OTL Tube Amps-- are they superior to everything?

Saxo, as a multiple Berning owner, I can only agree with what tubegroover and TWL said regarding its abilities and advantages.
However, I'm not quite sure what you mean by the amp 'appearing and disappearing'. If you mean availability of the amp to buy used here on Audiogon, they certainly are less commonly available used than more mainstream products, but that does not mean that there is something 'wrong' with them. There was one available a week or so ago here; it isn't there now, but that's probably because when they come up, they sell fairly quickly. Actually, I was thinking of buying it myself to compare a 'solo' 270 to my monoblocked ones, but common sense reared its ugly head. Both of my Bernings were bought through ads I first saw here on Audiogon, in fact.

OTL, means output transformer-less device, does not mean "nothing between the plate and speaker terminal", then it would be NPST or something like that, some OTL designs may use caps or resistors in between the plate and terminal, in fact some highly regarded units may do so.

The ZH-270 is an "OTL" amp in every respect of other "OTL" amps, just that it does it in a very high technology way, or different than anyone else.

Ok Allan, but it seems we have had this discussion before. But I ask you this, if an inpedance converter is not a transformer of sorts, what exactly is it? Also true that some OTL's have a cap or resistor between plate and terminal but none have an impedance convertor or transformer, agreed? ;)

Tubegroover, according to David's discussions, the ZOTL circuit is a transformer emulator circuit. It provides the impedance matching without the turns-ratio limitations and saturation limitations of traditional output transfomers. The circuit emulates the "perfect transformer" and doesn't suffer from the sonic limitations of these transformers. Therefore, the audio signal is not subjected to the performance limiting factors associated with output transformers. But the impedance matching is done in a much more efficient way than is possible with a transformer, without degrading signal integrity. This allows virtually vertical rise times in square wave response well beyond the audio range, and this is exhibited by the incredible speed of this amplifier. No audio output transformer can deliver this type of performance.

As far as comparing the ZOTL method to traditional OTL methods of impedance matching, the traditional OTL circuits use multiple sets of tubes in parallel to reduce the output impedance of the amp to a level which could be practical for driving a speaker. Due to the fact that they don't get the output impedance very low, they do better into higher impedance speaker loads. The ZOTL circuit gets the output impedance down to 1.8 ohms, and provides better damping factor into normal speaker impedances. And it does so with fewer tubes in the signal path.

To take this one step further, the Single-Ended ZOTL amps have only 2 tubes per channel(1 driver and 1 output triode). No traditional OTL can do that. The coherency provided by using one output triode in OTL, instead of a multiplicity of output tubes, is something that has never been heard, outside of the Berning circuit amps. So this goes beyond just impedance matching. There has never been a true single-ended triode OTL before this. That, in itself, is a major breakthrough, and is worthy of accolades for allowing closer insight into the music.

Tubegroover, Yes, the RF conversion transformer, is a transformer but Dude, again, like before, it's not in the signal path. So the signal is that of a direct plate to speaker output but this is achieved in the most absolutely ingenious and mind boggling way.
If you really study the design, you'll see that in fact it's superior to a regular OTL.
So maybe the best way to describe the ZH-270 is to say, it's like no other OTL, or there is no other OTL like it :)