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It is shown that the regulation of the radio-frequency exciter of a grid-modulated amplifier can be greatly improved by connecting across the exciter a diode biased so that at the peak of the modulation cycle the diode is just beginning to draw current. The exciter can then be designed on the basis of the peak exciting power required, and without regard to regulation. The method is also extended to include compensation for the flattening of the positive peaks of the audio-frequency modulating voltage. This is done by using a triode limiting tube that is so arranged that grid current in the modulating stage causes the load on the exciter to be reduced. A method of applying feed back to an isolated linear amplifier stage (or stages) is described. This consists in rectifying samples of the input and output radio-frequency voltages, balancing the resulting audio frequencies in the rectifier outputs against each other, and modulating the difference that results upon the system in such a way as to tend to correct for the distortion causing the unbalance. This system can be applied to linear amplifiers that are added to a transmitter subsequent to installation without disturbing in any way the negative feedback system of the existing transmitter.