This is a design for a circuit for a small 386-based amplifier called the Ruby
which is very well suited to our needs here. That’s worked great with hot
pickups but did not have enough gain for lower-output pickups. Fortunately,
they also have a preamp design called the Fetzer valve which is enough of a
preamp to get sustain with about any pickup I've tried.
The driver took me a couple of
hours to wind and is probably the most labor intensive part of this build.
Still, it's really not too difficult. My driver was built from a stainless
steel bar that originall measured 6.2x6.2x61mm as the core. I rounded the edges
of this so the corners wouldn't hurt the wrapping wire. I then cut two bobbin
pieces (top and bottom) from the black plastic from the back of a CD case. Just
cut them in a pickup shape as you can see in the pictures below, then cut out a
little rectangle in the middle for the steel bar to poke through. The steel bar
and the bobbin top and bottom are then all epoxied together to make the bobbin
structure on which the wire is wound.
I used 30AWG wrapping wire from Radio Shack for the windings. I
squeezed some Castol’s glue all in the bobbin and then started wrapping the
wire, keeping it tight. The glue squeezes up through the windings as you proceed
and this hopefully pots the driver and avoids squealing. Just wrap it until you
get around 8 Ohms, and then solder some wire on to the ends of the wrapped
wire. I epoxied this connection into the bobbin, too, just so that the wrapping
wire does not flex too much and break.
