Variocoupler Radio With A Subminiature Tube
Here we go again! I made another little radio using those cute little 6418 tubes that I am trying to unload. Oops, I suppose I shouldn't have said that. But if they are good enough for me to build, they are good enough for you too. :)
After building a few radios with these, such as my first 6418 radio,
and then my loop radio, I really fell in love with these little tubes.
This time, I wanted to do a mechanical regeneration control set up, like in my
first tube radio. I doubt that this will be my last 6418 radio.
This little tube, besides being really tiny, only draws 10 mils of filament current at 1.25 volts. The maximum plate voltage is 30 volts, but they work well at lower voltages.
For visual interest and for easier operation of this radio I use a vernier drive. The one I used also provides some eye candy because of the old 1930's dial look. The front panel look is followed through by the use of Garolite®, and the old time knobs and power switch. Although some modern parts are used, the general old time look prevails. Now lets get started
The circuit is a very standard regenerative detector that has been in use since the early twenties. The signal comes via your antenna to the antenna input coil. There are several taps on this coil to compensate for the length of your antenna. The total turns is 35, and the taps are at 10 and 20 turns. The tap you choose depends on several conditions, including your antenna, how loud the local stations are coming in as well as the part of the band you are tuning. In some situations, you might want to put a small value capacitor in series with your antenna lead. A3 works best at the low end of the band or when the highest sensitivity is needed. If you place your coils further apart, you can increase the number of windings on the antenna coil
I like using a separate antenna coil because if there is no antenna coupling capacitor connected to the top of the detector tank, the tuning range is wider. This is because the ratio of maximum to minimum capacitance in that circuit is greater.
The feature that stands out most about this 6418 tube radio is the mechanical adjustment of the regeneration. Regeneration is a method of coupling a little bit of rf energy that is received by the tube detector (in this case) and sending a small portion of that signal back to the input to be amplified again. This is usually done by placing a coil in the plate circuit of the tube detector and allowing the field form this coil to be near the input coil. But the amplification must be controlled. There are several ways of doing this, but I chose in this radio to move the coil so the coupling is varied. This adjusts the signal feedback. As the coils are placed in phase, the amplification increases. As the coil is turned, the coupling is reduced and can even go into an out of phase condition. The best adjustment is when the radio is nearing the point of oscillation but not quite there yet.
Power for this little radio is from two batteries. First the filament power is supplied by a single AA cell. The power switch breaks the filament connection. Since there is no dc path in the plate and screen circuit when the tube is off, no other switch is needed.
There is a small 22 ohm voltage dropping resistor in series with the filament. This is done to "lose" a quarter of a volt and make the battery voltage drop to what the filament is happy with seeing.
The B+ voltage is supplied by a single 9 volt battery! When built this radio I used two 9 volt batteries. The volume to my headphones was very loud and I didn't like the regeneration control I was getting. Dropping the voltage to 9 volts made the set work better. I measured the current used by the radio plate and screen circuits. It was 0.0002 amperes. That is 0.2 ma folks. That 9 volt battery is going to last a long time!
The tube is an old 1950's type of sub miniature tube called the 6418. This is a pentode tube (My diagram does not show the third grid as it connects internally to the filament. This keeps the view simple). Other battery tubes, such as a 1T4, 3S4 and many others can be used as long as the filament voltage is correct. A second or third 9 volt battery may be necessary.
For those of you that have been following my receiver build progress, you will not see many changes here. I found a good building method that is easy to make and looks great. The chassis an panel are made from 1/8 inch (3 mm) thick black Garolite®. Both pieces are 8-1/2 inches (21,6 cm) wide. This size was selected because it matched the width of my oak wood base. The width of the two panels is 6 inches (15 cm). This is because my Garolite® stock is 12 inches wide. Nothing scientific here friends. Three small angle brackets are used to tie the two pieces together. A curved top makes the radio less bulky looking. I made the curve by measuring down each side a distance I think would make the project look good. I also mark the center of the panel. Then a french curve is used to find the curve that I want. I use the curve from the center to one edge. I then flip the curve over and mark that side. That way the curves stay the same on both sides.
The antenna and ground connections are made by using thumb nuts. The coils are made from HDPE. I use 40/44 litz wire for the antenna input and tickler coils. The tickler coil should be made with litz because it will stand a lot of flexing as the knob is turned. The coil is quite small and has 25 turns of litz on it. This is about right. 20 turns was too few turns. The number of turns that work the best depends on the distance the two coils are separated. Changing the antenna input tap also has an effect on the regeneration.
There is a detailed picture of the ticker mechanics shown below. A panel bushing is there so the shaft is held as it goes through the panel to the knob on the front of the radio. Behind the bushing is a shaft lock. This keeps the tickler shaft from wobbling. The locking nut is only slightly tightened. The HDPE coil form has holes drilled through it in 4 places. I used some cable ties to attach the shaft to the coil form. Before I did that, I filed a small flat area where the shaft would rest on the coil form. All this makes a very nice way to keep the tickler coil steady. The shaft lock was added after the main pictures were taken, so you can see the "before and after" pictures.
The main coil uses 165/46 litz. I like using this size, but 40/44 properly wound would also work fine. Magnet wire can also be used for the antenna and detector coils.
The first thing I do when wiring is to use a piece of masking tap to indicate the locations of the tube wires. I also mark if the pin is a B+ point or ground. This keeps the confusion down. My method is shown in the bottom view picture. It keeps me out of trouble.
When I wire these little sets, I like to wire all the ground connections first. Then I do all the B+ wiring. After that is done, then the rest of the wiring is done. I do the soldering each time all the wires are connected to a point. The exception is the tube. I did all the soldering to the terminal strip that the tube connects to, but I made sure to leave a hole to insert the tube lead. After putting enough insulation (spaghetti) on three of the 5 wires, I then wire the tube to the terminal strip, watching the lead dress. I like to have the tube stick above the chassis for a visual effect. You will never see it lit because of the extremely low power the filament requires.
There isn't much alignment for this radio. Make sure you can tune the entire band. The 410 pf variable capacitors make this easier than the usual 365 capacitors. The detector coil as wound is about 225 microhenries. If the radio doesn't tune the full band, perhaps you can remove a coil turn, or add one, or use a trimmer capacitor across the detector coil.
The operation is pretty easy too. Hook up the batteries, antenna, ground and headphones. Turn the power and by adjusting the regeneration control and main tuning until you hear your favorite station. Occasional switching the antenna coil taps will help improve reception. If you get a lot of overloading, try reducing the size of the antenna, place a small capacitor in series with the antenna or use a wave trap.
I hope your project turns out as well as this one did. Happy building de N2DS!