The time to experiment with receivers has come, finally. I refrained for many years to work with receivers because they are much more difficult to make them working decently. A transmitter for Radiotelegraphy is a matter for kids. A receiver not.
Mr. Crystal suggested to try building a 'REGENERODYNE', which is a frequency converter followed by a regenerative detector. Some fellow Radio Amateur did very nice jobs and even rough measurements speak of sensitivities in the range of 0.2 - 0.5 uV / 10 dB Signal to Noise ratio.
I decided to try, absolutely.
Here below my attempt: ECH83 working as Frequency Converter, crystal oscillator as Local Ocillator. An ECL84 working as Pre and Final Audio Amplifier. A 6E5 'Magic Eye', to check the audio envelope level and a VR90 Voltage Stabilizer tube, for the ECH83 plates.
It took 3 gross days, i.e. 6 working hours (2 hours a day) starting from the flat aluminum chassis to the first test. Holes included.
The results are here below ...
The performance is ... astonishing. This is my first frequency-conversion receiver ... and almost the same stations can be heard as those with a professional receiver. Ok: there is practically no selectivity (yet ...) and no Automatic Gain Control. But the overall performance is sufficient to perform reliable traffic. Also local Radio Amateur stations on AM could be received just fine. For sure, it is perfectly suitable to Full Spectrum, where you don't have to fight against other stations for a tiny piece of band on which making your own traffic.
The regenerodyne concept has many less compromises as compared to a 'REGENERATIVE', because it is resettable and it is much more immune to strong signals or to overload when connected to a changeover relay of a transmitter. The fixed frequency Local Oscillator and the relatively low frequency regenerative stage, in my prototype working from 1.5 to 3.0 MHz, make the tuning stable as a rock.
It would take very little to replicate this circuit in solid state fashion, but I prefer the glow of the electron tubes ...
One very great feature is that the ECH83 triode section is made to work as regenerative detector. It start oscillating at a plate voltage from 5 to 7 Volt DC in my prototype! Which means that the regenerodyne can be replicated in versions working at a voltage as low as 12 Volt also for the anodes! ECH81 could do the same, but in my prototype the ECH83 seems to work better. ECH84 needs higher voltage and the russian military version of the ECH81 seems to be half way between ECH83 and ECH84.
The ECH83 is preceded by a simple parallel tuned circuit as preselector, which receives the signal from the antenna by a 2-3 turns link over the coil.
A final remark on the tuning variable capacitor: it is from a scrapped BC221 frequency meter. I will not stop saying that this capacitor is the best for making tuners or VFOs.
The frequency conversion has been a beast I have hated for decades as a circuital topology for homemade receivers: I thought it was too crytical. After seeing my receiver working so well (even though under the chassis there is a real mess made of wires and hanging components) ... I don't fear it anymore.