MAC Autoformers?

In case you are thinking I made this stuff up, this is really all about history; take a look at this Google search, in this case one for a Fisher A-55:
https://www.google.com/search?client=ubuntu&channel=fs&q=fisher+A-55&ie=utf-8&oe=utf...

One of the first hits is an image from YouTube, showing the damping control on the amplifier, which is a dual-gang pot. If you look closely, it is labeled "constant voltage" with the control all the way down, "constant power" with the control at noon, and "constant current" with the control all the way up. In this case, voltage and current feedback are balanced against each other in the noon position.

I dont think you are making it up Ralph. Do look more closely at the video. 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

 http://audiophool.com/Schem_A/Fisher_100_schem.gif

One can clearly see the the switch that grounds out the current sense resistors and returns the amp to conventional voltage feedback. The damping spec on this amp is 26, quite good for an amplifier of this vintage. It is interesing to me that although the Z matic control was of interest they made the click to entirely remove it and its losses. 

@bifwynne
Roger has one of the best tube matching services in the country.

I suspect that one reason you have issues with the taps is that the ARC amp really needs a few more db of feedback; 15 db is slightly on the low side to get right.

Thanks for the compliment. I did start the whole computer matching program and feel the only correct way to do it is at fixed plate current which I can enter in the test program.

Whatever feedback they use is not of importance, its the resulting output impedance that matters. For instance, a triode output tube has inherently low output impedance and provides a damping factor typically of 4-5 without feedback. This is childs play to figure out. Its just the output transformer primary impedance divided by the plate resistance. For a 300B tube that is about 3500/700 ohms= 5.

For a pentode output tube its much higher for a 6550 about 3000/27,000 = 0.11 ouch, less than one. Now we need a lot of feedback to get that down. Even triode connected it is only 0.22. Thus not a good triode comparred to the 300B.

This is why SET ampifiers can be made without feedback. These tubes were created to do this job. The 45, 2A3, PX4, PX25 all have similar characteristics as does the 6EM7 which I use because it is very close to a 2A3.

There are three classes of output tubes. Low impedance triodes as mentioned above, Pentodes (EL34, EL84) and Beam Power tubes (6550, 6L6, and all KT series). The pentodes and beam tubes all have very high plate resistance, so high that we don’t even try to match it.

When I was a young lad the idea of measuring feedback was a total mystery until I found out how easy it is to measure. Heres how you do it.

Simply connect the proper load to the amplifier, set the output to a very low voltage, remove the feedback and see how many dB the output rises. ARC looks at another way, which I appreciate and use in my thinking. If you think of an amplifier as a power supply then you can easily measure its regulation, as power supply specs do. It can be noted either in percent or dB. This is measured by noting the output voltage with the load connected and then with no load. We want to see the smallest change, as does the speaker.

This is a great way to demonstrate what is going on with your speaker. If you pull the load on an amplifier with a damping factor of 1 the output will go up exactly 6dB. It will do about the same on your speaker when it goes above 20 ohms!

@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