Acorn Tube DX Receiver
This kind of looks like the last tube radio I built, doesn't it? I plead guilty here. Hope you aren't disappointed. My last radio was very successful. In the first week, I was logging stations as fast as I could identify them and write them in my log. The use of large litz wire dual coils, and Bogen transformer made that one tuber into a killer dx radio. Can I improve on what is near perfection? I don't know, but I can try.
My new radio uses a 954 acorn tube. I like acorn tubes. I never built anything with acorns until last year. They have a lot of eye appeal. Acorn tubes were developed in the early thirties to quench the thirst of people that wanted to use the ultra shortwave frequencies. They never seemed to make it into the medium wave area. In an old QST magazine from the late thirties, there was a discussion on how tubes could destroy the Q of coils by the grid circuit loading them. This seemed to be more of a problem 14 mhz or higher. I extended the thought down to the medium wave broadcast band. I felt justified in this as the Q of the coil I was using would be so high that it would be sensitive to any loading. It was found that the acorn tube was superior as an rf amplifier. Since grid leak detection requires a small amount of grid current to flow, this loading could be much worse, with a 6K7 type tube, for example.
The circuit is a straight forward regenerative radio for the most part. The main coil is a big difference between this set and grandpa's golden oldie. The coil is made with 660/46 litz wire. The coil form is styrene and has a 3 inch (7.6 cm) hub. The outside diameter of the coil form is 7-3/4 inches (19.7 cm). There are 41 turns of wire on the coil making an inductance of slightly over 250 µH. If you have a greater than 440 pf tuning capacitor, the coil can be reduced to 40 turns and still be able to tune to the bottom of the band.
The tickler coil has 10 turns of 40/44 litz. This is not critical, The
hub diameter is 2 inches (5 cm).
The antenna tuning coil has 33 turns of 660/46 litz wire on a spider form with also a 3 inch (7.6 cm) hub. The outside diameter is 6-1/2 inches (16.5 cm). Details of the antenna tuning unit are described on my #50 crystal set page. If the big litz is unavailable, smaller litz will work alright too.
I left the "Hobbydyne" circuit out of this set. Regenerative radios, due to the positive feedback system tend to "multiply the Q". With 660 litz, the selectivity is already very decent. Also with my attempt at lower grid loading, I felt the Hobbydyne advantage would be minimal.
I attempted to keep the coil loading down by increasing the value of
the grid leak resistor. I also decreased the value of the capacitor below
what is normally used. The detection is still very good.
The output circuit uses an auto-transformer to achieve a better impedance match to the headphones. I used a 3 position switch to select the most likely transformer taps I was likely to need. This switch is mounted next to the headphone jack and only needs to be changed if you are switching to another set of headphones.
Also on that board is a high impedance jack for connecting to an
external audio amplifier. Regen sets sound pretty nice through an amplifier.
That little piece of Garolite® has another purpose. That is to keep the high voltage connections from being touched. All the components relating to the 180 volt supply are under the shield. The only other high voltage connection is the plate connection. The connector is fully covered by heat shrink tubing. Safety is an important topic and you should always make your sets as safe as possible.
The operation is like a standard design regen set. The main difference is you will want the coils to be nearly 2 feet (60 cm) apart. This keeps the coupling of the coils to a minimum. If your set is just into oscillation, by tuning the antenna tuning unit, will cause the receiver to fall out of oscillation. That is because the resonant circuit from the antenna is loading the main coil a little, causing the radio to stabilize.
The variable capacitor should be of high quality with ceramic insulators as a minimum. If you can find a "straight line frequency" type variable, your tuning at the high end of the band will be spread out better. The capacitor in this radio is a capacity linear type. The use of a vernier dial drive helps the tuning a lot. Don't build without one!
The power supply required is 180 volts dc at 15 ma and 6.3 ac volts at a half an amp or so. The power supply I built is described here. This project should not be attempted by people that are beginners. The voltages described here can hurt you. Beginners should build a battery operated version of this set.
Good dx de N2DS!