Making a Simple Tube FM Radio

Making a Simple Tube FM Radio

Power amplifiers made of electron tubes are more attractive when playing digital sound sources. Since FM radio and TV (accompaniment) have abundant high-quality sound sources, many electron tube enthusiasts have begun to have a strong interest in electron tube radios. Based on this, the author has also made some attempts. Now there are many FM radio music channels. Using electron tube FM radios to receive and restore music signals, and then using electron tube audio amplifiers to play music, the sound quality and timbre are even better, which is fascinating!

Since FM broadcasting (TV audio) operates in the ultra-high frequency range, under amateur production conditions, if a superheterodyne circuit is used, the circuit is relatively complex and the device is difficult to adjust. The author uses a super-regenerative reception method, which has a simple circuit and is easy to make. The production situation is briefly described as follows.

1. Component configuration

The circuit of this machine is shown in the figure. It is a self-extinguishing super regenerative receiver circuit. Since most channels operate in the range of 88MHz~108MHz, Shanghai Art Channel (intermediate frequency 96.8MHz) is used as the reference point. L2 is made of Φ2mm silver-plated copper wire, and 5 turns are wound on the outer diameter Φ15mm mold with a circle spacing of 2mm to form a hollow type. L1 is made of (Φ2mm silver-plated copper wire) and 2 turns are wound on the outer jacket of L2. The distance between the circles is 2mm. The distance between the two coils is about 2mm to 3mm. The high-frequency choke is made of Φ0.32mm enameled wire, and 150 turns are wound in three sections of honeycomb on a 10mm diameter paper tube, with a section spacing of 2mm. A one-meter-long trailing wire is sufficient for the antenna. The electronic tube uses J-level 6K3P or 6K4, and 6J1 can also be used, but its gain stability is not as good as 6K3P and 6K4 (variable μ tube).

It is better to have a finished variable capacitor. If there is no finished product, you can find an old radio to dismantle the double variable capacitor and modify it. The method is to remove some of the moving pieces and fixed pieces. The relationship between the capacity and the piece is: the capacitance of the moving piece and the fixed piece is 20pF, the capacitance of two moving pieces sandwiching a fixed piece is 47pF, and the capacitance of three moving pieces sandwiching two fixed pieces is 100pF. The specific value used depends on the receiving frequency.

2. Assembly and debugging

Since the FM part works at an ultra-high frequency, these components should be installed on a plexiglass or plastic board. The coil should be installed 10mm to 15mm away from the board, and the contacts should be firm to prevent high-frequency machine vibration. Each component should be installed as compactly as possible in an appropriate position near the tube to reduce the impact of distributed capacitance and leakage inductance, so as to facilitate smooth debugging.

The adjustment of this receiver is relatively simple. After checking that the wiring is correct, turn on the power supply, adjust the power supply voltage control potentiometer W to the appropriate position, and you will hear a slight super-regeneration noise. Then adjust the variable capacitor C and the adjustment coil L (stretch or compress) to receive the broadcast. At this time, the super-regeneration noise will be suppressed. Then adjust the grid resistance and capacitance of the electron tube 6K3P to the appropriate value, and finally adjust the power supply voltage control potentiometer W to make the sound clearer and louder. Repeat the entire adjustment process once more to make the sound quality more pleasant.