We are going to take our unmarked black wire and our marked red wire and solder them together to create a series link between the batteries, and create our 18-volt power supply. We slip on a piece of heat shrink tubing onto the wire before we start. After giving a moment for the solder to cool, we slide the tubing down over the connection. Using either a heat gun or your soldering iron heat the tubing to protect the connection.
Then we go over to the jack and disconnect the black wire from the previous battery connection. Here is where you solder the marked black wire. Once again using heat shrink tubing to protect the connection.
That takes care of the neutral connection form the battery to the jack, that’s how the preamp gets turned on and off as you insert the plug into the jack to play the bass. It grounds the battery and turns it on.
Then after that the only thing that’s left is the unmarked red wire, our hot lead, from our new 18-volt power supply.
On the preamp we remove the connection from the old battery setup, solder the hot lead to that tab, protecting the connection with heat shrink tubing.