Ed's JFET Convertible Amp
Ed Ganger sent this to me, and I only changed his Ascii
to a Clarisworks drawing cuz it made more sense to me. You can load
from the antenna the same way the convertible does, with a transfer coil.
Since this detector has such high impedance, you probably don't need a
detector tap on coil L1, but it might help selectivity. I haven't
tried this yet myself, but will, since it too has only 3 parts. Don't
know if the 47k resistor/crystal earphone will work, but why not give it
a try? Ed suggests that you can fiddle with the value of the
47k resistor, but not to go below about 5 k so as not to saturate the jfet.
And hey, on this and my version, you can really go pretty ugly on the wiring;
I mounted my components on a 5 pole terminal post, but you can do it anyway
that works. The circuit, such as it is, only has 3 parts; the rest
is power and other "off the board" danglies.
"I think that the MPF102 is a NEAT part to use in a "convertible"
radio --- the "grid-leak" detector, (feed the gate/grid directly off
the coil through a 1Meg//200pF Cap) is about the single most sensitive
detector around, take signal from the Drain/plate. The "infinite
Impedance" detector is similar, but signal is taken at Source/Cathode similar
to a "Source Follower", "Cathode Follower" or common cathode
amplifier.
Fig 1. Grid leak detector ---
Self biases to most sensitive
spot using rectifying
junction between S&D ...
Lots of detector gain!
G goes negative, play with 1Meg
and 200p for best sensitivity!
Fig 2. "Infinite Impedance" detector
VERY Light loading of tank.
I call out MPF 102 because I have a good supply --- The "cockroach"
of
FETs -- 2N3819, or most any JFET will work."
