Single Miniature Pentode Vacuum Tube Grid Detector Audion Radio Receiver

This Audion detector circuit radio receiver is just like the old ones from the 1920-s before the feedback was invented. It uses a seven-pin, miniature, all glass, modern pentode. The high input impedance of the pentode allows less loading on the input L-C circuit resulting in better selectivity. This radio works well when a local, or high power transmitter is operating nearby, especially with a good long wire, outside antenna.

Circuit operation is simple - Figure 1. L1 & L2 are made on the same coil form. L2-C1 tuned circuit used to select the stations. The antenna-coupling coil L1 is wound on top or near L2. Rotary capacitor C1 is a good quality air insulated unit. Capacitor C2 feeds the RF signal to the control grid while keep DC potential away from it. Grid leak resistor R1 is to remove trapped electrons from the grid. C3 is a RF signal bypass around the high impedance headphone set. Battery B1 is selected to operate a more modern, seven pin, all glass, miniature battery operated vacuum tube like those belong to the "1" set. Tubes like the 1T4, 1S4, 1R5 etc. are work well here and B1 can then be a single 1.5 Volt AA, C, or D cell. B2 as a minimum, may be three to five pieces rechargeable 9V transistor battery connected in series. Five is better, but with a strong transmitter three works as well.

             Parts list for Simple Audion Grid Detector One Tube Triode Radio:

                 B1  - Filament Battery - 1.5 Volt - function of tube used
                 B2  - Anode Battery - 9 Volt - 3 to 5 pcs. - Function of tube
                 C1  - Rotary Capacitor 15-500 pF
                 C2  - Grid Coupling Capacitor, 100 pF, Disc, 20 VDC
                 C3  - Shunting Capacitor - 10 nF, Disc, 20 VDC
                 L1  - Coil 1 - 100 uH, Air Core, Multiple Taps
                 L2  - Coil 2 - 200 uH, Air Core, Multiple Taps
                 VT1 - Vacuum Tube - Seven-pin, Miniature, 1T4, 1S4, 1R5, etc.
                 R1  - Grid Leak Resistor, 2M, 1/4 Watt, CC
                 Magnetic headphone, or a small high impedance speakers