Subminiature Tube Regenerative Radio
Here is a little weekend project you might want to try. This is a regenerative radio covering the MW range (530-1700 khz). The radio centers around the use of a 6418 sub miniature tube. These little tubes came on the scene about the time that transistors appeared. At the time, transistors couldn't handle much power or high frequencies. So this tube became the in between step between regular tubes and transistors.
The 6418 is very well suited to the hobby builder. The filament power is very low, 1.25 volts at 10 milliamps! This is 20% of the current that a 1T4 draws. Also this tube is meant to work on B+ voltages less than 30 volts. In the old days, this might have been used in a hearing aid with a mercury cell filament battery and a 22.5 volt battery, perhaps an Eveready type 505. Since these type batteries are no longer with us, I chose a D cell and two 9 volt batteries as my power source. A smaller AA cell can also be used.
Since the tube filament wants 1.25 volts, I used a 22 ohm dropping resistor in the A- lead.
I used a 6 point terminal strip that I had in my junk box to provide a place to solder the tube leads to. I arranged the leads so the filament was on the two lugs that bolt to the panel. This was not done by accident. These provide outside test points to check the voltage across the filament. Since one of the earphone connections is directly to the 18 volt source, I could take advantage of this as a B+ test point. I show the test points in the picture below. This is not a big deal but shows how I try to take advantage of everything that comes along.
This tube has 5 wires. Notice that there is a red dot on one side of the tube near the wires. This is pin 1, or the plate lead. Pin 5 (Filament minus) is at the other end.
The antenna is coupled through a 7-45 pf capacitor to the tuned
circuit. The adjustment of this capacitor will depend on your antenna and
any local strong stations that could overload the radio. Presently I have
the trimmer set to maximum. It will be turned back if found necessary to do so.
The coil is made from 40/44 litz wire. Litz is recommended in this project as it will give you a little extra selectivity over regular magnet wire. The main winding is wound on the spider form first and then the tickler winding. The outside of the main winding is connected to ground. This provides a little bit of tickler isolation.
The detection is done using a grid leak resistor and blocking capacitor (along with the tube itself). I found 2.7 meg and 100 pf to work well. This is not real critical. The regeneration control adjusts the voltage on the screen grid. Myself and others really like controlling the regeneration of tetrodes and pentodes this way.
The 27 mh choke is to block the rf to the headphones. This also isn't critical. I have a lot of these chokes around from my crystal set projects. I tried one "just for the heck of it", and found it to work very well.
I put a 47k ohm resistor across the headphone connections so that a crystal earphone can be used. If only magnetic type headphones are to be used, this can be left out.
This turned out to be a sweet little radio. There aren't a lot of bells and whistles here. A vernier drive between the capacitor and the tuning knob would be a great help. The tuning at the top end is a little tedious, but not impossible.
This project is recommended for the beginner. Nothing is real difficult and the radio should work right off the bat. If the set doesn't go into oscillation when the regen control is turned up, reverse the tickler coil connections. The voltages used are very low, so you can build a tube project without the fear of getting a shock. This could make a good shortwave radio, if a metal chassis is used along with a band spread capacitor. You are only limited by your imagination.
Happy building de N2DS!
PS Take a look at this radio built by Bram van Zoelen based on the design of this radio.