Seconded.
The post in Audiogon Forums gets into some things that are not recognized terms in the field of electronics. The term "back impedance" is new to me. From reading this post, and its not easy,
he is talking about how the speaker impedance is reflected back through the transformer. The author states the following:
"The explanations I read on some of the older Forum posts seems to go like this. One of the key functions of output transformers is to match the impedance load of the speakers to the optimal operating range of a tube amp's output tubes. So, in a crazy way that I still don't understand yet, an output tranny works two ways -- (1) it steps-down the output tubes' plate voltage and high impedance to match the speakers, and at the same time (2) it steps up the impedance loading presented to the output tubes through the interaction of the tranny's primary and secondary windings (or, back impedance). In the latter case, the impedance step-up relates to the speaker's impedance presented to the tranny's secondary windings."
Points 1 and 2 are saying the same thing. The misconception is that although output tubes may have sweet spot where distortion nulls at some particular power it is a very small spot which cannot be held by a speaker whose impedance varies. It's an easy mistake to make if one interprets the tube manual incorrectly. David Manley made this mistake in his letters to me in Stereophile concerning my RM-9. If one reads the design portion of the RCA manual and understands load lines, one will see that the output transformer primary impedance (the load the tubes sees) is not determined by the particular tube but the voltage, current and power that is expected from the circuit.
Now somethings I discovered when designing the RM-10 is that "light loading" always reduces distortion and improves damping. This has also been confirmed by my listening and reported by many of you in your listening experiments. The OP goes on:
"So, if I got this halfway correct, the inference that one should always hook his/her speakers up to the 4 ohm tap just because it generally presents the lowest output impedance to the speakers is flawed. The fallacy is that blindly using the 4 ohm tap may not result in an optimal impedance match for the output tubes. "
