Jack's One Tube BC-221 Regenerative Radio
Front view of set with plug-in coils for 80M, 160M, and the Broadcast bands. The broadcast band coil in combination with the rather small BC-221 variable cap does not cover the whole band. The 160M coil overlaps the top of the band.
While regenerative sets are a pretty common approach to a one tube radio design, there are a number of design features that can make the set a great performer. A regen set is usually operated near or at the point of oscillation. Small changes in temperature or supply voltage can affect the set's tuned circuit, causing frequency shift in the set. A good approach is to treat the regen set as a VFO and incorporate both mechanical and electrical stability.
-Mechanical stability: the BC-221 Frequency meter is a great basis for a regen set. These WW2 vintage units are built with a heavy aluminum chassis, good shielding, a high quality tuning capacitor, and a 100:1 tuning reduction mechanism with a calibrated readout. When building a regen set in a BC-221 case, all the frequency determining parts of the circuit should be firmly soldered and should not vibrate or flex. Other parts should be firmly held down with appropriate nuts, bolts or screws.
A well shielded set provides both mechanical stability and frequency stability. Hand capacitance can be a problem with regen sets. Shielding via use of a metal front panel and preferably a metal chassis will help tame hand capacitance effects. Toroids also provide self shielding and are useful in these sets.
The tuning knob is an important part of the mechanics of a set. It is sort of like a steering wheel in a car. A quality tuning capacitor with a calibrated dial and a smooth gear reduction can make tuning a joy.
-Electrical/electronic stability: Power supply voltage changes will cause the active device to move in frequency. I like to use a regulated supply, especially for lower voltages. My favorite tube for the regen application is a 12DZ6. It requires almost 200 ma of current at 12 volts for the filament, supplying plenty of electrons for a regenerative circuit. The plate also operates at 12 volts.
The frequency stability will be affected by the choice of components. Air core coils tend to be the most stable, if care is taken with proper form selection and winding techniques. I am a fan of toroids, and the proper core can provide good stability. I have had good success with Amidon products. The feritte 61 mix is fine for the broadcast band and 160 meters. For higher frequencies the iron mixes work well. I use a number 2 iron (red) mix for the 80 meter coil. The number 6 (yellow) or number 7 (white) are even more stable and would be a good choice for 40 meter and 30 meter coils.
For fixed caps, the NPO ceramic are hard to beat. I have a good selection of silver mica caps, so I ended up trying those. They worked fine in my applications, but I would definitely recommend the NPOs above 80 meters.
The choice of a grid leak cap and resistor will affect the circuit performance. I chose a small cap (22 pf) for the grid leak cap. This helps isolate the tuned circuit from the grid of the tube. At 1.0 MHz, a 22 pf cap has an impedance of about 7,200 ohms. At 4.0 MHz, the impedance decreases to about 1,800 ohms.
In the detection process voltage is developed across the grid leak resistor. I chose a value of 10 meg to hopefully maximize this voltage. The resistor can be placed in parallel with the grid leak cap or directly from the input grid to ground.
-Audio output: audio matching is important to get the most out of a simple receiver. A Bogen T725 can be wired as an auto-transformer. A cap in series with the headphones provides DC isolation. A 12 position switch allows matching to the impedance of the phones for maximum signal transfer. A important point about the audio transformer is that it should have enough inductance to work well in a regen set. The Bogen transformer works fine. An RF filter in the plate circuit will help keep RF from "floating aroung". A pi-network consisting of a 2.5 mH between two .001 uF caps works nicely. This does help keep RF out of the audio section (headphones).
You may also find that the combination of low impedance headphones and a low impedance tap on the Bogen will give you a nice low pass filter for CW work This worked out great with my particular phones.
-Regeneration control: There are a number of approaches that will work for feedback (regeneration) and regeneration control. Old issues of QST (from the 1930s) or the booklets from Modern Radio Laboratories are good references. I like the control and isolation that is available by varying the voltage on a control grid. When using this approach, remember that regulated B+ will help stabilize the frequency drift and regeneration operation!
By making the feedback loop from grid to cathode (as in a Hartley oscillator), strong oscillation is easily obtained. The coil in the tuned circuit only a needed a tap or two above the ground side of the coil to sustain strong feedback.
Light coupling into the tuned circuit will help keep the effects of antenna movement (swaying in the wind) from detuning the set. An RF gain control helps isolate the antenna from the tuned circuit. In my version of the set, I used only a one turn link on the toroid for the antenna coupling. I also used a 500 ohm pot for the RF gain cotrol. A 50 ohm fixed resistor in series with the pot also adds a bit of antenna isolation.
-So how well does it work?: This set works great! The RF gain control lets me reduce the intensity of strong signals and prevent circuit overload. The set is stable and works well on AM, CW and single sideband signals (SSB). There are three stages of gain that I have found with the set. Here is an example with an AM station. The first stage is with the regen control turned down. As the control is advanced the signal will get louder. The next stage is by further advancing the regen control so that the Q of the tuned circuit in the grid goes much higher, and the AM sidebands starting getting cut due to the increased selectivity and subsequent reduced bandwidth. In this stage feedback is taking place, but not enough for the circuit to produce a beat note. The next stage is when the set goes into oscillation, and you then zero beat the set to get best copy on the station.
For CW or SSB, the set is tuned to the point of oscillation. The input signal level is adjusted as needed. Sideband cutting can even happen on an SSB signal, similar to AM. In the CW mode, the filtering added by choosing the best audio tap is a great feature.
I have made a number of CW, AM, and SSB contacts using the set as a receiver. It is amazing to me what can be done with such a simple circuit.
Rear view of set
Bottom view of set
Schematic of the BC-221 regen set (right click mouse to choose a larger view)
Schematic updated 11/09/09
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