The three-electrode valve as an electron relay and rectifier.

From the book 'Vacuum Tubes in Wireless Communication' by Elmer E. Bucher. Published in 1919.

We shall show more in detail that the three-element valve may be employed as a relay or as a combined rectifier and amplifier.
A complete circuit for taking advantage of the non-uniform characteristic of the three-element tube is shown below, where the plate P as usual is charged positively by the battery B-2 in series with which is the telephone P-2. Filament F is incandesced by battery B-1 and the strength of the current regulated by rheostat R. The potential of the grid to filament is fixed by battery B-3 shunted by potentiometer P-1. Condenser C-2 provides a path for the radio frequency currents about the potentiometer P-1. The primary winding of the usual receiving tuner is indicated at L-1 and the secondary winding at L-2

Showing one method of connecting for the three-electrode valve as an oscillation detector in wireless telegraphy. Connected in this manner the valve is termed an assymetrical relay

Use of the three-electrode valve with grid condenser.
The circuit shown below differs from that of the former figure, in that it employs the so-called grid condenser, C, connected between the grid and the upper terminal of the secondary coil of the receiving tuner. The function of the grid condenser is to store up the currents which are rectified by the valve action between the grid and filament. The charge and discharge of this condenser during reception of damped oscillations in radio telegraphy decreases and increases the plate current at an audio frequency.

The circuit for the use of the valve as an combined rectifier and amplifier. Incoming oscillations are rectified and stored up in the grid condenser, the charge and discharge of which varies the telephone current at an audio frequency.

Further amplification of incoming radio signals can be obtained by tuning the plate circuit of the vacuum tube. Armstrong's method is shown below. In addition to the apparatus of previous diagrams, this circuit includes a plate circuit inductance L-3 and a condenser C-3. The coil L-3 , in conjunction with the electrostatic capacity of the valve (between F and P) constitutes an oscillation circuit of variable frequency. The condenser C-2 acts as a by-pass for the radio frequency component of the plate current around the head phone P-1 and the battery B-2. Condenser C-2 may be of fixed capacity.
In brief, as the incoming radio frequency oscillations are repeated into the plate circuit, the counter E.M.F. of coil L-3 either assist or opposes the plate circuit battery according to whether the latter decreases or increases. The effect of this counter E.M.F. is to alternately decrease and increase the potential difference between F and P, and since the space between F and P constitutes a condenser of small capacity, the charge accumulated therein is varied in accordance. The internal or self-capacity of the valve acts as a coupling to transfer energy from the plate circuit to the grid circuit, the energy for which is supplied by the reactance voltage of the coil L-3. Marked amplification are thus secured.

The three-electrode vacuum valve as an oscillation detector with a tuned plate circuit. 

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