What's your OTL tube amp experience and suggestion?

Where are you seeing semi-conductors?  In the tube section of the Alieno, or in its power supply?

I don't. I just googled it and started reading. The top hits say pretty much the same thing.

Alieno swears up and down that semi-conductors are not doing any heavy lifting, and that the amp is not a hybrid amp, and that only tubes are providing both the voltage and the current amplification. Somehow the big power supply allows the KT-150 allegedly to generate the current for the 300B.

What is your theory about how this might be possible?

If what you say is true, no need for the semiconductors then, right? But apparently they are needed for something. Think about it.

A single 300b is good for about 7 watts. That's it. When you look at those amps, you don't see a lot of 300bs, so the math is not hard to work out- the 300b isn't making the power. Something else it, and its not the KT150...

Is anyone familiar with the Alieno Ltd. 250 OTL (from Italy)?

Its not an OTL, no 300b can make 250 watts. The power is being made by semiconductors.

@c

The heat of an OTL is like it is with any other amp- the heat is based on the class of service rather than the number of tubes. The exception might be if you run an amplifier using 6C33s (OTL or not), which run unusually hot!

If our amps are on Standby, after a day of that you can easily grab a power tube and hang on without burns. But start the amp from stone cold take it out of Standby to full On and after a minute of operation, its too hot to touch!

If you had a solid state amp of 225 watts that run class A, you would find it making a very similar amount of heat!

@rusty_jefferson 

Just to be clear, I'm not Trolling you, or your company, just answering the question in the thread title, "What's your OTL tube amp experience and suggestion".

No worries- I didn't think you were. But most people don't realize the ZOTL amplifiers have not only an output transformer, but also semiconductors in the output section. It is quite unique!

By stating you made the first reliable OTLs does indicate that some or all of your predecessors were in fact, not reliable. And by safe, I didn't just mean not catching on fire, but rather safe and reliable in our systems. The Berning ZOTL amps have no chance of passing DC, no popping noises, and extremely long tube life, again, on the order of many years to decades of daily use. If your amplifiers are capable of similar safety and reliability, I apologize for not knowing so.

Sooner or later, if a tube fails the Bering will make a popping sound. But it unlikely to threaten a speaker.

You might think about this for a moment: If our amps were damaging speakers, we would not be able to stay in business for very long, instead of the 41 years we've been around. I'd be quite nervous if I thought for a second that one of our amps could threaten my speakers- the drivers in them are quite expensive. That sort of thing has a way of coming home to roost!

We protect the speaker with a simple mechanism which is highly effective. But that is not the only reason our amps are safe- we are the first OTL manufacturer to make an amplifier wherein the power tubes are in fact properly under control (which is really important; think about what that means for distortion too) and also, the first OTL that is unconditionally stable (which is to say: stable regardless of input condition or output load).

We too get long tube life (we warrant all the tubes in our products for a year) often on the order of many years to decades... so apology accepted- no hard feelings  :)

And if some think ZOTLs aren't really OTL amplifiers because they don't meet some engineering criteria that fits their needs, well, I don't know what to say about that. I don't think anybody has ever sued Berning for false advertising. Here's a nice discussion from the Berning website, including pros and cons of different OTL implementations, for those interested.

When the amp first appeared I read the patent, as it is patented. This is from the abstract of the patent:

"A linear audio amplifier includes a push-pull pair of vacuum tubes operating in a linear amplification mode coupled through a pair of dc-dc switching power converters to an external load impedance. Each power converter includes a **transformer** with one or more secondary windings that drive rectifier circuits, and the resultant dc voltage sources are loaded by their respective tubes."  (emphasis added)

Although connected in a quite unconventional manner (hence the patent), it is the transformers that effect the transformation from the high impedance of the power tube to the low impedance of the speaker, which otherwise is a rather conventional way of describing the operation of an output transformer.

In a letter to the editor of Positive Feedback back in 1997, David mentioned that he was originally going to call the amplifier a Zero Hysteresis amplifier but he found that to be difficult to explain. When he told people it was similar to an OTL then the glazed expression went away. It didn't help that Harvey Rosenberg suggested the ZOTL acronym, which Harvey told me specifically didn't mean anything as he knew full well the amp was not an OTL.

@rusty_jefferson 

Our (Atma-Sphere) claim to fame is we made the first reliable OTLs. That was in 1976. I don't know of any OTLs that were safety issues, including early designs. I've heard this said before, but as far as I know its some sort of myth.

FWIW, the Berning amplifier is not an OTL. It is a uniquely different design and does employ an output transformer (unlike an OTL; 'OTL' stands for Output TransformerLess).

Yes- apparently we're on the same page here, if you'll pardon the expression :)

@mikelavigne , I remember when that happened, but even back then the story I heard was they were only doing the transistor amp. In all that time, I've yet to hear about a Tenor OTL that was made after the 1st company went under. I suspect what you are seeing for sale are older amps that have been refurbished.

@mikelavigne ,

back to the Tenor OTL’s; eventually Tenor went bankrupt; but the good news is that another group bought the assets and raised it from the dead, and the 75 watt Tenor OTL’s got their design cleaned up and did become reliable......and i would strongly recommend a set if you can find one (and make sure it was built after 2005 or upgraded.

This statement seems to be incorrect. To the best of my knowledge they only make solid state amps; I'm sure we would have heard about it if they still made OTLs.

FWIW the MA-2 Mk2.2 dates from about 17 years ago...  and in the intervening time we have managed to make some progress :)  The current incarnation of the MA-2 is Mk3.3. People have described the improvement as a 'transformation'.