Heterodyne regenerative receivers

It is difficult to beat the simplicity/performance ratio of a one-tube regenerative receiver. For long and medium wave broadcast band listening, my only receiver is a rebuilt 1920s model with a single triode and a variocoupler. For short wave listening, I use a modern subminiature pentode with screen grid regeneration control. It is possible to use the same receivers for reception of amateur CW and SSB signals, although the generally weaker and narrow-band nature of these signals is starting to show the deficiencies of a simple one-tube regen. There are multiple ways to improve this basic circuit, such as adding one or two tubes for audio amplification, an RF amplifier with good isolation between the antenna and the tuned circuit, audio filtering, etc. But perhaps the most important improvement in my mind is the addition of a heterodyne oscillator.

Heterodyne regenerative receivers, where the detector is operating below the threshold of oscillation and a separate oscillator is tuned to provide a beat note for an incoming CW or SSB signal, have been discussed since at least the 1930s (see e.g. QST February 1933, December 1934, February 1936). This modification is often justified in terms of improved performance, but I would like to point out that it also allows keeping other parts of the station simple:

• Higher RF gain: less need for AF gain.
• Better RF selectivity: less need for AF filtering.
• No blocking with strong signals: less need for RF attenuation, and easier to use with a separate TX that has a "spot" function.
• Non-oscillating detector: you can couple the tuned circuit directly to the antenna, instead of adding an RF amplifier for isolation. This also means frequency stability is less of an issue, which reduces constraints on tuned circuit design.
• Dual use for BFO: to convert the receiver to a CW transceiver, connect the BFO output to a power amplifier and add some means for T/R frequency shifting.
• For a minimalistic QRPp transceiver, zero beat the incoming signal while oscillating, turn back regeneration, then connect the key to maximize regeneration. Full QSK with side tone provided by the BFO!

In addition, there are a few other advantages that are rarely pointed out:

• Option of crystal control: a crystal oscillator (or VXO) can be used for increased frequency stability, making it useful for higher frequencies or digital modes.
• Redundancy and backwards compatibility: turn off the BFO and operate as a regular regen, for AM reception or as a backup if the BFO tube breaks.

To be fair, there are a number of weaknesses even with a heterodyne regen. At least on higher bands (which includes 40m) filtering is not sharp enough to eliminate nearby QRM, or cut off one of the sidebands. You need to build and use two frequency controls instead of one. The increased RF gain is typically less than whan you would get with even a single additional AF amplification tube. That said, I still think the simplicity/performance ratio is nearly as good as a classic one-tube regenerative receiver!

I do sometimes use my general coverage HF 5678 miniature pentode regen with a 40m transistor-based VFO as a BFO, which makes a nice setup for listening to 40m CW. At some point, I would like to try out using this kind of setup more regularly on the air, and also to try it out on the higher bands with a VXO.