Homemade 3 Tube All Wave Radio
Homemade 3 Tube Radio
Well folks, I am up to a 3 tuber. I have revised
the circuit since I built this is 2002.
The first tube is a type 32 used as an untuned RF amplifier, another 32 as a regenerative detector and a 31 as the audio output. With 90 volts on the plates it drives headphones nicely. With 135 volts, it will be able to drive a speaker.
I call this an "All Wave" radio as it has plug-in coils for the various bands. Pictured is the broadcast band coil. This is the largest. I also have one that covers 1.8 to 4 mhz and a 5 to 10 mhz coil. The coil forms are old as the tubes and sockets. The rest of the items are newer, some even brand new.
You will notice this radio does have a little bit of metal. I have found from a previous shortwave radio that if you put your hand near the tuning capacitor, the frequency will shift. This is due to my hand capacitance changing the circuit values. This can be minimized by putting a metal plate (copper in my radio) between the front panel and variable capacitor. This plate is grounded to the capacitor. This radio has very little hand capacitance problems.
The building of this set was pretty traditional. Not much planning and just building on the fly. I designed the circuit myself as I have in most of my sets. I don't consider this a great feat as the design is pretty universal anyway. This radio is designed around what is in my junk box. I hope you are doing the same!
The tubes have 2 volt filaments and draw a combined current of 240ma. I use two D cell batteries with a 3.3 ohm dropping resistor. The high voltage is 90 volts for the audio output stage and 45 volts for the rf amplifier and detector. This is my first receiver to use a "C" battery supply. I am not talking about a 1.5 volt C cell, but a power source that is for grid bias. I used a pair of kind of semi dead 9 volt batteries to give me a 15 volt grid bias. The current is nil and the batteries can stay in the circuit all the time. I really didn't understand C battery supplies until I measured the output tube current. It was nearly 18 milliamps! This would flatten my battery in short order. The two early stages draw about 2 ma at 45 volts, which is ok. By putting a negative bias on the grid (through the volume control) the plate current is reduced down to about 2 ma at 90 volts. The audio is still the same strength and there is no more distortion with the bias battery.
Another new feature of this radio is the use of an interstage auto transformer. This is much like the interstage transformers of the old days, but this only has one winding. I use a coupling capacitor to connect to the grid of the audio output stage. I used a commonly available Bogen T-725 speaker matching transformer.
The main problem I had was getting this set to regenerate. After all this is where the gain multiplies and also allows me to receive cw and ssb signals. It turned out that the circuit using the 32 tube is about as sensitive as a marine drill sergeant. I had to add a lot of extra windings on the tickler coil to make this regenerate. My thanks to Mike Peebles for giving me the hint.
What can I hear? This last weekend I sat here listening to W1AW doing the code practice and earlier I listened to W3DUQ and a couple other 75 meter AM stations. Sure was pretty neat! By the way, am I ruining the sets with those Brother Label maker labels? I try to use them only when necessary. I feel they are important to label where the voltages connect. How about the front panel? The reason that I ask is I am getting ready to purchase a label maker that connects to my PC. I will be able to make very special labels. But if I am ruining the effect, maybe I should use my dymo. :-)
73, Dave - N2DS