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Low-voltage tube super-regenerative FM-receiver without an output transformer from the sandbox

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Hello.

Note

At the end of the article, there are two videos that roughly duplicate the contents of the article and demonstrate the operation of the device.

I can assume that many local residents are attracted by electronic devices based on electron tubes (I personally enjoy the warmth, pleasant light and monumentality of lamp constructions), but the desire to design something warm and lamp-like often breaks down due to the fear of communicating with high voltages Or problems with finding specific transformers. And this article I want to try to help the suffering, i.e. Describe the lamp design with low anode voltage, very simple circuit, common elements and no need for an output transformer. This is not just another headphone amplifier or some overdrive for a guitar, but a much more interesting device.

"What is this design?" – you ask. And my answer is simple: " Supergenerator !"
Supergenerators are a very interesting variety of radios, which is characterized by simplicity of circuits and good characteristics comparable to simple superheterodynes. Sabzhi were extremely popular in the middle of the last century (especially in portable electronics) and are intended primarily for receiving stations with frequency modulation in the VHF band (ie for receiving those same conventional FM stations).

The main element of this Type receivers is a super-regenerative detector that is both a frequency detector and a radio frequency amplifier. This effect is achieved through the use of adjustable positive feedback. Detailed description of the theory of the process does not make sense, since "everything is written before us" and without any problems it is mastered by this reference.

Further in this set of bucoffs, emphasis will be placed on the description of the construction of the tested design, because the schemes found in the literature are often More complex and require a higher anode voltage, which is not suitable for us.

I began to search for a scheme that satisfies the set requirements from Comrade Tutorskii's book "The simplest amateur transmitters and receivers of VHF" of the 1952 model. There was a superregenerator circuit, but I did not find a lamp that I was asked to use, but with an analog circuit, I was so normal and did not start, so the searches were continued.

Then this article was found. It already suited me better, but it had a foreign lamp, which is even harder to find. As a result, it was decided to start experiments using a common analogue, namely a 6n23p lamp, which feels great in VHF and can work at not too high anode voltage.

Taking this as a basis:

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And after a series of experiments the following circuit was formed on a 6n23n lamp:

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This design works immediately (with proper installation and a live lamp), and produces good results even for ordinary headphones.

Now let's take a closer look at the elements of the circuit and start with the lamp 6n23p (double triode):

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To understand the correct location of the legs of the lamp (information for those who previously did not have lamps with the lamps), you need to turn it with your legs to yourself and the key down (sector without legs), then the beautiful view presented to you will correspond to the picture with the lamp pinout (works and For most other lamps). As can be seen from the figure, there are as many as two triodes in the lamp, but we only need one. You can use any, there is no difference.

Now let's go on the scheme from the left to the right. The coils of inductance L1 and L2 are best wound on a common circular base (mandrel), ideally a medical syringe with a diameter of 15 mm is suitable, with L1 preferably being wound over a cardboard tube that moves with a small force over the syringe body, thus providing adjustment of the coupling between the coils. As an antenna to the terminal L1, you can solder a piece of wire or solder the antenna jack and use something more serious.

L1 and L2 should be wired with a thick wire to increase the quality factor, for example, with a wire of 1mm or more in 2mm increments (no special accuracy is needed here, so you can not bother with each turn). For L1, two turns must be wound up, and 4-5 turns for L2.

Next come the capacitors C1 and C2, which are a two-section capacitor of variable capacity (CFE) with an air dielectric, it is an ideal solution for such circuits , CPE with a solid dielectric is used undesirable. Probably, CPE is the rarest element of this scheme, but it is fairly easy to find in any old radio equipment or on flea markets, although it can be seen with two conventional capacitors (necessarily ceramic ones), but then you will have to adjust with an improvised vario Inductance). Example of the CPE:

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We only need two sections of the KPE and they must be symmetric, i.e. Have the same capacity in any adjustment position. Their common exact position will be the contact of the mobile part of the KPE.

Then, a quenching sequence is performed on resistor R1 (2.2 MΩ) and capacitor C3 (10 pF). Their values ​​can be changed in small limits.

The L3 coil serves as the anode choke, i.e. Is not allowed a high frequency to pass on. Any choke (only not on an iron magnetic circuit) with an inductance of 100-200 mH, but it is easier to wind the 100-200 turns of a thin copper enameled wire to the case of a spent resistor.

The capacitor C4 serves to separate the DC component at the output of the receiver . Headphones or an amplifier can be connected directly to it. Its capacity can vary within rather large limits. It is desirable that the C4 should be film or paper, but with a ceramic one also works.

The resistor R3 is a conventional potentiometer at 33 kOhm, which serves to regulate the anode voltage, which allows changing the lamp mode. This is necessary for more precise tuning of the mode for a particular radio station. It can be replaced with a permanent resistor, but this is undesirable.

This ends the elements. As you can see, the circuit is very simple.

And now a little about the power and installation of the receiver.

Anode power can be safely used from 10V to 30V (and more, but it's already a little dangerous to connect low-impedance equipment ). The current there is very small and for power supply BP of any power with the necessary voltage, but it is desirable that it was stabilized and had a minimum of noise.

. Another important condition is the power supply of the lamp (in the picture with pinouts it is designated as heaters), since without it it will not work. Here already currents are needed more (300-400 mA), but the voltage is only 6.3V. Suitable as a variable 50Hz, and a constant voltage, and it can be from 5 to 7V, but it's better to use the canonical 6.3V. Personally, I did not try to use 5V on the heat, but most likely everything will work fine. The pressure is applied to the legs 4 and 5.

Now about the installation. Ideal is the location of all elements of the circuit in a metal casing with a ground connected to it at one point, but it will work even without a casing. Since the circuit operates in the VHF band, all connections in the high-frequency part of the circuit must be as short as possible to ensure greater stability and performance of the device. Here is an example of the first prototype:

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With this installation everything worked. But with the metal case-chassis a bit more stable:

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For these schemes, the mounted mounting is ideal, as it gives good electrical characteristics and allows to make without any special amendments In the scheme that the board is not so simple and neatly obtained.

Now about the setup.

After you were 100% convinced of the correctness of the installation, they applied tension and did not explode and did not catch fire – this means, That most likely the circuit works, if the correct denominations of elements are used. And you will most likely hear noises in the headphones. If in all positions of the CPE you do not extend the station and you are sure that you are receiving broadcast stations on other devices, then try changing the number of turns of the L2 coil, by this you rebuild the resonance frequency of the loop and possibly get to the desired range. And try twisting the knob of a variable resistor – this too can help. If nothing at all helps, then you can experiment with the antenna. This completes the adjustment.

At this stage, the most basic has already been said, and the inept narrative presented above can be supplemented with the following rollers that illustrate the receiver at different stages of development and demonstrate the quality of its work.

Purely Lamp model (on the mock level):

Variant with the addition of ULF on the IC (already with the chassis):

In the latter version, the tube lightness is a little lost, since the IC is applied. This turned out to be the only solution, since the second triode did not work for me at the anode 20V in the ULF mode, although I can find the suitable mode, but I could not find it.

The amplifier was used as ULF PAM8403 amplifier, Which is powered by a linear voltage regulator L7805 (popularly called krenkoy, by the name of a Soviet analog).

In the plans for the development of this project, there is the creation of another superregenerator on a lamp 6c6b, but already portable, since it is very tempting to have a lamp Portable receiver.

Thank you for your attention. I am ready to answer questions on the topic.

Sources:

1. Superregeneration
2. Super-regenerative receiver
3. Documentation for the lamp 6n23p
4. Tutor "The simplest amateur transmitters and receivers of VHF" 1952

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