[RodB]

Bob, AGC is Automatic Gain Control. The rf amplifier tubes are specifically designed as variable gain tubes. At the detector tube, the duo diodes generate a negative voltage of a couple volts depending on the strength of the rf signal being received. This signal is also the audio being passed on to the audio amplifier. The negative voltage is fed back to the grids of the rf amplifiers where it adds and subtracts to the gain of a tube depending on the strength of the radio signal. Strong stations reduce the gain and weak stations increase the gain. If the AGC isn't working properly then this feedback circuit could cause the motorboating. In your radio parts 44, 15 and 6 are providing the AGC. Be sure part 15, the 0.05 mfd cap, is the correct value and both resistors are within 10 percent tolerance. No cold solder joints, anywhere.