The Dual Triode Shortwave Regenerator
What? Another radio? So soon? When the stock market goes south, I build more radios. Keeps my mind occupied. This radio started differently than it ended up. I had lots of trouble with this set. It wasn't working up to my standards. I had to keep at it. Oh well, fringe howl happens.
The basis of this design was to build a two tube shortwave receiver. I wanted a low gain rf amplifier to proceed the detector stage. This was to isolate the detector from the antenna. This has a two fold advantage. The antenna swaying in the wind won't affect the tuning. Also, the energy that the detector emits won't make it to the antenna, due to the one way valve action of the tube. Don't want the DHS breaking down my door whilst I'm listening to the BBC. This radio tunes from 3.2 mhz to about 10.5 mhz. This covers the 80, 75, 60, 40 and 30 meter ham bands, as well as the 90, 75, 60, 49, 41 and 31 meter shortwave bands. This makes a suitable night time receiver.
Looking below at the schematic drawing, I will explain the circuit and why the heck I did things that way! This radio was built to run on a 12 volt ac wall wart transformer. These are available on the surplus market, and as the Frugal King Schmarder sez: "Build em Cheap"!
The 6C4 tube heaters are wired in series and with a diode and small filter capacitor, receives it's 6 volts per tube. I had straight ac feeding the heaters, but there was some hum that appeared. Using a diode and large filter capacitor would have brought the voltage to about 15 volts dc, or 7.5 volts per tube. By using a small capacitor, I am able to do most of the filtering but the peak voltages aren't as high. If you want to use a big filter, add a resistor to drop around 5 volts.
The antenna is connected to the arm of a 1000 ohm pot via a small coupling capacitor. The pot is the cathode bias resistor of the first 6C4 tube. This amplifier operates as a ground grid amplifier. This has a low gain and is very stable amplifier. Remember, this tube is here more for isolation than amplification.
There is an rf choke on the plate to keep the rf from traveling to the power supply. Now, here is something important: The capacitor that connects the rf stage to the detector has to be a very small value. I first had a 12 pF capacitor in the circuit and bad things were happening in the detector. The capacitance on shortwave only needs to be a few pico-farads. I decided to use a "gimmick capacitor". This is just two pieces of wire twisted together making a very low value capacitor. I used some wire wrap wire as it is easy to handle and small. Use what you have. Adjust this for the best regeneration, while having enough signal at the low end of the dial.
Although not shown in the circuit, there is another small gimmick capacitor across the main variable capacitor. This is one of those fine tweaks that I do on my radios to adjust the tuning range. I cut about a half Megahertz of unwanted tuning range by doing this. The top end of the band is just above the 30 meter ham band. Nothing squawks and burps until the 25 meter band anyway.
The coil is wound on a small ceramic coil form with a slug for adjusting the tuning range. It is around 1/4 inch in diameter and has nearly 40 turns of small magnet wire to provide the 9 µH coil value. The tickler coil is wound directly over the main coil. About 6 turns is enough. Many use toroids with great success and this may be the route you want to take.
This is a grid leak or a grid rectification detector. The high value resistor and small parallel capacitor is the circuit giveaway. Grid leak detectors were very popular in the 1920's. The detector gets a big signal boost from regeneration. Edwin Armstrong found in 1912 that a detector could be made very sensitive by sending some signal from the plate back to the grid input. This changed the whole radio scene. It was also the basis of the multi decade lawyer full employment program.
The regeneration is controlled by varying the dc voltage applied to the plate of the detector 6C4. This is done with a 10k ohm voltage divider. Between the plate and the regen pot is the famous Bogen transformer. I added a single pole, 9 position rotary switch to allow for output impedances from 8 ohms to 20k ohms. This means that the more comfortable earphones, such as the Koss Sparkplug types can be used for listening. In my case, I use my trusty ole sound powered headphones.
This radio is built the way that no shortwave set should be built. That is on a plastic (HDPE) chassis. This chassis is 9½x6 inches (24x15cm). The panel is made from Garolite® which is 9½x4 inches (24x7,5 cm). It would be better if the chassis and front panel were made from aluminum. The radio would have greater stability and free from hand capacitance effects. Although, I must say that this radio does not suffer greatly from these problems. Maybe someday I will find someone willing to do a little metal bending for me for future projects.
The chassis is held up by four plastic standoffs. They are 4 inches long, which
gives a ¼ inch (6 mm) clearance to the wood base. I use red oak. It is my
favorite wood. It is fairly cheap and stains well.
The only special thing about the layout is to try to keep the rf leads as short as possible. Notice that the coil is very near the variable capacitor and that is close to the detector tube.
One nice feature of my little radio is the s-l-o-w tuning rate of this radio. This is accomplished by using two vernier drives in tandem. This gives me a 36:1 rotation reduction, or 18 full turns of the knob, end to end. This eliminates the need of a separate band spread capacitor. If you don't want to do the vernier thing, just add a 10 to 20 pF capacitor in parallel with the main tuning capacitor.
This radio will pick up AM shortwave stations and CW and SSB transmissions, mainly from ham operators. The antenna and regen control act together, especially with SSB and CW transmissions. Keep the antenna control as low as possible and adjust the regen control until everything sounds right.
This is a nice little receiver, once I got the many bugs ironed out. It was well worth sticking with the project. I learned a few things about these sets on shortwave. I hope yours turns out well too
73 and happy radio building ~ Dave, N2DS