One Tube Regenerative Radio Circuit

A regenerative radio works by feeding back a small amount of amplified output of the detector back into the input. Thus it achieves sensitivity far beyond what only a detector could alone. This simple regen radio uses a single tube as it's detector and amplifier; the "Audion". It's a great first project for those wishing to bring back some nostalgia by building one of the first amplified radio sets. Built on a board using point to point wiring and a set of period headphones, it can be a great functional conversation piece.

Schematic

Parts

Part Total Qty. Description Substitutions R1 1 50K Linear Taper Pot R2 1 2.2 Meg 1/4W Resistor R3 1 10K 1/4W Resistor C1 1 250pF 100V Ceramic Disc Capacitor C2 1 365pF Air Variable Tuning Capacitor C3, C4 2 120pF 100V Ceramic Disc Capacitor C5 1 0.1uF 100V Ceramic Disc Capacitor V1 1 3A4 Audion Tube 3S4, 3Q4 (See Notes) L1 1 30 Turns 26 AWG Magnet Wire (See Notes) L2 1 80 Turns 26 AWG Magnet Wire (See Notes) L3 1 20 Turns 26 AWG Magnet Wire (See Notes) S1 1 SPST Switch HEADPHONE 1 High Impedance Headphones (2K or Greater) ANT1, ANT2 2 See Notes MISC 1 Board, Wire, Sockets For V1, Case, Knob for R1, Clips for Antenna and Batteries

Notes

1. L1 - L3 are constructed on the same coil form. A toilet paper tube will be the coil form. Secure the 26 AWG wire to the form by punching two holes close together and winding the wire once around the "bridge" between them. Alternately, just a drop of hot glue can be used. Leave about 6" of wire. Wind on twenty turns close together but never overlapping. Make a tap by securing the wire with a drop of glue and twisting a loop. This is L3. Wind 80 more turns in the same direction and then secure the end, leaving about 6" of wire at the end. This is L2. Now secure the 26 AWG wire 1/8" from the end of L2 and wind 30 turns in the same direction as the other coils, making sure that this coil is spaced 1/8" evenly from L2. Secure the end and leave about 6" lead. This is coil L1. Now trim the extra length of the coil form and spray the coil with several coats of lacquer to hold the wires in place.
2. R1 and S1 can be one unit if you purchase a pot with a built in switch. This will avoid turning the set on with full regen and causing a nasty squeal from the headphones.
3. The 3S4 or 3Q4 can be substituted for the 3A4 in the connections of pin 3 and pin 4 are swapped.
4. An antenna of only a few feet can be used for ANT1 for easy indoor installation. Far better is an antenna of 50 feet or more outside, which would be connected to ANT2.
5. Note that this circuit requires a good Earth ground. A suitable location is a metal cold water pipe.
6. To use the set, place C2 at about midway, set R1 fully counterclockwise and then turn on S1 and allow the tubes several seconds to warm up. Increase R1 until the headphones squeal, then back off. That is when the set is most sensitive. Now tune C2 to the desired frequency. You may have to readjust R1 as you tune different frequencies. With some practice you will figure out how much regen you need as you change stations.
7. If R1 seems to have no effect, swap the connections to L1.