I bought this little radio kit during a visit to Berlin, at the gift shop of the Museum of Technology (Deutsches Technikmuseum), a museum with quite interesting exhibitions and well worth a visit.
I found it to be a rather curious kit, since nowadays it’s quite unusual to find a kit that lets you build a valve radio, so as soon as I saw it, I grabbed it to assemble at home.
Interestingly, it’s a radio that only tunes shortwave, specifically from 3.5 MHz (80-meter band) to 12 MHz (25-meter band). It uses a single valve as a regenerative receiver, and it operates at low voltages, which allows it to run on batteries and also avoids dangerous voltages for the builder.
The first step in the construction is to open the kit box itself, which includes everything needed inside:
The kit’s aesthetic design is quite good, and it includes all the necessary components as well as a small printed circuit board, so there’s no need for point-to-point wiring.
It also comes with an instruction manual, although one downside is that it’s written entirely in German. So if, like me, you don’t know much German, it won’t be of much help. In any case, the assembly is fairly intuitive, and the included diagrams are more than enough to follow the build.
If we take a look at the schematic included at the end of the manual:
The circuit:
- First, it’s noticeable that this is a hybrid radio. It uses a 6J1 valve, equivalent to the EF95 (also known as 6AK5), as a regenerative detector, followed by a transistor (T2) acting as a preamplifier, and then a power amplifier (the well-known LM386) to provide enough audio power to the speaker (it doesn’t include a headphone output, which is a downside).
- Two batteries are used: a 6V battery for the LM386, and a 9V battery which, combined with the 6V, gives a total anode supply of 15V—enough for the 6J1 to operate.
- The transistor T1 acts as a simple switch, so when the 6V supply is turned off, it also disconnects the potentiometer P1, preventing unnecessary drain of the 9V battery, which is not switched off by the main power switch.
- Regarding the regenerative receiver stage, the feedback is controlled by P1, which simply adjusts the plate voltage to the 6J1, thereby controlling the feedback level. Although the 6J1 is a pentode, it is connected as a triode, which in turn is wired like a typical regenerative circuit with a center tap on the tuning coil—except for R1, which is connected to the anode to ensure a certain positive voltage on the control grid.
- This function is important, so the kit includes a small dedicated socket for R1, allowing you to alternate between three possible resistor values (100K, 330K, or 1M). The lower the resistance, the higher the positive voltage generated on the control grid, which forces a greater flow of electrons from cathode to anode, thus affecting the regenerative stage’s operation. The best value varies depending on the specific valve, as no two work exactly the same, hence the option to try different resistors to find the optimal setting for the receiver.
- Finally, there are three possible antenna connections: A1, A2, and A3. A1 is completely useless because connecting the antenna to the cathode kills all the circuit’s feedback. A2 and A3 are more useful, with A3 recommended for long antennas, as capacitor C1 compensates part of the reactance that a long antenna might have. For testing with short indoor antennas, A2 has worked best for me.
Assembling the radio is quite simple. After a while soldering the components, the radio is ready for its first test:
For this, I connect the ground to the E terminal, and attach as long a wire as possible to the A2 terminal. I turn the volume knob to switch on the radio, wait a few seconds for the 6J1 valve to warm up, and then:
¡It works!
As seen in the video, the operation is quite simple, but there are some drawbacks or possible improvements I need to mention about the radio:
- The frequency stability is very poor. If you get close to the antenna, the tuning changes. This is because the capacitance of the antenna changes when you bring your hand close. A similar effect happens when you bring your hand near the variable capacitor to adjust the tuning; the capacitance added by the hand is enough to change the tuning. This effect could be significantly reduced if the case were metal instead of cardboard, and if a transistor configured as a common-base stage were used in the antenna circuit to isolate the antenna from the tuning coil.
- The antenna coupling would improve if, instead of using a fixed-value capacitor like C1, a variable capacitor were used to finely adjust the antenna coupling with the tuning coil.
- The fact that the tuning knob acts directly on the variable capacitor does not allow fine frequency adjustment. This makes it quite difficult to tune stations that are close to each other (which is quite common in shortwave). A reduction drive system or even a small adjustable trimmer capacitor as a complement to the main capacitor would have been better to allow finer adjustment.
- The radio does not include a headphone output, which would be appreciated especially to listen to weaker signals. It would also be useful to have the option to add an external power connector, since using batteries frequently drains them relatively quickly.
Overall, this kit performs well as a basic radio to teach the fundamentals of regenerative receivers and as an introduction to valve radios, but not as a “serious” receiver for shortwave listening. Still, it’s quite fun to build and use.