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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. 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!
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 have 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.
Good dx de N2DS!
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