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?

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.
I spent about four hours using Google translate to translate from Italian to English dozens of posts about the Alieno LTD 250 amplifier on an Italian audio forum. The translated posts did not tell me anything. 

But then I found this:


. . . a large excerpt of which Google translated to English as follows:

The LTD implementation has nothing to do with the traditional circuits, let alone with those defined hybrids that require the use, in addition to the tubes, also of the semiconductors in the amplification line.

The hybrid amplifiers, in fact, try to reconcile the softness of the tube with the imperiousness of the transistor, entrusting the first to the amplification in voltage and the second to the current one. In doing so, however, the "magic" of the tube sound is only partially expressed, since only the modulation of the voltage supplied to the loudspeaker has a valve imprimatur, while all the current supply remains the typical one of the transistor (of course with all its pros and cons).

The new technology LTD is inspired, instead, to a different design philosophy: the valve section, Single Ended in pure class A, provides the load as much voltage as the current, the latter supported by a particular process of power supply semiconductors.

Short technical study

We report in the attached figure an extremely simplified circuit that helps to understand how the LTD system works. Let's start by examining the right half of the circuit, the most innovative one. The output current from the final pipe drives the load through a very low value resistance (in the figure indicated with "R"). The voltage drop at the ends of this resistance is used to control the modulation of the output current at the power supply section (which therefore works as a current and not voltage generator). The output of the feed section, as shown in the figure, is connected to the load in parallel with the output of the final valve. By operating in this way, the tube drives the load both in tension (directly) and in current through the sophisticated feeding system indicated above.

On the left side of the circuit you can see the extreme simplicity of the signal path, which crosses only two stages, the one equipped with the driver valves (ECC82), configured in asymmetric SRPP, and the one using the 300B finals (one per channel, in Single Ended Class A configuration with cathodic output). The coupling between the stages is direct, without any capacitor. Finally, the KT150 (high power and latest generation valve), used as an active current generator, feeds the 300B in the most linear way possible, allowing it to express itself in the best possible way.

Last technical note: the mass (indicated with GND on the extreme right of the circuit in the figure) has the particularity of being floating: it preserves the safety of the speakers as it inhibits the transit of the signal in the event of any functional anomaly.


I interpret the translation to mean that the load (i.e., the loudspeaker) is seeing current delivered to it directly by the transistor power supply.

I think I would characterise this design as a "semi-hybrid" design in that the voltage side is tube driven with a tube output, and the current side is tube driven but with a solid-state output. I think this topology is somewhere between a hybrid amp and an all-tube amp.  I would not call it an OTL.

My understanding is that the speakers are connected directly somehow to the voltage output of the 300B, while the current output of the 300B is fed to a solid-state amplifier.

The solid-state stage amplifies the current output from the 300B. The speakers are connected directly somehow to the current output of the solid-state amplifier.

Somehow the voltage output of the 300B and the current output of the solid-state stage are summed, and the summed effort is connected to the speaker terminals.

After scanning all of the prior posts all i can say is my experience with an original Julius Futterman designed H3 amp purchased in 1972  using  6KG6A tubes has been going strong powering my KLH model Nines. I have had the output caps doubled by Julius himself in 1981 and  actually replaced in 2021 along with other aged capacitors by Mark Weiss of Amplifier experts in Ct. Never had problems with this amp driving  KLH Model Nine, Martin-Logans CLSIIZ, Soundlab A-3, KLH model 5, Janszen ZlA2.1. I am using using it currently driving a refurbished pair by David Janszen of my orig Model Nines purchased 1971. I still find this extremely lifelike in musical reproduction of music no matter the style played.