MAC Autoformers?

@ramtubes,  I was in contact with Anthony today.  I will ship off 2 quads of KT-150s next week that I bought from a 3rd party tube vendor.  Hopefully, Roger can get some good matched pairs out of these tubes.  Damn expensive!!  Otherwise, it's back to ARC.

Anthony advised that I should "light load" the amp off the 4 ohm tap, which Atkinson measured as having an output impedance of .55 ohms.  Presumably, FR should be flatter and DF higher. The impedance in the bass saddle is a smidge south of 4 ohms. I estimate the DF to be approximately 7 in the saddle.  Hopefully, that will be enough to get the bass kick I like.

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.  In fact, it will make for less strain on the tubes and lower distortion.  The cost, … about 3db of power.      

I can live with that.  My speakers have a sensitivity rating of 92 db.  They play plenty loud.

Thanks  Roger and Anthony

To clarify a bit I made reference to Class A3 operation as implemented by Jack Elliano and used in the design of his A3-500 300B/6A3 amplifier. As Jack does not design amplifiers with multiple taps, this Class A3 operation achieves similar results as light loading. Jack achieves this with a combination of the transformer winding, as well as changes in the plate dissipation, load requirement, and grid drive of the circuit.

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.