Dave's First Code Practice Oscillator
Dave's First One Tube Code Practice Oscillator
Am I too late? Darn! Yes, I know that hams don't have to know the
morse code anymore. But I built one anyway. I would like to dedicate this to my brother,
Les Schmarder. He held the call signs WV2IQY,
WA2IQJ, and WA2AEA. He really liked CW. Not the super fast types with the electronic
keyer cranked way up, but the straight key type of wonderful morse code.
Now to the project. My little CPO (Code Practice Oscillator) uses one #30 triode tube. This is a battery tube, so I am using my battery power supply to make this go.
Before I forget about it, the tube in my model is actually a VT-67 military tube. It is like a #30 tube, except that the filament draws 100 milliamps instead of 60 ma. Both will work, but the 10 ohm series filament voltage dropping resistor should be changed to 18 ohms if you use the #30 tube.
This project uses a handy-dandy Bogen T725 transformer. It is perfect for this oscillator as it has plenty of taps to choose from. Pictured below is the breadboarded version of my CPO. I wasn't sure of the circuit, or the transformer taps that I would need to use. My little breadboard made it easy! I just fiddled around until it sounded the best.
The tone is controlled by the 50k ohm pot. I added a series 10k ohm resistor to
limit the low resistance value. That allowed the pot to be varied across the whole range
without the tone cutting out, and not getting the dog mad with the super high tones.
This resistor is optional. If you want a single tone, the
resistor and pot can be taken out and a 33k ohm resistor used instead.
The speaker is one that came out of an old Johnson CB radio that I stripped. I used it as it fit the front panel material. The little secret is that the Garolite® front panel and HDPE base were in my scrap box.
The CPO only took part of a day to build. I drilled, mounted and wired. Now I am having CW fun right in my little room. The volume is decent for a medium sized living room. The #30 tube might be a little weaker in volume as there are fewer electrons boiling off the filament, compared to the VT-67. It still will give you or your kids a lot of fun.
So --... ... -- Dave N2DS
Code Practice Oscillator Schematic
Dave's Second One Tube Code Practice Oscillator
Hi CW freaks! I am back again with another code practice oscillator. At noon today, I had no idea what I was going to do. After a couple of e-mails about my previous oscillator, I decided to make another. The box was prepared and the panel was fit to the box, so all I had to do is cut the holes and wire it up. Truly an afternoon project.
The box was purchased a while ago at Bed Bath and Beyond. They have some cool boxes there and when the coupon shows up at my door, I'm down there. There are two strips of pine, a soft wood that hold the panel to the box. First I drill two holes for the wood screws in the side of each piece of wood. The wood screws bite into the side of the box. Be careful as the wood splits easily. If you can, drill a pilot hole in the side. You can also drill from the outside and put the wood screws in that direction too.
After mounting the two pieces, place the cut panel on top of the box, measure and drill the 4 mounting holes. I then take off the panel, and drill the freshly drilled holes one size larger so the T-Nut fits underneath. The screw then draws the points of the T-Nut into the soft wood. I used this method on some of my earlier crystal sets.
The circuit is much the same as the code oscillator shown above, except this one uses a power supply and a indirectly heated tube (heater and cathode). The type is a #43, an old time audio output tube. The reason that I selected this tube is that it is old time, and has a 25 volt heater. These tubes are widely available and usually aren't too costly. 300 milliamps is required to operate this circuit. All Electronics is a good source for this transformer.
Only a SPST switch is really needed for the power, but the DPST has extra terminals for connecting the the wires needed.
Not too much more to tell. The output is enough to fill a medium size room with sweet cw sound. As a side note, I found it interesting that even though the cw requirement to get an Amateur Radio license disappeared over two years ago, there are still prospective hams that learn the code. Good for them!
Here is a mp3 sound file of this latest oscillator.
Update: To mellow out the sound, I placed a .05uF capacitor from the plate to the cathode. This took off some of the "edge" off the waveform.