Hello again friends. My last project was a crystal set. Now I am back to the tube radio construction again. I wanted to try a tube set with the one knob tuning system that I had used on that crystal set. This is not a hard set to build. I imagine that a smart 15 year old could put this together.
The down side is it will help if you have a couple pieces of test equipment around your workshop. That is, a signal generator and inductance meter.
I always wanted to build something with a loctal tube. They always looked very cool with it's mirror top and how the tube would click in to place. I expect to build more loctal sets. While the audiophools are fighting over 6V6GT, the 7C5 which sells for almost nothing. These tubes are for me! The 7C5, according to Sylvania is a 6V6 but in a different package.
Starting at the antenna end, there is a 1000 ohm pot that acts to attenuate the signal from the antenna. It is more or less a volume control. The outside arms connect to a three turn coil (L2) wound on the first tuned circuit toroid coil (L1). This is a FT114A-61 toroid. At slightly over an inch in diameter, it is just the right size to wind the tuned circuit coil. That is, a 36 turn winding made with 165/46 litz wire. The other tuned circuit (L3) has the same winding on another toroid.
Between the two coils there is a FT-50B-61 toroid (T1). This is to couple the two tuned circuit windings. The 3 turns of each coil add to the inductance of the main winding. Some have asked me about putting a loop that passes through both big toroids? Unfortunately that would cause a shorted turn, thus ruining the coil. The third shared toroid is used so there are no shorted windings on the main coils. This is how J.W. Miller made their band-pass tuning systems on their famous Hi-Fi Crystal Radio Tuner.
Speaking of shorted windings, or near shorted is the tickler winding on the second big toroid. This tickler connects to the cathode of the 7C5 and provides the rf feedback needed for regeneration.
Notice the 22 and 12 ohm resistors connected with the tickler. This is to reduce the feedback. Even with the tickler just looped through the toroid, there is too much feedback. The idea with my designs is to have the smallest amount of tickler coil but still will allow the receiver to go into oscillation. This improves the performance of the circuit and provides better volume to the magnetic earphones. You may want to adjust these values to your own taste. The 33 ohm resistor was added to reduce the "shorted turn" effect on the second toroid. I felt the resistor was better than lowering the 22 ohm resistor that is across the tickler coil even further.
Moving along further, most of the circuitry around the tube is very common. The regen control adjusts the screen voltage of the tube, and thus the gain.
The power input is 9 volts ac provided by a wall wart transformer. Why 9 volts? That is what I had. A 6 volt ac transformer would work too. If you use a 6 volt transformer, take out the 6.2 ohm two watt heater voltage dropping resistor. I made the value of this resistor with a 10 and 15 ohm 1 watt resistors in parallel.
The two diodes and electrolytic capacitors make a voltage doubler circuit, which provides around 25 volts dc to power the plate and screen circuits. The 330 ohm resistor plus the 470µF electrolytic provide some extra filtering. Even with this extra filtering, there is a little hum that gets through that can be heard on the weakest stations. Just the nature of an ac operated regen set.
Starting from the bottom up, I used the same type of box as my #76 crystal radio only smaller. The box is 9x6x2.5 inches (23x15x6,3 cm), and was purchased from Bed, Bath and Beyond. I prepared the inside of the box the same way as the #76 project.
Also as in the crystal set project, this one uses a dual vernier for super slow tuning. I did cut the shaft on the inner vernier so that the outer panel would be closer to the calibrated dial.
Speaking of the front panel, I had some special situations to contend with. That is, I had to be able to see the dial markings but I also needed enough space for the four front panel controls. That is why I cut the notch in the center top of the front panel. I followed through with similar angles on the edges to give the front a special look. Since I got my scroll saw and sander, I never make a plain front panel. The smaller white disc in the front is to cover the vernier hole.
The placement of the main variable capacitor is the most important physical detail of this project. Pretty much everything else can be placed where it fits. So I worked on this first. I raised the capacitor so the shaft is almost two inches (50mm) above the chassis. This allows for easy placement of a 3.5 inch in diameter dial.
Everything else gets mounted where it fits. A metal chassis can be used, if you can work with metal easier. If you do, make sure to move the toroids slightly away from anything metal.
The wiring is not real critical. I use old time tie points to wire this project. I don't mind a few extra terminals, but hate too few! I mark the places on the schematic where I need a connection point, keeping in mind where everything will be placed. You may want to make some layout drawings.
I check the coil inductance with my trusty AADE L/C meter. Winding the two coils close the same inductance is important. The main coils in this project are 180µH. My dual gang variable capacitor is 520pF. The radio tunes from 1900 kHz down to 530 kHz. My #76 crystal set uses a dual gang 440pF capacitor. That radio tunes 1700 to 530 plus a little. The lower the capacitor value, the more difficult it will be to cover the entire band. I think that 400pF is about the lowest you could go and still cover the band.
The alignment of the Loctal-Regen is pretty easy. The LC circuit (L3) coupled to the tickler
is sensitive to frequency. The antenna coil only peaks the levels without moving
the resonant frequency. This works out very well and makes the alignment a snap.
It is helpful to have a signal generator for these steps. Set the generator to 1700 kHz and adjust the trimmer connected to the second coil until the signal is heard. Then check the bottom of the band (530 kHz). Adjust this trimmer until you get the full band coverage or the best that it will do.
Tune around 1400 kHz and with the input control turned down at least half way, peak the trimmer associated with the first toroid. I pick 1400 kHz because there should be reasonably aligned through out the top end of the band. The bottom is not that dependant on the setting of the trimmers.
There you have it. Just make sure the tuning range is right before you calibrate the dial.
This radio operates a lot like my other regen sets that use a separate antenna tuner. This radio is nice because of the one knob tuning. You may have to fiddle with some of the component values to optimize your radio, but the values I show will get you close.
73 and happy radio building ~ Dave, N2DS