Autoformer vs Speaker impedance Curve

Back in 2010 I asked Ralph about that statement in this thread.  Here are excerpts from our discussion:

Atmasphere: Power cords: a 2V drop across a power cord can rob a tube amplifier of as much as 40% of its output power! Cripes! You're trying to say you can't hear that?? [That comment was addressed to someone else].  So this is very measurable and audible as well. On lesser transistor amps, a power cord will be less audible as the drop across the cable is reduced, but a class A transistor amp will easily bring out cable weaknesses.

Almarg: Ralph, could you provide a technical explanation of why that would be so? I don't doubt your statement, but I'm interested in understanding why that would occur.... Re your other points, all of which strike me as excellent, I think that it should be stated that none of those points NECESSARILY mean, to cite an example, that a $2,000 power cord will outperform a $200 power cord in any given system.

Atmasphere: Al, the reason a power cord can have this effect is simple. If there is a 2 volt drop in a power cord, the filaments of the tubes will run cooler and the B+ will be reduced. Since this is a voltage, the result is we get less voltage output out of the amp. Less voltage=less power. Depending on the amp this can be pretty profound. and I have seen it with my own eyes. I do agree though that that does not justify a $2000 power cord, but it **does** justify one that has decent connectors and conductors that will not heat up at all. That has to cost something, probably not $2000 though. One thing about audio is that if there is a phenomena, there is also snake oil for it.

Almarg: Thanks, Ralph. That would also seem to say that the value of the ac line voltage at each listener's location can be a very significant variable in the performance of a tube amp (assuming it does not have regulated filament and B+ supplies). Which in turn emphasizes how easy it can sometimes be for extraneous variables to lead to incorrect sonic assessments.

Atmasphere: No doubt! But it extends to anything that can draw significant power- and bigger transistor amps can! Imagine the peaks just... not... making it.

This taught us a lesson... when we set up an amplifier for test, we test the AC line voltage from the IEC connector. The meter on the variac (a bit of test instrumentation) cannot be trusted. 

A 2 volt drop in a power cord of typical length seems like a lot, of course.  But if the amp draws say 5 amperes a resistance of 0.4 ohms, including the resistance of the connectors and the contacts as well as the combined resistance of the hot and neutral conductors, would do it.  I'd imagine that could happen in some situations.

Regards,
-- Al  

I am enjoying the debate among experts, but am still not understanding how an autoformer works. The last question from the OP was a good one.  I Think including ss adds to the confusion and would prefer explanations stick to tube amplifiers where they are more practical.

How does an autoformer work? Does the speaker cable mate with the windings? How is the signal changed at this point?

@recluse---look at the 8-29 and 8-31 postings by ramtubes (Roger Modjeski of Music Reference) above.

I would like to know how to measure that. 30% is hard to imagine. I test amplifiers all the time with ordinary power cords and have never seen this. This reminds me too much of Bruce Brission's white paper which was a total embarassment to his distributor who promptly dropped his line.

Set up the amp on a variac with an appropriate speaker load and sine wave input. Measure the voltage drop across the power cord with the amp at full power. A 3 1/2 digit DVM is sufficient for this.  Then boost the variac voltage to compensate for the voltage loss. You may have to do this several times because as you increase the output power of the amp, the load on the variac increases too.

This came to our attention with our MA-1 amplifier back about 1998 or so. Bascomb King had measured the amp as only 100 watts where we get 140. I was wondering where the measurement error was and found it easily enough. We spec the amp at 120 volts (which is what Bascomb claimed he had) since these days that's more common than 117V. We normally set the incoming voltage by measuring it at the IEC connection, since there is typically a voltage sag in our variac. We have IEC connections on all of our products since we know that audiophiles will want to use or audition different power cords from what we supply; the idea that power cords make an audible difference is nothing new- its been around for decades (this is also why our MA-2 employs *two* power cords per chassis, to reduce the effect of the power cord). So the power cord isn't regarded as part of the amplifier in our test. Bascomb King didn't do it that way- thus the difference. By our measurements he was seeing about 117 volts at the AC input of the amp.

So it follows that the heavier the load on the power cord, the more audible the differences between power cords will become. This ignores the higher frequency aspects of any power cord's current delivery of course, but cynics of the fact that power cords can make a difference are usually silenced by the simple revelation that they never bothered to measure the power cord's voltage drop! It is easier to measure the effects the cord has on the product with which it is being used.
For example on our MP-1 preamp, which has heavily regulated power supplies, even though we can measure differences between power cords, we can't measure any differences in the performance of the preamp, nor can we hear any differences. So we can also conclude that regulation in the circuit plays a role here as well.

Ralph, would I be correct in thinking that the low B+ voltages that must be used for the 6AS7G power tubes in your amps, compared to the much higher B+ that is supplied to the tube types used in most non-OTL amps, would tend to increase the degree to which output power capability is sensitive to AC supply voltage variations?

Thanks.  Best regards,
-- Al