Cascade Amplifiers.

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

In a circuit where the radio or audio frequency components of the plate current of one valve is impressed upon the grid and filament of the second valve, that is where the output circuit of the first valve is coupled to the input circuit of the second valve and so on, the valves are said to operate in cascade. In practice as many as six valves have been thus used, but three usually suffice for practical operating conditions, and little gain usually results from employing more.
The complete circuits of a cascade radio frequency amplifying system are shown in the figure below where a coupling transformer M with the windings P4 and S serves to impress the radio frequency component of the plate circuit of the first valve upon the grid and filament of the second valve.
The antenna circuit of this diagram embraces the antenna loading inductance L, the primary winding of the receiving transformer L-1 and the short wave variable condenser C-8. The secondary or grid circuit comprises the secondary coil L-2, the secondary loading inductance L-3, the shunt secondary condenser C-1, and the fixed condenser C shunting potentiometer P-1 which has a resistance of about 400 or 500 ohms.
B-3 is a battery from three to twenty volts. B-1 is the usual filament battery, and B-2 the high voltage battery of the plate circuit of the first valve. The plate circuit also includes the radio frequency circuit P-4, C-4; P-4 being the primary of a radio frequency transformer which acts inductively upon winding S. L-4 is the loading inductance for the grid circuit of the second valve and S id the secondary coil. C-5 is the shunt secondary condenser. The condenser C-6 performs the function of C in the first valve. The plate circuit of the second valve comprises the battery B-5, the head telephone P-3, and the shunt condenser C-7.

The successive groups of incoming (damped) oscillations may be rectified and stored up in a grid condenser and during the pile up of this charge the plate current decreases. At the termination of the incoming wave train the charge leaks out of the condenser and the plate current returns to normal value. This variation of the continuous current in the plate circuit is termed the audio frequency component of the plate current.
The audio frequency component may be amplified through the medium of a second valve. One method is shown in the next diagram, wherein the plate circuit of the first valve and the grid circuit of the second valve coupled inductively through the iron core transformer M. The primary and secondary coils of M consist of several thousand turns of relatively fine wire such as No. 32 or No. 34.

By means of the connections shown in the last schematic, the radio and audio frequency components of the plate or output circuit of one valve may be amplified simultaneously through a second valve.
In this system, the audio frequency component of the plate circuit is impressed upon the grid of the second valve through transformer M. Condenser C-3 serves as a bypass for the radio frequency currents around the the audio frequency inductance L-5. Condenser C-2 serves to tune the plate circuit to the incoming oscillations (the radio frequency component of the plate current) and C-5 tunes the grid circuit of the second valve to the same frequency, the radio frequency current of the plate circuit being impressed upon the the grid circuit of the second valve through the coupling L-3, L-4. Condenser C-4 acts to by-pass the radio frequency current in the grid circuit of the second valve around the audio frequency coil L-6. Potentiometer P-2 shunting battery B-6 enables the operator to obtain the best relaying characteristic of valve No. 2.
As usual, a grid condenser can be inserted in the circuit of valve No. 2, and the relaying action obtained without the potentiometer. The battery B-6 is preferably shunted by a condenser of fixed capacity.

At first sight, the circuits shown seem complicated in structure and difficult to adjust, but it should be understood that if the tuner is designed for a given range of wave lengths, it is not necessary to re-adjust each element of the circuit to select a new wave lenght. 

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