From Kit to Custom Design: Building a Tube-Based FM Radio -- Episode 680

Mark takes on the challenge of building a fully functional FM superheterodyne radio without transistors or SDR, relying instead on vacuum tubes and a custom PCB design. After finding major flaws in a DIY kit, he scraps the schematic and creates his own design from scratch, covering everything from oscillator design and frequency mixing to tuning and final testing. Follow along as he assembles, debugs, and tunes the radio into a working stereo receiver.

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From Kit to Custom Design: Building a Tube-Based FM Radio -- Episode 680

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Building a modern FM radio receiver is usually straightforward. A single chip like the TDA7000 or a software-defined radio (SDR) module can handle most of the work with just a few supporting components. But where’s the fun in that? Using ready-made solutions means missing out on the design process, sourcing unique parts, building prototypes, and debugging problems. To make this project more challenging, Mark decided to design and build a vacuum tube FM receiver without transistors.

(Note: Mark did use a stereo decoder IC, though it’s optional. For mono audio, it can be skipped. He also included an LM7809 regulator, which of course contains transistors internally.)

The Kit That Wasn’t

To get started, Mark ordered a DIY tube radio kit from AliExpress. It cost around $220, including shipping, and came with essential components such as vacuum tubes, intermediate frequency (IF) coils, and a variable capacitor. The idea was that it would save him time tracking down parts.

When the kit arrived within two weeks, he quickly found issues: the power transformer was missing, and one resistor was broken. The supplier promised replacements in a later shipment, but the bigger problem was the documentation. The manual was only four pages long, entirely in Chinese. After running it through Google Translate, it became clear there were no instructions on tuning or adjusting the frequency stages. Worse, the schematic had errors: pin numbers were swapped, some required connections were missing, and the PCB contained components not even listed in the bill of materials. At that point, Mark knew the kit was unusable.

Still, it wasn’t a complete loss. He had the critical parts he needed such as tubes, coils, and the capacitor. The rest he would redesign himself.

Designing From Scratch

Looking at the salvaged components, Mark concluded he could make the radio work by replacing or adjusting capacitors and resistors while reusing the tubes. With that decided, he set out to design a proper superheterodyne receiver from scratch.

Input Band Filter

The first stage is a band-pass filter that limits the antenna input to the desired frequency range. This filtered signal is then amplified before being sent to the mixer.

The Oscillator

Next, Mark designed a Hartley oscillator to generate a tunable sine wave frequency. To tune a station at 95 MHz, the oscillator can be set to either 105.7 MHz (station + 10.7 MHz) or 84.3 MHz (station – 10.7 MHz). He found that using the lower frequency provided better results with less interference from nearby airband signals.

Mixing the Signals

The mixer combines the input signal with the oscillator signal, creating several outputs: fInput, fOscillator, fInput ± fOscillator. The only one of interest is the difference (fOscillator – fInput), which falls into the intermediate frequency (IF) range.

Intermediate Frequency (IF) Stages

The mixed output is filtered at 10.7 MHz, which is the standard IF for FM receivers. Multiple stages of filtering improve selectivity. In Mark’s build, four stages are used, with the final one incorporating an FM detector.

FM Detector

Finally, the IF signal is demodulated to recover the original audio. This is done with a dual-diode detector known as a Foster-Seeley discriminator, which produces the audio signal that can then be amplified.

Reflections

What started as a faulty DIY kit turned into a rewarding scratch-build. Along the way, Mark revisited many fundamentals of analog design such as oscillators, mixers, filtering, and FM detection. The end result is a working vacuum tube FM superheterodyne receiver, built from a mix of salvaged and custom-sourced components. Despite the frustrations, it was a valuable reminder that sometimes the best projects are the ones that don’t go as planned.

Supporting Links and Downloads

- Schematics and Bill of Materials

Bill of Materials

Additional Parts