Thought on OTL tube amps

Xonex77, as someone with multiple degrees in electrical engineering and multiple decades of experience designing and managing design of analog and digital circuits for advanced defense electronics, I can assure you that I am well aware that placing an impedance in parallel with some other impedance (of similar type, i.e., not an L in parallel with a C) results in the total impedance being less than either of the two individual impedances, at a given frequency.

The point, though, is that you and Realdeal appear to be envisioning autoformers as behaving as if they were inductors, and they are not inductors. While autoformers (as well as transformers) possess some degree of parasitic (undesired) inductance (and also resistance), they are different animals. And as I indicated, an autoformer can transform voltages and impedances just as a transformer can.

See the section in this Wikipedia writeup on the equivalent circuit of a real-world transformer, and this Wikipedia writeup on autoformers.

Some brief excerpts from the latter writeup:

... The voltage and current ratio of autotransformers can be formulated the same as other two-winding transformers.... As in a two-winding transformer, the ratio of secondary to primary voltages is equal to the ratio of the number of turns of the winding they connect to.... In audio applications, tapped autotransformers are used to adapt speakers to constant-voltage audio distribution systems, and FOR IMPEDANCE MATCHING [emphasis added] such as between a low-impedance microphone and a high-impedance amplifier input.

Surely you don’t deny that a transformer can transform impedances (in proportion to the square of the turns ratio)? Well, an autoformer ("auto-transformer") can also.

Regards,
-- Al

Upon re-reading my previous post, to be more precise it occurs to me that I should re-word the second paragraph as follows:

The point, though, is that you and Realdeal appear to be envisioning (incorrectly) that autoformers behave as if they are inductors, and nothing more. But while autoformers operate based on inductive principles, they behave differently than inductors, assuming that a load is present. Just as the primary winding of a two-winding transformer behaves differently than an inductor, assuming that a load is present. And as I indicated, an autoformer can transform voltages and impedances just as a transformer can.

Regards,
-- Al

I had a pair of Fourier Panthere monoblocks with 8 6C33C Russian military triodes each that powered a pair of Essence Super Gem and later Wilson Sophia speakers through an autoformer.  When things were right, the sound was glorious.  Not the lean, hyper-detailed, flat-soundstage, micro-image sound you hear so often, but big, transparent, harmonically-rich sound that washed over you and left you mesmerized.  The problem was:  you never knew what you would get when you turned these beasts on and watched the lights in the house dim as the amps doubled as space heaters.  Sometimes everything went right and on others bass authority seemed light and a check of the amps indicated one or more blown tube fuses.  Sometimes in the middle of the music something seemed askew and I noticed one channel had popped its little red circuit breaker.  After I took the amp apart and cleaned and tensioned the tube sockets, things seemed more stable, but finally I realized that I wanted to escape the world's problems by listening to music, not add to them with a problematic amp.  Perhaps the newer OTL designs are more stable, such as Atma-Sphere, but with older designs listening often became a project.  Perhaps this is why almost every manufacturer of these designs went out of business.