AM Converter
This is the test setup. It is a cigar box with the lid and a side panel removed. The variable capacitor at the back holds it steady. This makes it easy to experiment and construct the radio bit by bit.
The tube is an ECH81. It is a popular tube for AM superheterodynes, because it has a seperate triode part for the oscillator. On the bottom right of the picture you can see the small protoboard PCB which holds all the parts (except for the tube). The schematic is shown below.
Winding the inductors is something that many people seem to resent, but I found it quite easy (though perhaps a bit fiddly). I simply used a plastic pen to wind the inductor on, keeping the wire tight by pressing it with my finger. After the inductor was wound I put a single layer of common household tape on it. Then I repeated this for the other inductor, which I wound on top of the first. If you press the tape down firmly you can slide the finished inductor off the pen without it falling apart. Unfortunately I have yet to figure out how to neatly hold the ferrite core in place.
Below is the voltage waveform at the grid. As you can see in the schematic, the oscillator is self-biasing: it "generates" a negative bias voltage at the grid thanks to the DC blocking capacitor and the grid resistor.
Download the ECH81 datasheet here.
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