Using 6K4 tube to make FM radio

A very simple tube FM receiver can be made by using a 6K4 tube and an air 365P double-connected variable capacitor. The air capacitor needs to be modified by removing most of the moving plates and fixed plates, leaving only two moving plates sandwiching a fixed plate. At this time, the adjustable range of the variable capacitor is about 3P to 50P.

The antenna coil L2 uses 2mm diameter enameled wire, which is wound 5 times on a 16mm diameter plastic round tube with a spacing of 2mm. Heat shrink tubing is applied to L2, and it is shrunk and shaped with a hot air gun, and then several layers of heat shrink tubing are applied, with a thickness of 2mm to 3mm, and L1 is wrapped around it. L1 is also wound 2 turns with 2mm diameter enameled wire. The high-frequency choke coil L3 uses 0.25m diameter enameled wire, which is randomly wound 150 turns on an I-shaped frame with a diameter of 10mm and a width of 15mm. The purpose of the random winding is to reduce the distributed capacitance. The antenna uses a 1.5-meter rod antenna or a soft tow line. The FM receiving band can basically cover the frequency range of 88MHz to 108MHz.

Debugging method: First use a high-impedance headset of about 600 ohms to listen. After the wiring is checked, adjust the voltage control potentiometer w. At the appropriate position, you can hear the "slight" super-regeneration noise in the headset. Then adjust the variable capacitor C, and cooperate with the stretching or compression coil L1 to receive the broadcast sound. At this time, the super-regeneration noise is suppressed. Adjust the grid resistance and capacitance value of the electron tube 6K4, and adjust the power supply voltage control potentiometer w to make the sound clear and loud. Repeat the debugging to achieve the best radio effect. Finally, input the signal to the power amplifier.

Note: Pins 3 and 4 of 6K4 need to be connected to the power supply, which is not shown in the picture.