Thought on OTL tube amps

"Blow a tube with your OTL and you could fry your speakers. Great sound, but some risk if you listen loud, or run tubes till they drop "

Properly designed OTL amp represent no more danger to speakers then properly designed regular tube or transistor amps.

For example, in many tube amp designs, output tube is connected to resistor which work as a "fuse of last resort". if output tube blows then it takes out this resistor.

I never heard that OTL amps by Joule-Electra and Athmasphere produce any troubles. I suspect that with proper maintaince these amps can work half century or longer - at least I see not a single reason why not?

Jallen's comment above reflects a popular myth.

OTLs can be as reliable and safe as any other amplifier. Its all in the design. The only OTL design out there that really did have issues is the one that has generated a lot of the mythology around OTLs years ago (and in the case of that particular design, generally known as a Futterman, those stories are often true). The three myths thus generated were/are speaker safety, amplifier reliability and sensitivity to load. I'm only addressing the speaker safety issue here but will cover the others if anyone is interested.

Speaker safety is a design issue and nothing else with OTLs, the same as it is with a transistor amplifier (the concern is what happens if a tube fails/shorts).

A common myth is that OTLs run the same high voltages that regular tube amps do. This is false- if you think about it, the fact that the amp has no output transformer means that the tubes have to generate voltages and currents that are normal for loudspeakers, to do this requires a tube-type that runs at much lower voltages and higher currents. For example in our amps the B+ is only about 140Volts.

All it takes is a little testing and experimentation to find the limits of any OTL design, so its easy to set up the amplifier with a scheme to protect the speaker in the event of a tube failure. Otherwise the company goes out of business. I hope this is obvious for what it is: common sense! After over 33 years of business, its safe to say that we have built more OTLs than any other manufacturer so I think we can agree that if there was a problem it would have manifested by now :)

I have also owned and auditioned a variety of amps. Solid state; Classe, Krell, Gamut, Plinius, Threshold... Hy-bred; Lamm... Tube; Jadis, Lectron, Conrad Johnson, Convergent Audio... OTL; Atma-Sphere, Joule. I have found, as with nearly every piece of audio, gains and concessions in each approach. Presently, I find the Joule amp to be fantastically musical, yet capable of reaching the frequency extremes. Bass nearly as deep as Classe and Krell with greater body and density, shimmering highs, plus a near CAT midrange (not quite as lush as the Jadis amps). These amps, both the A-S Ma-2's and Joule Rite of Passage, have adequate power to drive the power hungry Avalon Eidelons to a very high listening level, which I require.

I will respectfully disagree with Jallen regarding damage to speakers from blown out tubes. With both the A-S and Joule amps the internal protections have arrested any harm from reaching the drivers on my speakers. I too witnessed removal of an output tube from an MA-2. Not to be done on the Joule, though. The Rites of Passage require all output tubes for proper function.

I have much enjoyed the few solid state amps, especially with tube preamp, but ultimately found the midrange less fullfilling. The Jadis along with the other tube amps left me wishing for more slam and top extension, although they likely would have benefitted from additional tweaking.

The OTLs have come closest to achieving what I demand from the particular system I now own. Do keep in my mind, they run very, very hot and devour electricity. Certainly more than any other amp I have owned. As is typical, the associated components, listening space, and your tastes will best dictate whether these amps prove satisfying.

FWIW,I've owned an Atmasphere S30 for 10 years almost since it's introduction currently updated to the latest 3.1 configuration.
Over that time there have been no significant mechanical/reliability issues. IMHO, these "OTL concerns" date back to the Futterman amps and their clones. Unfortunatly, modern day OTL manufactures are unfairly lumped in with them.
AFC, I have not heard the ZU Essense but on paper it's a match made in heaven with the S30. With the Zu's minimal crossover there would be very little between the output tubes and the Z4 driver except the speaker wire.
Have fun.