Regen in a BC221
In the group of friends with whom I share my radio-experiences and knowledge, there has been a recent stir in the pot with Regenerative receivers.
Mr. RegenMan threw the stone in the lake insisting on making a receiver with only 12 V on the plates. On the web there are many examples. I made mine and he made his version, but inside a recovered old BC221 Frequency Meter.
The schematic is rather self explaining, other that Mr. RegenMan used a 12AU7 pre-RF/Detector + 12BH7 audio section. The adopted tubes are not 'space charge' type, but rather meant to be used with 100 ... 200 Volts on the plates. We wanted to see how they perform at much lover voltages.
The optimization has lead to a device that can drive a normal High-Impedance headset without need of further amplification, even though the audio level would actually need some help ...
The operating point at the RF preamplifier is set by the 'rf set' potentiometer. One should find the proper setting noticing the noise raising, but not too much. The preamplifier has a chocke at the grid, and feeds the main tuning tank: this configuration, if the amplification is too much, would generate self oscillations and mixing cross products with the incoming signals.
Mr RegenMan uses only a 4 mt whip on his window, and by the whole setup we can easily carry out radio-traffic down to 3.8 MHz, being our preferential frequency around 5 Mhz though ...
Along the audio path, a parallel resonant circuit has been added, exploiting a 220/6 V transformer as inductance, to narrow a bit the very wide frequency response typical of these receivers: the Radiotelegraphy reception is better.
On some evenings, the overall performance of this receiver has comparable (if not better) reception quality of a modern 'rice box'. Again: the key of success is the lack of the Automatic Gain Control and a very minimal number of active stages, resulting in a high signal-to-noise ratio.
I can confirm the performances as I made a similar receiver, with 12 V on the plates, but with different tubes, i.e. pentodes instead of triodes on the RF path: EF805S at the preamplifier and the detector, and an ECC802 double triode on the audio section, special version of the 12AU7 (... first to come in my hand when digging in the boxes ...). My prototype is still a bit far from being properly optimized, and as I prefer loudspeaker listening, I added an external audio amp.
On these nights we use our respective '12V B+ regeneratives' to make some good QSOs ... on the Shortwaves.
Radioman, Aug. 2019
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