Installing an Aguilar preamp

This week my friend Robert asked me to install an Aguilar OB3 preamp into his favorite Fender P Bass. If you’ve ever taken the pickguard off of a P Bass, you know that there’s almost no room for any extra stuff in the control cavities, least of all an additional couple of knobs, preamp, and 9v battery. So, when installing an Aguilar preamp, I know I’m in for an all day adventure. Here we go!

Robert and I discussed at length what he wanted out of his bass, and how he wanted his controls laid out. He wanted a Volume, Blend, stacked Bass/Treble, and Midrange control (with switches frequencies from 400hz to 800hz by pulling up on the knob), in that order. However, his original pickguard only had 3 holes in it, and the body cavity only had enough room for those three pots. So I first pulled out my trusty Festool 1010 router, and with a flush cut bit I routed out some extra room for the controls. This was pretty straightforward, as the bit would just follow the pre-existing control route around towards the neck pickup. [A note on preparation: I always double and triple check the tightness of the bit in the router, the bit’s final depth, clamp the body to my bench, and wear protective gear for my eyes and ears. One can’t be too careful!]

Here’s the first of the flush cut bits I used, to take out most of the wood:


This bit wasn’t quite deep enough, so I switch to another follower bit, which doesn’t run quite flush with the walls, and left a small shelf in the bottom of the control cavity. This would be fine to leave in there if I didn’t need to squeeze that Aguilar preamp in there, so I cleaned this up with a chisel, and after I took this picture I painted the whole thing with shielding paint:


The next part wasn’t nearly as easy: installing a battery box on the back. This is really tricky, and there’s only one chance to get it right: it’s got to be dead straight, not overly deep, and can’t intersect with any cavities on the front. There’s absolutely NO room for mistakes, or I’ll be buying Robert a new bass!

Robert supplied a horizontal facing battery box, which unfortunately nobody makes a commercially available routing template for. Now, I could spend an hour or so mapping out a custom template for this style battery box, or I could get sneaky and use one of the laser cut templates I already own. I found that a humbucker routing template could work – it was the right shape, just far too big. So I installed a series of overly large bushings into my router, with undersized bits: this would offset the cutter from following the template a few millimeters, and thus cut a smaller route. I experimented on a piece of scrap wood until I found the correct combination to perfectly match the battery box’s dimensions:



Satisfied with this arrangement, now I had to transfer this to the body itself. I opted to place the battery box right between the bridge pickup and control cavity, which would mostly keep it away from the player’s body, and with any luck would intersect with hole that the bridge pickup’s wires travel through to get to the main control cavity, which would eliminate the need for me to drill another tunnel through the body. Next, I mapped everything out on the front of the bass, relative to the center line of the body and at right angles to the holes of the string-through bridge:


Then I took careful measurements, and transferred all those measurements to the back of the bass. This is tricky stuff – there’s no reference points on the back of the bass beyond the center line and the 4 holes for the string ferrules, so I double, triple, and quadruple checked my work before going any further. I’m always a little nervous when doing routing jobs like this, but I consider a little bit of healthy paranoia to be a good thing sometimes!

With the body again clamped to the workbench, the template firmly affixed to the body, the vacuum hose attached to the router for dust collection, and my eye and ear protection in place, I took a deep breath and plunged the router into the body. When I’d routed it down to the appropriate depth, I pulled off the template and tape, and inspected my handiwork. Perfect!


I also painted this route with shielding paint, and popped the battery box in there:


Now that the sweating bullets part of the job was done, it was simply a matter of drilling a new hole into the pickguard and installing the preamp. Squeezing all of this into the still small control cavity takes some work, but it helps that I try to keep all my leads short and well organized, so there’s not a ton of extra wire floating around in there. (Note that I collar all my leads to prevent the wire from melting when solder is applied. Also note that I installed two 10M ohm resistors across the mid switch to keep it from popping when engaged.)


My poor soldering iron just about gave up the ghost on this job. Whoa!


And that’s it! That’s how I go about installing an Aguilar preamp. It took me most of the day to complete the work – hopefully Robert digs how it sounds, or I’ll have to pull it all out!

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