The Hikers One Tube Radio
Ok kids. Put down your Ipods and pay attention! Today we are going to take a hike into the past. In the 30's, people took hikes. When they got where they were going, they wanted a little entertainment. Some took along a little homemade radio that looks something like the project on this page. The original Hikers One radio had one tube. A flashlight cell was used to light the tube and 4 penlight cells provided the B+. Only six volts? Yes. This can be done if the tube is operated in a space charge mode. I won't go into big detail here, but by putting a little positive bias on the first grid and using the second grid as a grid leak detector, only a small amount of plate voltage is needed. The best site on the internet for space charged tubes is at Jeff Duntermann's 12 volt tube page.
Ok, now to my little project. I had this on my to-do list for a couple of years. Finally it is a reality! The radio operates well. Before I built my Hikers, I looked over all the information that was available to me. Modern tube radio builders such as Mike Peebles provided me with a bunch of schematics and plans, including one of his own. Take a look at his more recent page about the Hiker's Radios. Otherwise, my Google searches didn't turn up much else.
I started my project by making the chassis and front panel. These are made from Garolite®, a tough and old time looking material. The chassis is 7-1/2 inches wide (19cm) by 5-1/2 inches deep (14cm). The height of the front panel is 4-1/2 inches tall (11,5cm). The chassis size was based on the size of the wood base I decided to use. After these pieces were made, the first part to be mounted is the variable capacitor and vernier. They should be centered, and then the other parts are sprinkled around where they will fit.
The coil inductance is about 175µh. This coil was wound and tweaked with the value of the other components, especially the variable capacitor in mind.
My variable capacitor is a dual section 270 pf with the stators tied together to make one 540 pf capacitor. I can cover the entire 530-1700 khz broadcast band with this capacitor and coil. I like this capacitor and thought it would work well in this little radio. If you only have a smaller capacitor, you may not cover the entire broadcast band. But if you do the following, you can get by with a smaller capacitor. First, couple the signal with another coil, instead of the 5-50 pf trimmer capacitor. Also reducing the value of the grid leak capacitor as much as possible will help. By increasing the value of the grid leak resistor, the grid leak detector will still work properly. A single gang capacitor usually has a lower minimum capacitance than a multi gang type.
Getting the regeneration control to work properly took a fair amount of tweaking. In the end, I arrived at what is shown on the schematic as the best. Placing a variable resistor across the tickler coil is not the best way to control regeneration. Many times the full regeneration control is over a small portion of the variable resistor. By placing the entire resistance of the pot across the coil and connecting the fixed throttle capacitor to the pot arm, gave me the best regeneration control. This was tried after making several other attempts of taming this beast.
The battery arrangement is different than the original Hikers radio. I use a single D cell to light the tube. It was recommended to me that the full filament voltage not be applied to a tube in space charged mode. The next battery is a double-A cell. This is to bias the control grid. It is a positive bias which causes the tube to operate as it does. The last battery is a 9 volt block battery. I thought that a 10.5 volt plate voltage would be what I wanted. I'm sure that swapping three double-A batteries for the 9 volt battery would work fine.
This is one project that just took off and started working right away. One gamble is how the tickler coil is wired. It seems that there is a 50/50 chance of getting the connections right. If the radio won't go into oscillation, try reversing the tickler windings.
My Hikers radio works about the same as any one tube battery powered regen set that I built. The nice feature of this set is the low battery voltage requirements. I hope that me showing you my project will encourage you to make one for yourself. Feel free to change the design and looks to suit yourself.
Happy building de N2DS!