The original Fairchild 670 limiters used a saturation transformer to provide a harmonic free, stablized supply for the 6386 heaters in the gain reduction stage. Supply stablization is one of several areas that help to maintain a stable signal level at the output stage. Too little heater voltage and the signal drops, reducing incidently the performance of the tube, and too much and the signal increases. Consequently signal level and performance at the o/p of the gain reduction stage are connected to these key areas. Original 670's were supplied with a saturation transformer or ferroresonant transformer an unusual device where the core is so strongly magnetized that any further changes in current have no effect to the magnetic flux, these devices operate in complete core saturation and work well in certain circumstances providing a clean harmonic free supply, however they do have some limitations, they are intolerant of frequency variations and are very inefficient wasting a lot of energy, a simple schematic is shown opposite.