I had the pleasure of using and seeing the schematic of a two stage magnetic amplifier that was used to control the magnetic field of a large electromagnet, during my graduate work in superconductivity at the University of New Orleans. It had a large DC gain and with a resistor for feedback produced a gain of near 1000. The DC input, I presume was from a spinning gauss meter coil in the magnetic field, some few millivolts and the output of the amplifier was some volts, used to control the magnet (3 phase AC input)power supply. As I remember the AC power source for the magnetic amplifier was a few volts at an audio frequency and there were two sets of reactors (maybe two toroids). The output from the first set was fed as input to the second set to produce the large power gain. The intermediate connection did not to my knowledge involve any nonlinear parts such as transistors or diodes and the output was a voltage source proportional to the input voltage similar to a DC op-amp circuit. Unfortunately I did not copy the page and it is likely lost from the University of New Orleans Physics Department. I am trying to figure out how this worked and possibly recreate a crude replica. The final output of my (school's) amplifier was DC. The device was a commercial bought circuit about the size of a book. Supposedly the Germans used amplifiers of this type on their naval guns during WWII. Memory is a drawing of what I remember of the schematic that I saw in college. DCAmp1a is almost at the level I am building at home. Navserva is part of a navy device which is three stages and more sophisticated than I can understand or reproduce. My school's device had a minimum of non-magnetic parts, and, I believe no diodes. If you can find anything on this or even advise on experiments please help.