Abstract: The present invention concerns a grid amplifier tube producing with an approximately constant gain at least a combination of a first signal for a short duration and a second signal for a longer duration. The maximum voltage of the first signal is higher than the maximum voltage of the second signal. To improve the efficiency of the tube, the anode polarization voltage is adjusted to a level enabling the tube to operate as a class B, AB or C amplifier and to produce the maximum voltage of the second signal but not the maximum voltage of the first signal. The class of the amplifier is made to vary towards class A for the duration of the first signal so that it produces the maximum voltage of the first signal. For this purpose at least one of the polarization voltages of the electrodes of the tube is made to vary. The present invention finds particular application to television transmitters operating in negative transmission systems.

Abstract: High frequency amplifiers require neutrodyning to prevent the risks of self-oscillation generated by the existence of stray capacitances among the electrodes of the active component used in the amplifier. Grid tube amplifiers (such as triodes, tetrodes, pentodes etc.) are more particularly concerned. Instead of simply providing a variable inductive element, in parallel, on the stray capacitance between the input electrode and the output electrode, there is provided a star connection of three reactances between the input electrode, the output electrode and the reference electrode. Only the first reactance is variable. The others are fixed and are in a constant ratio independent of the frequency. Preferably, the variable reactance element is an inductive element, and the other two are capacitive elements. Thus, by means of this single, variable reactance element, it is possible to make a setting, at the same time, of the input or output frequency tuning of the amplifier and of the neutrodyning.