL

To add to that I have an Audiovalve RKV OTL amp that sounds really wonderful, but 3 watts per channel was never going to be enough so am looking to upgrade soon to a little bit more power, hence the question.

The guy perhaps most equipped to answer questions about OTL is Ralph of Atma-sphere (apologies to you other lurking OTL experts), who sometimes finds the time to post on Audiogon. Until he gets here, I will attempt to not embarrass myself unduly.

SET means Single-Ended Triode. A triode is a three-electrode amplification tube, and a single-ended triode amplifier is one where only a single triode tube acts as the gain stage. This is in contrast to a 'push-pull' amplifier where the signal waveform is 'split' into two parts, and one half of the signal is amplified by one device, and the other half by another device.

OTL stands for "Output-Transformer-Less". This means that while most tube amplifiers are coupled to the speaker by an output transformer (that big hunk of steel which makes the amp so heavy), the OTL amplifier is not burdened by the hunk of steel and the way that it might affect the signal. OTL amps are coupled to the speaker load either by (to my knowledge - and I dearly hope Ralph of Atmasphere can jump in if I miss something here) direct coupling (which prevent DC through circuit topology (and tube choice?)) or capacitive coupling (which does so with large-value capacitors).

Reading between the lines, this means that SET and OTL are not necessarily mutually exclusive by definition. It would be possible to have a SET OTL, however, the Cary 300SE is not one of them.

The Atma-sphere website has a section for white papers and there is some interesting reading to do there. From a quick look, it appears there was a 300B OTL project.

I also know that David Berning also has developed OTL and ZOTL (a variation on the OTL theme) technology and he probably has some interesting info on his website. At one point, I think he made a SET OTL amp for someone (if I remember correctly, it was an Audiogon member).

As to your question:

Is it true that despite its operational inconveniences, a good OTL (eg Tenor Audio) will always sound more "natural" than a good SET (eg a Cary 300SE)?

I don't have a clue. I expect that implementation counts for more than topology, and besides, I do not have enough experience comparing OTLs (which when I have heard them have been wonderful) with 'regular' SETs.

With regard to your follow-on post, several of the extant OTL mfrs offer higher-power OTL-topology amplifiers - many of them a LOT more than 3W. If OTL is what floats your boat, it can be found with enough power to drive somewhat insensitive speakers (as long as their impedance is high enough and the load is relatively flat across the frequency spectrum). In fact, if you wanted 50W, it would likely be a lot cheaper to do this with OTL than SET (because with SET you need to find a single tube which can put out 50W (there are a few, but not many, and the tubes themselves are expensive, and the OPT (output transformer) will generally be really big (and expensive)).