Wednesday, November 26, 2014

Lite Em UP! Setting up for a Totally Tubular Winter Ham Radio Experimentation

I'm building a single tube MOPA. Yes, the same one I seem to almost build every year. :-)
This year, however, I have a plan! When you have a plan, you can win!

Sometimes.

The tube is a 10DE7, a higher heater voltage tube version of the 6DE7.
The circuit is a Crystal controlled Colpitts Oscillator. The only reason I'm doing this as a Xtal Colpitts is because I want my first foray into the world of tube design to be easy. I've built somebody else's tube Colpitts (The NoGA Twin Tube 80), so I figure that this will be a matter of scaling the design to the tube. Next I'll try a PTO Hartley. I've got the brass screw and nylon tube to wind a coil on for that!
I will handle this project in three phases:

1. design power supply. This tube has an interesting heater voltage requirement, so I have to solve that lil problem first. Also, need something that will give me around 300vdc for B+. I'm thinking voltage doubling circuit at first, just to see if I can do it. I'd use the same back to back radioshack 12.3v transformers to isolate the AC and then double the result. that should get me around 330v. I can drop voltage with a resistor if necessary. I want to QRV, not optimize right now.


2. Design the Oscillator and make it oscillate. I want to design the circuit in such a fashion that the compenents are accesable, but not dangerous. I'll socket the crystal somehow, and make sure all B+ voltages stay tucked away, shielded from wandering and little fingers. I want the board layout to be identical to the schematic layout, so I can understand how things are working, and I can tinker with values at will.

3. Design and build the final stage, including a way to remotely switch between receive and transmit. If I've done my calculating correctly, I should be able to get 3-4 watts out. High hopes? Yes. But we will do it as we are able.

Once QRV with this radio, I'll work on a receiver, probably something DC, and involving that Hartley oscillator. It will glow in the dark too, and will be used primarily with this transmitter.

Like I said, that's the plan!
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