Hi DFrigovt:
OTL
Output TransformerLess
They name it for what it does not have! Sounds stupid but the main advantage of an OTL is that it does not have an output transformer.
A transformer is used in most tube amps because there are major problems matching tubes to speakers. A tube is a high voltage, high impedance thing (thousands of ohms) and a speaker needs current and has a nominal impedance of 8 ohms or so. These are real problems. You hear folks talk about "power tubes" whcih usually means current. But even many "big tubes" do under .5 amp or so. The output circuit and the transformer creates the power. A transformer steps down voltage and steps up current and matches the amp's and speaker's impedances. Thus it solves some problems. You can see from Tubegroover's answer re speaker selection that the OTL, while gettin rid of hte tranny, has not solved all these issues either.
But the tranny creates problems too. John Broskie over at Tube Cad Journal says "The best transformer is not as good as the worse tube." Basically transformers inevitable cause distortion for a number of reasons. I'd be happy to outline the basics but I think I'd bore you to death.
OTL's tend to be expensive as Mark notes. Bruce Rozenblit's are not to bad though. They tend to be made in small shops so the economy of scale isn't to good and by people who are quite smart (Berning) and believe they deserve to make a decent living.
Sincerely
I remain,
