David's Duodecal Delight —
12AE10 Regenerative Radio
Hello honored visitor! Thank you for your visit. Today's radio is built with a Compactron tube. While the compactron was the big idea of some Wunderkind at GE in the early 60's, it has become the unwanted stepchild of radio amateurs, restorers and audiophools alike. This means they are available very cheap, and hence has grabbed my attention
I used to sell a lot of compactron tubes where I worked. For an old school guy like me, these were somewhat odd, with their 12 pin duodecal tube base. I never thought about building anything with them. I had seen magazine articles that used them to build small one or two tube radios. So now is my turn.
The center of this circuit is the 12AE10 compactron tube. Please download the
data sheet from Frank's tubes. Make sure to drop Frank an
e-mail thanking him for his efforts. As a little tip, I would suggest saving anything you get from Frank's
set to your hard drive. That way, when you want to see it again, you don't use the bandwidth again.
The 12AE10 is a dual section tube. One is a low power sharp cutoff pentode and the other side is a beam power audio pentode. As logic dictates, the first section is the regenerative detector, while the other section drives the speaker.
The circuit is not really unusual. The regenerative feedback is accomplished by a tickler coil in the cathode of the detector. A standard grid leak circuit offers grid rectification detection. The audio is recovered at the plate, after the signal travels through the LC filter. A 25k ohm pot controls the screen voltage on the detector, which controls the amount of regenerative feedback. The operation of this control is very smooth. You will have no problems setting that exact point just before oscillation for maximum sensitivity.
The power supply is a voltage tripler. It starts out with a "wall wart" transformer with an output of 24vac at 650 ma. This is part number ACTX-2465 available from All Electronics. The center tap is necessary to provide voltage for the tube heater, which requires 12 volts.
The full 24 volts ac then goes through a voltage tripler. The voltages ends up to be 80 volts dc under load. I have to tell you that I did have a lot of hum problems at first with this radio. I eliminated the hum by using more capacitors than I had expected. The B+ voltage is lower than I had hoped for, but there is enough to get moderate volume to the speaker. There is moderate hum when headphones are used.
The audio output transformer is the famous Bogen T725. While I had speculated that this would make a nice audio output transformer in small radios for years, this is my first time to try it. The problem is that the Bogen has a solid iron core, rather than a laminated metal core. This means that under moderate dc current flowing, the transformer core would become saturated. I decided to take a chance and I wasn't disappointed. It is probably true that a true tube audio output transformer would work better, this is a good way to go. I used the black and violet wires as the primary connections. This gave me the loudest sound.
The radio is tuned by a 540 pF capacitor. This is available from Leeds Radio. It is really a dual section 270pF with a nice 2:1 vernier drive. That added with the 6:1 vernier reduction drive gives 6 full turns of the knob to tune end to end. You can use a smaller capacitor, such as a 410pF, but the bigger capacitor lets me tune part of the 160 meter ham band.
The main coil, also heavily involved with the tuning of this radio is wound on a HDPE spider coil
form. The outside diameter is 3-½ inches (90 mm). The hub diameter is 1-¾ inches (45 mm).
I used 40/44 litz wire to wind all the coils. There are 51 turns on my main coil tuned with the 540pF capacitor.
Using a 410pF tuning capacitor, you should start with 55 turns. You can trim
as you feel necessary to get the tuning range the way you want it. To provide the best isolation with
the tickler, the inside of this coil goes to ground, while the outside goes to the grid leak circuit.
The tickler coil is on a 2 inch form (50 mm) with 1 inch hub. I wound 4 turns on this form. The tickler is mounted to the main coil at the center. The turns will need adjustment. You want good regeneration control through out the band. It is best to have as few turns as possible and still get the job done.
The antenna coil has 25 turns on a 2 inch diameter form with a 1 inch center. The antenna coil is connected to a 1/4 inch (6 mm) rod that passes though a panel bushing on the front panel. Two grommets (Thanks Mike Peebles) are used to hold the antenna coil on the shaft. Also, if someone starts turning the shaft instead of push pulling it, there won't be any pulling of the antenna coil wires. By pulling and pushing the rod, you can control the amount of rf that is picked up by the main coil. Some kind of front end level control is always necessary on a regen radio. This can be done mechanically, as I have done, or it could be a variable capacitor between the antenna and coil, or even a potentiometer. The number of windings on this coil may need to be adjusted, depending on your receiving conditions.
There is a LED that causes a blue circle to be shown through the white dial when the radio is on. It is not shown on the circuit, but involves a LED, rectifier diode and a 1000 ohm resistor, all wired in series and connected to the power switch. A nice little touch.
This radio is made out of (surprise surprise) Garolite®! It is made the same way as many of my other project panels and chassis. After the panel is connected to the chassis with a pair of small angle brackets, then the hole can be found for the vernier. I put a pointed piece of metal on the end of the capacitor and make my mark on the panel. A pencil connected to the capacitor shaft would also work. Then the vernier holes are drilled. After that, the capacitor mounting holes can be marked and drilled.
Then mark and drill the holes for the controls. After that, the standoffs (made from Delrin at 2 inches long) can be mounted. Those are all the critical holes. Now find room on the chassis for everything else. Don't forget the solder lug mountings. You will need a couple of holes at the front of the chassis so the wires from the controls can pass down under.
My panel is 6-½ x 9-½ inches (16,5 x 24 cm) and the chassis is 5-½ x 9-½
inches (14 x 24 cm). The reason for this size is that the Garolite® comes in 12 inch wide sheets. The
width was made to match the piece of wood that I had previously finished.
Be prepared to reverse the tickler winding leads if the radio fails to go into oscillation. Myself, I just connect them and see if it is right or not. I seem to get it right around 50% of the time.
This is a great project for the intermediate builder to make. I caution you that the 80 volts, while not particularly dangerous, can cause discomfort if you grab on to it. Make sure the capacitors are discharged before working on this set.
The performance is good with moderate volume. It sure isn't your kid's ghetto blaster, but will give you hours of listening pleasure on a radio that you built!
73 and happy radio building ~ Dave, N2DS