What is the best OTL out there?

I build and designed these types of OTL and they sound excellent.using KT120,KT88,VVT52b,300b Maybe you are just concentrated on your OTL.

Can you indicate the impedance of the speakers you use with these OTLs? These tubes were designed to work into impedances that are vastly higher than the 4 to 16 ohm impedances of most modern speakers.

For example, without an output transformer it seems impossible that a 300B, which in most and perhaps all versions can handle a plate current of at most about 0.1 amperes, could supply anything close to a reasonable amount of current (and hence a reasonable amount of power) into an 8 ohm speaker. Even if several such tubes were used in parallel.

On the other hand, if you were using something like the horn-type antique radio speakers that were made in the early 1920s, many of which had impedances of 1000 or 2000 ohms or thereabouts, I can see that such a design might work.

Regards,
-- Al

Al,
Intelligent and logical inquiry as usual. Xonex77 could you share more detail about your very unique OTL amplifiers using these unusual (for this application )tubes. This is quite interesting. David Berning makes a 300b SET /OTL amplifier  (Siegfried) to my understanding.
Charles,

Thank you, Charles.  While the Siegfried and certain other Berning amps are described by Mr. Berning as OTLs, and are considered by many to be such, and by all accounts I have seen those are brilliant designs and excellent performers, in between their output tube(s) and the output of the amp they employ a transformer operating at RF frequencies, and also a number of solid state switching devices.  Therefore, despite their description and despite the fact that they are considered to be OTLs by many, they are not truly OTLs for at least two reasons:

1)While the output transformer is not an AF (audio frequency) transformer, as it is in most tube amps, an RF (radio frequency) transformer is still a transformer.  So the design is not Output TransformerLess.

2)The use of solid state switching devices in the output circuit, which are "after" the output tube(s) in the signal path, arguably makes the use of the term OTL meaningless, since aside from a few McIntosh designs nearly all amps having solid state output stages do not have output transformers either.

I know from past discussions that Ralph/Atmasphere would strongly agree with what I have said.

The patent on Mr. Berning's "Output Transformerless Amplifier Impedance Matching Apparatus" can be found here (although it addresses a push-pull version rather than the SET version).   In the first figure note all of the "stuff" between the output tube and the "load" (i.e., the speaker), including a transformer and numerous solid state devices.  And note in the introductory text:

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....  The effective turns ratio between primary and secondary windings of these converter transformers determine the voltage/current step-up/step-down relationships between the tubes and the external load impedance. 

The last sentence is identically applicable to any conventionally designed tube amp that has an output transformer.  (Although in both cases it is the voltage that is stepped down and the current that is stepped up, rather than the opposite which is what the wording of the sentence might seem to suggest).

Best regards,
-- Al
   

Hi Al,
Thanks very much for your illuminating comments.  Well it sure seems that the Berning is an OTL amplif only in the sense that it lacks the "traditional "output transformer. Given information you provided calling it an OTL design is a generous stretch. I can see why Ralph would express reservations. 
Charles,