243,407. Brydon, S., and Fletcher, D. B. July 22, 1924. Thermionic amplifiers and detectors. - A back coupled valve is set at the " threshold " point withou appreciable backlash, so that it generates high-frequency oscillations contem porary with the incoming signals, the corresponding anode voltage-variations being applied to the grid and filament of a second valve, which contains a detecting or recording device in its plate circuit. The invention is distinguished from known arrangements which utilize changes in the value of direct, current in the anode circuit of a triggered valve, and from the super-regenerative circuits described in Specification 182,135. The first valve A is coupled through a condenser O, shunted by a resistance q, to the valve B which feeds a recording device R or a pair of phones.; The proper setting of the valve A on the threshold point is ascertained by varying the filament current and the coupling between the coils m and g against a step-by-step adjustment of a grid potentiometer h, i. The curve X, Fig. 2, represents the " oscillation " conditions, whilst Y corresponds to the cessation of oscillations, the distance between these curves indicating the degree of back lash present. For telephony reception the valve should be set near the converging point Z, whereas for Morse reception a certain amount of back lash is desirable as providing an impulsive effect, disproportionate to the signal voltage, in the recorder R. The frequency of the oscillations generated by the valve A is determined by the tuning of the circuit g, and may differ from that of the received signals.