Homemade Tube Reflex Radio
Hello friends. Today I am presenting to you my homemade reflex one tuber. It could be called a two tube set as there are two sections to the one tube.
The reflex radio has it's roots going back to the early tube radio days of the 1920's. A very good explanation is provided here. The information on the reflex radio is towards the bottom of the page. The basis of my radio is this circuit. Other than that excellent site, there isn't really much information about a reflex tube radio. One could build this set with semiconductors too, but my aim is to recreate the old listening experience and provide some interesting looking knick knacks to have around my house.
In the old days, tubes and battery power were very expensive. The tube count could be reduced by using regeneration and reflexing. The reflex radio works by passing a signal twice through the same tube. The RF signal comes in the antenna circuit, then is amplified by the first tube. Then the second tube detects the signal, also amplifying. Then that audio output is introduced back into the first stage, amplified again and then passed to the headphones.
My radio uses a 1J6G tube. This is a dual triode with an octal base. The filament voltage is 2 volts and draws a hefty 240ma. The plate voltage I use is 45 volts, made up of five 9 volt batteries. The total plate current is only a milliamp, so these batteries will last a long time.
I wound two identical coils. The primary is 25 turns of 30 gauge wire and the secondary winding is 48 turns of 23 gauge wire. The wire is insulated and the coil form is a 3 inch mailing tube. I painted the tube black with lacquer. There is no scientific basis for the primary winding of these coils. They just "looked right" to me.
This radio has two tuned circuits. They are tuned by a single dual ganged variable capacitor. For this radio to work it's best, the two tuned circuits have to move the same when the tuning is changed. This is called tracking. With my circuit there is somewhat of a tracking problem because of the audio transformer and capacitor connected in series with the first coil. The second tuned circuit differs from the first with the grid leak detector circuit. To compensate for all these differences, I added a trimmer capacitor to the first tuned circuit. This does help the tracking quite a bit, but is far from perfect. But this is only a demonstration receiver, and nothing that I would be using on a daily basis.
A fairly expensive part in this radio is the interstage transformer. It is best if you can find an old interstage transformer with a 1:3 ratio. There are a lot of those imported transistor type transformers out there that would probably work too.
Before you build this set, please note that this is not a very stable configuration. This is a 3 stage radio, all circuits out in the open, no shielding and so on. But it was fun for me to build! I did add that pot across the second coil to quiet this set down. This acts as a volume control. It isn't exactly "20's" but I am sure it was done a few times in the 30's. This set will squeal and motorboat all over the place if it is run wide open. This set will drive a high impedance speaker too! Nice to hear one of my sets without putting on the "cans".
If you know more about this than I (which can be very likely), and have better ways to tame this set, please e-mail me.
73, Dave - N2DS