12L6 One Tube DX Radio
With the 2008 1AD contest coming up soon, I had to throw something together. I still may use a different receiver as I have a couple more months to put on my thinking cap. (It also keeps my head from getting cold.) You have my permission to first look at all the pictures and the schematics. I'm excited about this one too!
This is one of those projects that started about two years ago at a local hamfest flea market. A fellow was selling a lot of tubes. I saw ten delicious 12L6 tubes, in their boxes setting there saying "buy me, buy me". I couldn't resist. I knew that the 12L6 was a nice beam pentode power amplifier tube. I had been building my 1625 tube radios for a while and I felt it was time to offer a different project.
In the 2007 1AD contest, I used a one tube radio with low plate voltages.
Regenerative radios work very well for me. I don't have any local flamethrowers, only a 400 watt BIC down the road about 10 miles. There are other annoyances such as the whistles and no real accurate frequency readout, such as what I have with my 1 Tube Superheterodyne Radio. The regen still makes a great contest radio.
My 12L6 regen set is a standard design. The antenna tuner has one small design quirk. That is the 20 pF trimmer capacitor across the coil. The dual gang variable capacitor I chose to use is only 360 pF per section. That is a very minimum that can be used and still be able to tune the entire band (530-1700 kHz). The antenna and ground are also part of the circuit. I added a little trimmer so that the full range of the band can be fine adjusted when using different antennas. (One small trick if the range isn't wide enough is to put a 100 pF trimmer across the A-G connection. This will give a larger range.
The detector uses a tickler coil for regeneration. This is a small coil placed near the main coil to produce some RF feedback. The feedback intensifies the signal for easier detection. Tickler coils are most of the time placed in the plate circuit, but work equally as well in the cathode to ground path. Since this tube has pretty high gain, I found that the tickler needed only 4 turns to work well.
The detection is possible partially due to the grid leak resistor and capacitor. These values are not critical. A 2.2 to 3.3 megohm resistor is fine. The capacitor can be between 100 and 200 pF In general, if you increase the resistor, you should decrease the value of the capacitor. Please refer to an excellent technical article on the Modern Armstrong Regenerative Receiver by Ramon Vargas.
The amount of regeneration is controlled by adjusting the screen grid voltage. A 5k ohm pot does this nicely. I added a 3.3k fixed resistor on top of the pot to give a better spread of the useful regenerative range.
The detected audio is recovered at the plate of the tube and is passed through the Bogen T725 autotransformer. The set will work by connecting high impedance headphones directly to the plate - B+ line, but I found the transformer improves the radio.
The headphones are connected via a 3 position rotary switch. You can choose the taps you want to use by experimenting. More information about the Bogen transformer can be found on my T725 page.
I drew the tube as a tetrode but in fact it is a beam power pentode style tube. Since no additional connections were needed, I left off the beam plates in the drawing.
My radio is built on a chassis consisting of a Garolite® panel that is 9-¼ inches wide (23,50 cm) and 6 inches tall (15,24 cm). The chassis base is made of HDPE that is 1/4 inch (6 mm) thick and 6 inches deep. These sizes were selected according to what would fit on my oak wood base, as well is the best use of the materials at hand.
The three coils are wound on HDPE forms that are 1/8 inch (3 mm) thick. I use 165 strand, 46 gauge litz wire for the larger coils. The tickler is wound with 40/44 litz. Your choice of wire is up to you. I do recommend the litz.
High performance capacitors aren't necessary in a regen set. The ATU capacitor is a dual 360 pF, but I would recommend that you use a higher value so you can be sure of tuning the whole band. I used a 540 pF (dual 270 pF) tuning capacitor. You can use as low as 410 pF with ease.
A drive reduction (vernier) is really necessary. I build very few radios without them anymore. It is real hard to divide 115 channels into a 180 degree rotation. I use a 6:1 vernier drive. That will give you a 3 full turn tuning. Since the surplus capacitor I used already has a 2:1 reduction, I get 6 full turns and a very large dial spread. Sweet, very sweet!
There are two posts with thumb nuts on the ATU for the antenna and ground connections. There are two more connections on the detector unit for connecting your 12 volt dc supply. I use a wall wart that is rated for 12 volts at 1 ampere. The tube draws 600 ma, so that is a good reserve. Wall wart supplies are not generally well filtered, so I added an extra 1000 uF filter capacitor isolated by a 1N4001 diode. The diode isolates the extra filtering from being bogged down by the tube heater. You can use 12 volts AC to power this radio too. You may want to increase the 1000 uF capacitor to 2200 uF if you decide to power this radio with ac.
There isn't much alignment to do, except for a possible coil adjustment. A signal generator and portable radio make a good way to adjust the ATU coil. The portable radio is used to tune in the signal and listen for a change in the level when the ATU is resonant. It is important to have the antenna and ground connected as this is part of the circuit. Check both ends of the band and make sure you have full band coverage. If the bottom of the band tunes easily, but not the top end, you can remove a turn or two off the coil. The beauty of a spider coil is that you can unwind a turn and leave it strung out on your bench. Then you can see what the range will be before you cut the wire.
The detector unit coil is adjusted in about the same way. For this, you need
only a signal generator. The ATU can be moved away. It is handy to calibrate your
dial in kHz for easier tuning. 0-100 doesn't cut it for me anymore!
You can start with 8 or 10 turns on the tickler. The amount of turns depends on how far the tickler is away from the main coil. Mine is 1 inch (24 mm) distant from each other. When your radio is working, adjust the number of turns until the radio goes into oscillation at 1/3 to 1/2 the rotation of the regen pot.
Connect your antenna, ground and headphones and turn the set on. The ATU and detector have to be set to the same frequency. The regen pot is adjusted to the point at which the radio is almost oscillating. There is some interaction as the ATU is tuned. At resonance, the Q of the detector coil is reduced and the right might fall out of oscillation. Not too much to explain about Major Armstrong's wonderful invention.
Happy building de N2DS!
Views Of My 12L6 DX Radio Detector Unit
12L6 DX Radio Antenna Tuning Unit
I modified my 12L6 DX radio for use with a one turn loop antenna. This is handy if there is no room or ability to erect an outdoor antenna. You can still enjoy this radio! Using the loop means you can set the antenna tuner section of this project aside.
This modification is easy to do. A simple loop can be built from two pieces of wood, plus another for the base. My simple loop has a 35 inch (89 cm) stick and a 29 inch (74 cm) stick. The dimensions are not very critical. For signal pickup, bigger is better. Wind a single turn around the loop and you are ready.
As shown on the schematic below, I broke the connection between the bottom of the main coil and ground. This is the best place to insert the loop. Luckily the dial calibration stays close to where it was without the loop.
That is about it. The one turn loop modification doesn't give you a super sensitive radio. It isn't even as good as my Russian Tube Loop Radio that I just built. But the reception is still decent. A couple of terminals can be added for this loop along with a shorting bar for use as a dx radio. It is my way of finding more uses for one radio. Good luck with your project