Low Voltage One Tube 6J1 SW-SSB SDR Radio (Works at 3.7V)

A vacuum tube radio, also known as a valve radio, is an early type of radio receiver that uses vacuum tubes to amplify and detect radio signals. These radios were widely used from the early 20th century until the 1960s. Nowadays with just one Vacuum Tube and Signal Generator you can make a super simple, yet sensitive and selective Shortwave/SSB Radio Receiver, thanks to the use of PC and SDR processing software. In fact, this project represents a kind of Software-Defined Radio where components that have been traditionally implemented in hardware (e.g., mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a personal computer or embedded system.

This allows for more flexibility and the ability to handle a wide range of frequencies and protocols simply by updating the software. Another advantage is that the radio uses a cheap 6J1 tube which is capable of operating at an extremely low voltage of 6.3V which we also use for heating. This Radio is actually a modification of the DRM Direct Mixer project published by Burkhard Kainka, but now using a VFO instead of a constant frequency crystal oscillator and also adding an antenna matching circuit to the input oscillator.

The 6J1 tube acts as a mixer with the oscillator signal been injected via the screen greed. In the project I use the frequency oscillator whose construction is described in one of my previous videos, but you can use any other frequency oscillator.

Now let's look at the way of making it and the components of which it is composed. The receiver is made in a simple way by directly soldering the components to the tube base, a technique that was also used on commercial radios until the middle of the last century.

The receiver is made in a simple way by directly soldering the components to the tube base, a technique that was also used on commercial radios until the middle of the last century.

The device consists of a cheap 6J1 tube, several capacitors and resistors and an input part consisting of a coil with 25 turns on a body with a diameter of 2cm, and a variable capacitor with a value of 400pF. The frequency range of reception depends on the value of the last two components and for the given data ​​it is from approximately 5 MHz to 15 MHz.

The SDR software I use is "SDRadio" by Alberto I2PHD, it is free and you can download it at the given address. It is very simple, does not require powerful hardware and contains only the basic settings and functions needed for such a simple device.

On the software we can monitor the reception signal very clearly and precisely, and we can also set the type of demodulation, namely AM, LSB or USB, as well as the bandwidth of the received signal. The stability of the frequency of the received signal mostly depends on the VFO generator, which in this case is very stable and precise. That's why we can perform demodulation of several digital radio signals with different software.

And now let's see how the device works in real conditions. The receiver should first be tuned to have the best reception around the desired frequency. For this purpose, we adjust approximately the frequency on the VFO, and without the antenna connected, we turn the variable capacitor until we get the strongest reception, that is, noise. Now we connect the antenna and start searching for stations. When we encounter a station, we again move the capacitor to the best reception. On the video I demonstrate reception at different frequencies, bearing in mind that reception varies greatly with the propagation conditions of the SW band.

During the testing I came up with the idea to reduce the supply voltage, and I came up with an amazing result: the receiver works perfectly well even at a voltage lower than 3.7V, so I decided to power it with a single lithium battery.

As you can see, in this case the receiver works quite normally and the total consumption is only 0.4W. It even works at 3V (2x1.5V), but with more modest results.

Unfortunately, during the recording of this video, the radio signal propagation conditions were relatively bad, so to give you an idea of ​​the quality of the reception, I made a simple comparison with the well-known commercial product SDRuno from SDRplay.

According to the tests that I performed in the previous period, the receiver has the best reception with the simplest antenna - a high mounted long wire with a length of at least several meters and proper grounding. With a LOG (loop over ground) antenna, the reception is clear with a better signal/noise ratio, but at the same time with significantly lower signal strength. I tested the receiver with an EF95 tube instead of the 6J1, and the results were about the same.

And finally a short conclusion. Although this is a simplest type of SDR radio, wich is a combination of very simple RF Front end, Soundcard as A/D converter, and PC software, it still represents an excellent example of how we can apply modern computer technology in radio technology. On the other hand, the reception quality and the many software options are excellent. And let me not forget, the radio operates on an extremely low voltage of only 6V. And even a curiosity is the fact that this tube radio can function at an amazing 3VDC.