Preamp Support
Installation and user manuals for Underhill electronics, and technical information will be posted here as it is created. For now, don't hesitate to contact me directly with any questions or concerns.
Wiring Diagrams
Battery Polarity and Cables
Preamps are tested and shipped with all of the cables connected. If you are re-arranging or changing modules or cables, MAKE SURE THAT "INs" ARE ONLY CONNECTED TO "OUTs" (labelled on the boards) and your BATTERY POLARITY IS CORRECT (going into the volume module) before you connect power. Incorrect polarity can damage the modules.
Q Trimmer - Hi & Med Q
Here are the trimmer positions and resulting Q boost for the HI and MED trimmers on the FS and FP boards.
Screw Terminal Connections
A few users have mentioned that the connection markings on the new boards look like there are two "+" connections. If you look close enough, the left wire is labelled with a tiny "⏚" symbol, for "ground". This is the same as the "-" connection on the old preamps.
Overview
Until the user manuals are written, the picture below (a labelled view of the Full English preamp) may help you to identify the various modules and adjustments on the preamp. All wire connections you will make to your bass go to the green screw terminals.
The preamps are quite robust and hard to kill with "wrong" connections (the idea is to experiment with different setups, i.e. putting a bass EQ on the neck pickup and treble on the bridge, with the mid EQ on both). If your battery polarity is correct and your boards are not shorted to cavity shielding or other metal parts, you won't hurt it by connecting things different ways. A useful thing to keep in mind is that an "OUT" cable connection must always be connected to an "IN" connection. Filter outputs, FT input, and the MD mixer use W4 (4-wire) cables for the additional treble boost signal. You can connect a 4-wire output to a 3-wire input with W43 cables.
Be Gentle with Pot Nuts!
Pot Nuts are powerful, with a lot more threads than a tiny ring nut, and they will pull the shaft right off the pot if you crank them with a 1/2" wrench. It only takes a light snugging up to hold the pot very securely from rotating or coming loose, and light pressure is less likely to damage the finish of your bass.
Knobs
All single pots have 6mm shafts. Stacked pots are 8mm for the bottom knob and 6mm for the top. Most stacked knobs will fit perfectly, but there are some short knobs that may leave gaps (both knob shafts are 10mm long). For stacked knobs, I highly recommend Luminlay numbered knobs for filter and EQ settings, and...they glow in the dark! You can get them here.
I recently found these knobs, which I like for single shafts. They have excellent fit and finish, and a recess that will clear 1/2" Pot Nuts. For a flush fit, the shaft needs to project no more than 11mm from the nut, so you may need to add some washers inside the cavity. And they have 1/4" holes, so use brass adapter sleeves for the 6mm pots. They have a nice low profile, and are particularly good for push-pulls, being easy to grab. Available on the Dopro Parts store on Amazon.
Battery Power
I don't supply or re-sell parts like jacks and battery boxes. You will need to provide the power to the preamp by using a 9V battery and clip in the control cavity, or an external battery box. I recommend using lithium batteries, as they will provide longer life and more headroom over most of the battery life. BUT... lithium batteries tend to die very quickly, without slowly starting to distort like alkaline batteries, so make sure you keep a fresh one in your bag for when the Stones ask you to open.
I personally use these nifty Keystone AA waterproof battery boxes with four 1/2-AA lithium cells for a 14V supply with about four times the life of a 9V alkaline. They are really tough and look nice on the bass. You can get them from Mouser, part No. 534-2438.
