MAC Autoformers?

The Class A3 amp sounded good too.

The pot is not dual but single with a switch attached, thus the click into the high damping condition which cuts out all the Zmatic circuitry resistor losses. It is indeed a combination of voltage and current feedback as seen in this schematic

Yes- any way that the current and voltage feedback is balanced is how you arrive at constant power. Other amps I've seen use dual controls.

Whatever feedback they use is not of importance, its the resulting output impedance that matters.

I agree that output impedance is important, but if you wind up using too little feedback to get there, it can be detrimental to the sound thru the process of bifurcation. There's a range of sorts- 8-15 db or so is where this sort of thing can occur. Here's a nice article by Nelson Pass:
https://www.passlabs.com/press/audio-distortion-and-feedback

I agree that output impedance is important, but if you wind up using too little feedback to get there, it can be detrimental to the sound thru the process of bifurcation.

What is bifurcation in feedback?

There's a range of sorts- 8-15 db or so is where this sort of thing can occur. Here's a nice article by Nelson Pass:
https://www.passlabs.com/press/audio-distortion-and-feedback

I read the Nelson's article, Figure 11 doesnt make sense to me as there is no reduction in the lower  harmonics with added feedback. I wrote Nelson to ask about it. 

I would not make any generalization about the proper amount of feedback. I built a Futterman to see what was going on. Sometimes that's the best way to understand something. In his circuit 40-60 dB of feedback is easily applied. Unlike most amplifiers the open loop bandwidth is so wide that the feedback is not a problem. I don't think we can dispute that the Futterman amplifiers are well loved for their sound when used in their power range. The OTL I did for Counterpoint is a Futterman style also. 

Also, Anthony echoed that even if the 4 ohm taps presents an impedance to the output tubes off the primary coil of the output tranny that is higher than the rated 3000 ohms output impedance of the tubes, no harm will occur.  

Lets get clear about matching plate impedance to tranformer impedance. We don't do that in amplifier design. The primary impedance reqired is determined by the voltage and current of the output tubes. For instance a EL84 amp with a 385V B+ uses a 8,000 ohm transformer while my EL84 RM-10 has a 13,500 ohm primary because it runs at higher voltage and lower current. 

While the plate impedance will provide some damping this is not a large effect on the damping when the loop feedback is applied. 

I have doubts about the plate impedance being 3,000 ohms in pentode. It should be much higher and I will measure it at my first opportunity. In triode it looks like 1 to 2 Kohms.

@ramtubes,

Roger, take a look at the New Sensor specs for the KT-150.  I believe it says the plate resistance is 3000 ohms.

https://www.newsensor.com/pdf/tungsol/kt150-tungsol.pdf

I thought you said before that if the primary impedance presented to the output tubes off the output tranny is less than the plate impedance, it can cause distortion and shorten tube life.  Ergo, use the 4 ohm tap. You called it light loading.  

So if the speaker impedance is higher than the nominal tab impedance, resulting in a primary impedance presented to the output tubes that is higher than the plate impedance, it will result is less distortion and longer tube life. Ergo, you said to use the 4 ohm tap.

Of course, the 4 ohm tap has lower output impedance and therefore a higher DF, which is a good thing. 

What did I not understand??

BIF

BIF,

I thought you said before that if the primary impedance presented to the output tubes off the output tranny is less than the plate impedance, it can cause distortion and shorten tube life. Ergo, use the 4 ohm tap. You called it light loading.

What I have been saying is that the primary impedance chosen has nothing to do with the plate impedance of the Pentode. Ideal plate resistance of a pentode is very high as demonstrated by the flatness of the plate curves above the knee. If the plate impedance of a KT150 were 3000 ohms it would produce something close to a 45 degree line on the curves. I just think the 3000 ohms is a mistake thought is it also shown on the KT-120 data sheet, which is curious it would be the same for both tubes.. Modern spec sheets are not done with the care of the good old days and there are few out there to check them.

RCA, GE, Tungsol and Sylvania all had identical plate curves or at least very close and such a mistake would have been found right away. The printed specs for most tubes are IDENTICAL from one maker to the other. There is no other maker for the KT 120 or 150 to monitor the situation.

Thus is the current state of tubes.

Here is a much better data sheet, made not by the manufacturer but someone else..

http://www.tubeampdoctor.com/images/File/data%20sheet%20KT150%20Tung-Sol.pdf

On the first page are the pentode curves and the plate resistance is the slope of the line above the knee. The top curve has a slope of about 20 ma/200 volts = 10K ohms, the lower curves, where the amplifier typically operates is flatter making the impedance even higher.

Also note on the last page the the the load impedance is given as 3,000 ohms. So that is likely the source of the mistake in the Tungsol data sheet, which was made by the Russians I would think. Perhaps something got lost in the translation.

The simplest way to explain light loading is to look at what heavy loading does. If the load is two heavy (4 ohms on the 8 ohm tap) the tube will have excessive current and voltage across it (ie it wont get to the knee but be to the right of it). That product of voltage and current does two things. It reduces the output of the amplifier and it heats the tube excessively.