Thought on OTL tube amps

The Zero autotransformer will not make  amplifier output impedance and the speaker input impedance higher as  Paul Speltz claim,It only decreases impedance on both input and output but it increase output  power due to less impedance and more current.There's a lot of Paul explanation about the Zero  are wrong.

Realdeal, as I indicated in my response to the very similar comment that was made by Xonex77, that is simply not correct. A suitably designed autoformer can transform voltages and impedances no differently than a two-winding transformer can. Although there are obviously other differences between the capabilities of the two kinds of devices, most notably the obvious fact that an autoformer can’t provide electrical isolation.

You (and Xonex77) may want to read up on autoformers a bit more. You might also want to reflect upon the various McIntosh solid state amplifiers which use autoformers at their outputs. Which by presenting the output stages with the same load impedance when a 2 ohm speaker is connected to the 2 ohm tap as when a 4 ohm speaker is connected to the 4 ohm tap as when an 8 ohm speaker is connected to the 8 ohm tap, enable those amplifiers to have identical power ratings into 2, 4, and 8 ohms.

Regards,
-- Al

Please review your your electrical circuits about series /parallel of resistance,inductance and capacitance,I don't like to trash anybody but there is no point in  continuing  the discussion if the  theory is not fully  understood.

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