MPF 102 with Diode

MPF-102 with Diode

What's an MPF-102 you ask? Well, they are a unique type of transistor but instead of an Emitter, Base and Collector, they have a D, S, and G for Gate. I have seen them used in mainly VHF applications but I wanted to see if they would work for broadcast band. Before I came up with the final circuit, I tried just the MPF-102 without the battery and found out it was actually rectifying!! Yes, this little gem without power will give you your local stations! Add a diode and a few volts and you have a 1AD radio that performs excellent. I am by no means an expert at radio design, I know just enough to experiment and have fun. I have used ideas from other builders to make this the best set possible. Take the 5 to 50 pF capacitor in conjunction with the choke, that was used in several radios made by my friend Dan Petersen.
The coil is wound with 28g wire @ 230 mH and the tickler is about 1/4 of the amount of turns with the same gauge wire spaced at 1/8" apart. The main capacitor is a 6 gang with all the gangs shorted. This gave it a value from 28pF to 416pF, just right for the broadcast band. The two studs in front of the base are for the headset. You can put a 47K resistor across the two for a crystal ear piece or even use this set with a Bogan to maximize loading and increasing audio quality. If you want to delve into powered radios but want to keep it simple, this is the radio to try! As always, if you have any questions, please feel free to email me.
Update: After some experimentation and a recommendation from Mike Peebles, I changed the resistor value from 47K to 15K. It took this set from a decent performer compatible to a nice crystal set to something I have never heard before! I literally could not move the dial without bringing something in. So selective and so sensitive it blew my mind. There was actually some regeneration that put the transistor into oscillation. This had to be calmed down with a tweak or two of the antenna tuning capacitor.