By using the right number of turns the toroids could be run very near to magnetic saturation. Applying additonal dc field would push them into saturation, resulting in greater current on one ac voltage peak. By using two in opposition, you get a dc in, ac out amplfier. Totally solid state, just two toroids. Of course the ac out was a weird shape, but no matter, it was sufficient to drive servo motors. The motors had an ac field and the variable ac input from the magamp, so reversal of the dc into the magamp would produce reverse rotation of the motor. Much of the Vulcan and Trident autopilots were constructed of magamps, dead reliable and free from parameter drift.
The magamps used on the Vulcan were about three inches cube. The case was plastic with connection tags attached, and the cores were suspended inside in foam rubber. Occasionally there were production faults and one of my apprentice tasks was to diagnose what had shorted and where, so I got adept at digging into foam rubber without severing the very fine wires.
Using four toroids and very few electronic components made a dc in dc out magamp. The Trident magamps consisted of four toroids less than two inches diameter potted into an aluminium 'bathtub' with a small tagstrip on top to hold the electronic bits.
Now a lost art, but then a remarkably effective way to do the job.