The Regen-o-flex Receiver
Welcome to the fifties! This radio is much like the little regenerative radios that Lafayette, Knight Kit and Heath sold those many decades ago. The circuit is a different than those I've have seen before. I want to thank Frank, K5DKZ for putting the circuit for this set on his web site. (Update: Unfortunately the project is no longer on his website.) When I saw the circuit, I knew immediately that this is the one for me.
The circuit is built around a 12AT7 and a 6AK6 tube. The 6AK6 is a conventional audio tube. There is nothing really special about this section. The detector is a different story. The first section of the 12AT7 is used as a ground grid rf amplifier. This provides a better impedance match between the antenna and the detector and also provides isolation so the regenerative detector doesn't bother anyone else's reception. (A regenerative detectors that is oscillating is a small transmitter.)
The signal then goes to a grid leak regenerative detector. The coil is tapped with the tap going to the cathode of the second half of the 12AT7. The regeneration is controlled by adjusting the B+ voltage on that stage. The audio is recovered at the plate of this section.
Here is where it gets interesting, folks. That audio signal is fed back to the grid of the first stage. This stage is grounded to RF but not to audio! This first part of the 12AT7 now acts as an audio amplifier. This is called "reflexing". This technique was popular in the first days of radio as a means to obtain more amplification without extra expensive tubes or more battery power.
Ok there is the theory. Would you like to build one of these also? Read on, brother. The chassis is made from aluminum and measures 9 x 7 x 2 inches (22.8 x 17.9 x 5 cm). The front panel is 9 x 7 inches (22.8 x 17.9). I used Greenlee chassis punches to make the larger holes. After mounting the components, it was time to wire the set. The wiring only took me a few hours. (Drawing the circuit diagram that appears on this page took me longer than it did to wire the radio.)
Once I get started, I tend to work through until it is finished. Having made a test circuit with the detector, I was sure that it would work in the finished unit. That is if I wired it properly. I turned it on, using the coil from my test setup. As soon as the tubes warmed up, I was hearing stations. Success!
Making The Coils
After I finished patting myself on the back, I started making the permanent coils for this radio. To figure out the values of this coil, I measured the value of the tuning capacitors and associated circuitry with my AADE L/C bridge. I found that the capacitance range, with the band spread capacitor set at mid range was about 65 to 310 pf. Using Professor Coyle , I figured out what inductance the coils should be and how many turns of wire were needed for each one. Below is a coil chart. I will be buying more of these nice coil forms from Peebles Originals to complete my set.
I tweaked this set in the last week. If you compare the two circuits, you can see what I have done. I removed a 180k ohm resistor that went back to the grid of V1a. This only cut the gain. This is not a loud radio, so all the help it can get is welcomed. I added two rf chokes. One in the plate circuit of the first tube. This greatly improved the regenerative qualities of this radio. Another choke and electrolytic capacitor in the cathode improved the audio gain a little. Not by a lot, so you can leave those out if you want.
I put in a little 100 pf capacitor just past the rf choke on the way to the volume control. There was some rf voltage that was causing some distortion problems when the volume was nearly at full level. I added a .01 uf capacitor across the B+ for bypassing unwanted signals riding on that line. Just some good housekeeping.
Besides using plug in coils instead of a band switch, the last change I made was to include an rf gain control. This is really needed if you intend on using this set for cw or ssb reception.
This set works pretty nice. This is the best regen set I have built. I highly recommend the circuit but I want to remind you that these voltages, while not especially high can hurt you. This project is recommended to intermediate level builders.
One of the confusing issues I have encountered is what is the best grid leak resistor and capacitor values to use? Right now my network stands at 1.2 meg ohms with a 50 pf capacitor across it. From what I understand, it is better in theory to have a very high grid leak resistor. In practice, I have found that the higher values introduce an audio hum. So that is the reason for the fairly low 1.2 meg ohm resistor. I had a 100 pf capacitor as the capacitor value but found a 50 pf worked a little better. The regeneration pot had a wider range from no regeneration to the squeal. That is the range that the control will always be at. I welcome comments concerning this topic. It is not hard to change the circuit drawing.
My special thanks go to some of the people that helped me with this project. Frank Kamp for posting the original circuit idea. Ramon Vargas of Lima Peru for ideas on perking up this set. Also Mike Peebles and Fred Wise were standing beside me through this. I dedicate this page to these fine people.
I found the original article from Popular Electronics at a site online. The article starts at page 59. It is a PDF file. American Radio History has lots of other free old magazine downloads. It is interesting that after I finished my project, running across the entire article and comparing my work with the original article.
Good dx de N2DS!