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Wednesday, May 29, 2024

A 1941 Simplified Superhet


One of my favorite homebrew receivers is the one I built  based on the "Two Tube Superhet Receiver" shown in the 1941 ARRL Handbook. This one has a regenerative detector running at 1700KHz with a tunable convertor stage (covering 80 and 40) and a single stage of audio.

After few minutes of playing with the two tube superhet on 40 meters I decided that a BFO was needed for reasonable CW and SSB reception. The two tube superhet became a three tuber. The regenerative IF remained. It is not typically run into regeneration but just below to improve IF gain and selectivity. As a modern safety measure I RC coupled the headphones into the plate circuit of audio stage rather than directly connecting them. Read about this project here.

I then decided to make a major improvement by adding a crystal filter. This gives this simple receiver almost single signal selectivity. At 6dB down, the bandwidth is only 250Hz. Inserting the crystal into the middle of the regenerative IF stage feedback loop allowed regeneration to make up for crystal insertion loss without adding an extra tube. Also modified from the original was changing the regenerative detector from 1/2 of a 6C8 to a 6J7, adding better bandspread and a front panel BFO adjustment. The attenuator on the antenna input was added after experiencing problems from strong signals overloading the receiver. This version is described here.

The single AF stage really did drive a speaker very well. As built this is a headphone radio. In addition it needed an external power supply. I built another AF stage (with a speaker) and a power supply into an external box. In addition I added a toggle / Transmit-Receive switch.

I typically use this receiver with my 6J5-6L6 Transmitter.