6. APPARATUS FOR AMPLIFYING AN ELECTRICAL INPUT SIGNAL, SAID APPARATUS COMPRISING, IN COMBINATION; A SATURABLE MAGNETIC CORE; A LOAD WINDING WOUND UPON SAID CORE; A PAIR OF SERIES-CONNECTED RESISTORS, THE END TERMINALS OF SAID SERIES-CONNECTED RESISTOR PAIR CONSTITUTING OUTPUT TERMINALS FOR SAID AMPLIFIER; AN ALTERNATING CURRENT SOURCE; FIRST AND SECOND DIODES WITH THEIR ANODES CONNECTED TOGETHER, THE COMMON POINT BETWEEN SAID ANODES BEING CONNECTED TO THE COMMON POINT BETWEEN SAID RESISTORS; THIRD AND FOURTH DIODES, EACH HAVING ITS ANODE CONNECTED TO THE CATHODE OF ONE OF SAID FIRST AND SECOND DIODES AND ITS CATHODE CONNECTED TO ONE OF SAID END TERMINALS OF SAID SERIES-CONNECTED RESISTOR PAIR, SAID ALTERNATING CURRENT SOURCE BEING CONNECTED BETWEEN THE CATHODE OF ONE OF SAID FIRST AND SECOND DIODES AND ONE END TERMINALS OF SAID LOAD WINDING, THE OTHER END TERMINAL OF SAID LOAD WINDING BEING CONNECTED TO THE CATHODE OF THE OTHER OF SAID FIRST AND SECOND DIODES; AND A CONTROL WINDING WOUND UPON SAID CORE AND ADAPTED TO RECEIVE AN INPUT SIGNAL AND DEVELOP IN SAID CORE FLUX IN AN AMOUNT DEPENDENT UPON THE MAGNITUDE OF SAID INPUT SIGNAL.