“Regens” are fun! They have been in use since Armstrong invented them in the early part of the last century. Like some other early technologies, they still have a place in the technical sphere, being used in remote controls and other simple applications.
We hams like them because of their simplicity and when properly designed, they can be extremely sensitive. Selectivity is not the greatest but our bands are usually not as crowded these days so this writer has no problem tuning to a single signal. Audio filtering can greatly improve selectivity.
The original Armstrong circuit uses a “tickler coil” close to the oscillator coil, and which provides feedback (“regeneration”) to increase the signal strength. The Hartley or Colpitts oscillators commonly use a tapped coil, with the Hartley being the most simple to build, especially if it is desired to cover multiple bands since the coil can have multiple taps and the tuning capacitor can have a switch-controlled fixed capacitor in series with it, allowing further range capabilities. In the Colpitts design, multiple plug-in coils are commonly used to cover multiple bands which complicates construction.
If an RF stage is included, the regen will not radiate spurious signals from the antenna, and selectivity will be improved.
Many, many designs have been published, using both oscillator types, tube-based or solid-state. A simple 12 volt solid-state design would be helpful in an emergency although it might not survive an EMP burst (a tube unit probably will).
There are kits to buy, or you can roll your own. Given their simplicity, a home-built unit should be easy to achieve. Robust construction is recommended. Nothing flimsy as it is important to prevent any movement of the chassis or components while tuning.
Links to several designs are below. There are many more and a Google search will reveal them but these are very illustrative and educational.
Many designs include audio amplifiers but the audio can also be fed into an outboard audio amp which might be available in your shack. It can also be routed to a home-brew audio filter of your choice.
For CW work, the regen is very good. Its oscillator can be switched to a transmitting RF amp, rather than the antenna when in a QSO, achieving a sort of QSK operation using a QRP transmitter of your choice.
Some links are listed below. The subject is a real rabbit hole when doing a web search – there are a lot of designs and I’m sure you will find one you like.
Here’s a very neatly-built and well documented, simple tube-based Hartley design. The coil & cap values can be changed to cover any desired band or multiple bands. It uses a 12AT7 tube which is quite sensitive and has a high amplification factor. 12AU7 and 12AX7 will also work, although with some loss of gain.
W7ZOI’s Regen web page has a lot of explanations, single-tube examples and is well worth a read.
Here is a link to the AA8V “Twinplex” regen receiver, with excellent documentation. This uses the venerable 6SN7 tube which is a full-sized octal unit, as opposed to the miniature tubes used in other designs.
The octal 6J5 tube is 1/2 of the 6SN7 and makes an excellent RF amplifier front end for these radios. The 6C4 serves the same purpose in the miniature tube designs.
This ARRL link provides a schematic and documentation for a much more sophisticated regen, including an RF stage and an Audio Amplifier. It is a solid state design.
K5BCQ’s design uses 1T4 and 3V4 tubes. It includes detector and audio stages.
This Hartley design is powered directly from the 120 VAC AC line. It uses discarded TV tubes. Be careful to ensure the chassis is not “hot” if you build it.
The transformerless design might also be built using the two stages of a 12AT7 as an RF amp and Detector, feeding two 50C5 tubes used as audio and filter tubes. This hasn’t been explored to my knowledge.
The ARRL design at this link says it’s for “beginners,” but it’s actually quite sophisticated. This writer built one many years ago and was impressed with its performance.
Charles Kitchen, N1TEV, has published an extensive article about regens and presents designs that are easy to build and quite effective.
K9XN presents his version of the “Kitchen” regen and provides yet more links to regen articles.
W7EKB’s page lists more links to regen projects and also to a very nice TRF design.
This one, from circuitsalad.com, features easy-to-construct RF, Detector and Audio stages. The design is very simple and clean. The LM386 audio chip could replace the TDA amplifier if it proves hard to find.
AA5TB provides yet another design. This incorporates an RF amp along with the usual detector and AF stages. His page also lists other links that might be of interest.
This writer built the regen illustrated here about 30 years ago. It worked well and might still be buried in a junkbox. I should look for it.
This design, using a Colpitts regenerative system, includes an S-meter circuit and tunes from 3.5 to 26 Mhz in three bands (requiring three plug-in or switched coil sets.