What's your OTL tube amp experience and suggestion?


Are OTL amps in general much less reliable due to their nature, or due to the implementation quality, or both?

Perhaps this has been discussed a zillion times in the past.   Perhaps, however, makers have now improve on past experience?  So it could be worth re-visiting.

My past encounters with OTL tube amps are among the most negative: Wonderful (but never great) sound during the brief period that they work.  Otherwise, major fire hazard.  Overheating, red hot plates, sparks, consistently toasted fuses, burning smell, you name it.  My past OTL amps are like crying babies on an changing table - don't you ever walk away from an arm's distance.   The used market seem to reflect such as well --- way more 'as-is for parts' or 'totally refurbished' units than reliable 'used' units that rarely need service.

Beg your pardon if it's just my poor judgement that keep getting the lemons.   What's your experience, and tips to keep OTL amps up and running happily?


bsimpson
@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).
Who made the amp?Do you know the tube topology?Looks like homebrew from inexperienced TUBE MASTER.
@atmasphere

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".

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.

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. http://davidberning.com/products/qz
It is accurate to point out that the Berning amplifiers utilize a "nontradional" output transformer that operates at very high radio frequency levels. This is a different type of transformer as compared to the common or traditional output transformer. This is different from the Atma-Sphere which does not utilize an output transformer at all. Both companies have earned reputations for excellent sound quality. Obviously each has their own unique character.
Charles 
@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.