Audion Grid Detector One Vacuum Tube Triode Radio Receiver

Figure 1. Audion Grid Detector One Vacuum Tube Triode Radio Receiver

Before 1906 almost all early radio radio receivers relied on the improved Branly type coherer wave detector. The weak signal reception required a large outside antenna. The Audion device immediately changed many things for the better. Higher level audio and real selectivity were the most important improvements. Because the lack of grid current in the new detector, input circuit loading disappeared.

The audion device invented and patented (U.S. Patent 841,386) in 1906 November 13 by Lee De Forest. This new RF (Radio Frequency) detector used an external control wire wound on the body of the glass "lamp" vacuum tube. This crude device became the much awaited, wonder detector. Unlike the coherer the Audion required no periodic manual or automatic mechanical tapping. In spite its complexity, the difficulty to produce good vacuum, the audion worked reliably.

Figure 2. Demodulated AM Signal

L1 & L2 are made on the same coil form. L2-C1 circuit selects the station - Figure 1. The antenna-coupling coil L1 is wound on top or near L2. C2 couples the RF signal to the control grid while keep DC potential away. Grid leak resistor R1 removes trapped electrons from the grid. C3 is a RF signal bypass around the high impedance headphone set. In old radios rheostat R2 controls the cathode emission. With current vacuum tubes R2 is not used. If seven pin, all glass, miniature battery operated tubes are used like those belong to the "1" set - 1T4, 1S4, 1R5, etc. B1 can be a single 1.5 Volt AA, C, or D cell. Anode battery, B2 as a minimum, may be three to five 9V battery connected in series.

As shown on Figure 2 the upper (red) trace depicts the AM (Amplitude Modulated) signal. The bottom (blue) trace shows the audio signal after the Audion detector removed the carrier wave. This audion grid detector circuit does not use feedback and its gain is low. It can be built the way it's shown but much better circuits are available.

        Parts list for Simple Audion Grid Detector One Tube Triode Radio:
            B1  - Filament Battery -  B2  - Anode Battery - Voltage is function of tube used
            C1  - Rotary Capacitor 35-500 pF
            C2  - Grid Coupling Capacitor, 100 pF, Disc, 20 VDC
            C3  - Shunting Capacitor - 10 nF, Disc, 20 VDC
            L1, L2  - Coil 1 - 100 uH/200 uH Air Core, Multiple Taps
            VT1 - Vacuum Tube - 1.25V, seven pin, miniature
            R1  - Grid Leak Resistor, 1M, 1/4 Watt, CC
            R2  - Rheostat, 100, 1 Watt, WW
            Headphone Magnetic, or a small high impedance speakers