One Valve Radio

This one-valve low-voltage hybrid radio circuit uses a 6j1p pentode in "starved conditions" for the RF detection and amplification stage, and a ready-made LM386 amplifier module for driving the speaker. For a hobbyist project, the design is highly affordable and simple to make. It is an interesting circuit because nobody has ever used this combination of components before. One major advantage of this valve circuit is that it is low-voltage (9 V). In addition, the pentode valve works extremely well and you will not need to make "reaction" type circuits that employ feedback to increase amplification. Reaction, or regeneration, can be difficult for hobbyists to get right, and by eliminating this, we can also eliminate a large amount of unnecessary components, complexity, and cost. However, there is the option to build a simple coil to introduce this concept. If I have any shekels in my pocket left, then I might add more stages, otherwise for the time being, this is all there will be. Please note, this circuit requires experience of radio engineering principles and is not for children.

One compelling reason for using a ready-made LM386 amplifier module was that using discrete components to make it is expensive, and increases the complexity and cost of the build. I bought this ready-made module for just 99p online and it saves time and money. One of the main reasons why the LM386 IC works so well is due to its internal gain of 200. Other amplifier ICs such as the TDA7052, which has an internally fixed gain of 40 dB, also worked and produced sound through a loudspeaker, but was not as loud as the LM386 module. I also tested an LM386 amplifier built using discrete components around the IC, however the results were not as good as this module. A homemade LM386 amplifier circuit on breadboard usually does not perform so well because when the gain is set to 200, the IC becomes very sensitive and picks up signals from any long wires resulting in feedback type noises. However, this LM386 module uses SMD components on a small PCB and short tracks, thereby keeping crosstalk to minimum, and consequently functions much better.

For the valve, I chose to use the 6j1p pentode simply because it is cheap and for its electronic benefits described in its separate article. It is available for one pound on eBay, which is extremely good for the cash-strapped student or hobbyist who wants to experiment with low-voltage valve radio circuits. I also did not wish the hobbyist or student to buy expensive pre-used valves that may or may not be operating properly and thereby adding an element of instability, or even worse a point of failure for the project. Although there are many Chinese sellers offering the 6j1p as a new valve, many of these valves are not actually new. I recently ordered a batch of valves from one seller, and I received used faulty valves that were coming to the end of their service life. If the valve has greenish oxide on the pins, if the labelling has faded, if the base has black marks, then chances are that the valve is not new. A used valve coming to the end of its service life will not produce sufficient gain and therefore your radio will not work.

If the 6j1p is anything like the 6z1p "Russian Valve", then the heater will guzzle approximately 175-mA of current. Therefore, a mains power supply is essential because battery use will become expensive. It is always good to use a conventional transformer, as majority of the modern adapters tend to employ switched-mode circuitry that produces RFI posing an interference problem. An advantage of this particular power supply circuit is that we use a 3-core mains wire and employ the earth, which provides greater stability for the tank circuit, thereby eliminating one unstable variable from our radio development.

One of the main reasons why this radio works so well is because of the strong earth taken directly from the earth pin. Consequently, the required aerial has had to be only 1-meter long telescopic whip type. Therefore, a strong earth is vital! Something like the earth pin, or a copper pipe of the central heating would work well. If you do not have a whip aerial, then a long piece of wire is good enough. You need it to be as high as possible to pick the strongest signal.

The anode voltage and its resistor is critical, therefore I added a 9 V regulator to ensure that the valve bias remains the same if the hobbyist decides to use other power sources. The anode is also very sensitive and when using a mains transformer it is possible to hear hum even after rectification and smoothing, however I noticed that using the 9 V regulator chip eliminated this hum, hence it plays a crucial part in the operation of this circuit.

The RF and AF components from a valve are usually extremely small and in the bygone days, engineers used chokes in the HT rail to prevent RF and AF dissipating into the power supply. Although this circuit does not have a choke, the voltage regulator appears to be blocking the RF, which helps considerably. Hence, a voltage regulator is vital in this circuit.

When I designed this radio, I wanted everyone to be able to make the best tank circuit possible no matter what variable capacitor they had. Therefore, I have a separate article showing how a standard 10 mm diameter ferrite rod antenna coil could be made accurately using standard CAT5e solid core wire. This article is the one to consult as it has extremely good information including calculators that help in building such circuits. I also provide the inductance of each coil and the number of turns required thereby eliminating the guesswork inherent in such builds.

I dedicate this circuit to a new friend recently made who inspired me to build a valve radio; hence, it is named after him. "The Clifford"

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