Dave's 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
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