The Hellenedyne Regenerator
Hello friends, neighbors and all the ships at sea. dit-dit-dit-dah. I bring forth news of a new tube radio design. It isn't my design, but some of the so called refinements can be blamed on me.
Not too long ago on my favorite forum, The RadioBoard, Phil Washere introduced some radio designs. One in particular, the Hellenedyne caught my attention. Here was a simple way to build a radio. Not a lot of parts and a claim of high performance. This is a shortwave radio. Since the unfortunate end of the cold war, and big budget cuts,all the good shortwave broadcasts have disappeared. That's not to say that the MW band in the US is that wonderful either. But since most of my sets are medium wave, AM band style, I kept with the tradition.
I'm not going to go into a lot of circuit detail as there is no way I can top the explanations in Phil's article. What I will do is explain what I have added to this wonderful design. Phil used a 3GK5 tube and mine has a 6GK5. Actually mine is a 6FQ5A but it is all the same. Since I am running my sets on AC power, I wanted to get the heater voltage as close to my supply voltage. BTW, the reason this radio works so well is because Phil spent hours looking at different tubes. This one was the best he could find, in keeping with a reasonable price too. His article explains.
I had two ideas on a power supply. First is a 12 volt wall wart (or even a large battery). The voltage of a wall transformer rated at 500 ma. is around 14 volts. This is a few volts less than the original design. At this point, I haven't actually built this version yet, but Phil is comfortable with the voltages.
The design that is shown in the pictures is the 9 volt ac input. This was the closest wall transformer that I could find to 6 volts. I used a 22 ohm at 1 watt dropping resistor to power the tube heater. The B+ voltage is produced though a voltage doubler circuit. The no load voltage is about 2.8 times the input rms voltage. This means that 27 volts is provided to the plate circuit. This is just below the 30 volt so called safe voltages. I like safe voltages!
I tested this radio with a 6 volt AC input, with the 22 ohm resistor removed and the radio worked fine. The B+ is only about 15 volts, but that was not a problem.
The other main area of discussion is how I bring the signal to the receiver. Phil uses a toroid with three windings, the main winding, tickler and untuned antenna input. I decided to go the same route as my 1AD superhet radio. That is, to use one of my crystal radio tuner projects as a front end. The volume can be controlled by moving the two units closer or further apart. The disadvantage is there are two knobs to adjust. The actual antenna tuner circuit will not be discussed on this page. Please look at the link above. I will test other input configurations too.
You may already have a suitable input device to go with this project. Just use one of your existing crystal sets, by moving the coil close to the coil on this radio.
After doing some pre-testing, it seemed that the bias wasn't right. I felt this because I found the receiver worked much better with about 3.5 volts, not 6.3 on the heater. Since the 1n34A diode was used for biasing, and not a detector, I replaced it with a 2700 ohm (2200 ohm in the 12vdc radio) resistor. I selected this value by placing a 5k pot in the circuit and, with a scope, adjusted it to the peak in detected signal levels. I picked the closest value to the resistance measured. The tube detects because of the non linear operation at the grid cutoff. (The classic grid leak detectors operate at the other non-linear point in a vacuum tube, saturation.) If you check the 6GK5 curves, you will see that at a 15 - 25 volt plate voltage, cutoff occurs at -1 volts on the grid. I calculated the plate current, at zero signal input, and 25 volts on the plate to be 0.21 ma.
Now, after doing all that explaining, I looked in my 1953 Radiotron Designers Handbook and found out that this is called a "Plate Detector", or "Anode Bend Detector". This is on page 1084 of this handbook or is explained at this Wiki article. This is a very obscure circuit. Isn't the internet wonderful?
The last change I made was the way the regen pot was connected. I like to place the
pot across the tickler coil and use it as a voltage divider. This way, I can go from
zero feedback, all the way to maximum. Phil's shortwave model isn't probably
as sensitive, but on the MW band, there is a lot more that can go wrong.
Whether you want to use my changes is up to you. I never build anything entirely based on a circuit I see, but try to add a little value to what was done.
I used pieces of Garolite® 8x6 inches (20x15 cm) that are 1/8 inch(3 mm) thick for both the panel and chassis. Two metal brackets hold the two pieces together. Metal standoffs that are 1 inch (24 mm) long hold the chassis up. These bolt to the stained oak wood base. It is a favorite design of mine.
The variable capacitor is supported by three standoffs. They are plastic and 1¼ inch (32 mm) long. Metal standoffs can be used too. Once the chassis is attached to the panel, the spot where the vernier drive goes can be determined.
I found a neat trick to locate the drilling position. I sharpened the end of a 1/4 (6,3 mm) shaft. With a shaft coupler, the pointed end sticks out the front of the capacitor. I place a piece of masking tape where I expect the vernier to be mounted. I then draw vertical line on the tape. After that, holding the capacitor tightly to the chassis, I jam the pointed end into the masking tape used to show the vertical line. Now I know where to drill.
Once the capacitor is mounted, then drilling holes for the rest of the parts is simple. Just make one thing doesn't bump into another.
The coil is wound with 40/44 litz, which is very suitable for a regenerative radio. I cut a spider coil form out of HDPE material The form is 3½ inches (9 cm) wide. The hub is 1¾ inches (45 mm) in diameter. Make sure there are an odd number of slits. That way, the windings are automatically spaced. 50 turns wound on the form will give you a little over 220 uH. This allows tuning of the entire band (and the 160 meter ham band) with the dual 270 pF variable capacitor. If you use a 365 or 410 pF single gang, just add 3 or 4 more turns. The tickler coil is wound over the main coil. I wound 5 turns to use as the tickler. It is likely that you will have to fiddle with the tickler to get the radio working it's best.
If you have a set of high impedance magnetic headphones, you may be able to use them in place of the Bogen transformer.
Well, there it is. Phil did a nice job with the design of the Hellenedyne. If you happen to build this set, drop by The RadioBoard and tell him how much you like it. If you need a tube, I have several NOS tubes for this project available. Click on the contact information below if you need one.
73 and happy radio building ~ Dave, N2DS