Created 8 September 2016. Modified 13 December 2016.

Some notes and observations about the Kubicki Ex Factor bass guitar. The opinions expressed are my own; use at your own risk.

Please do not copy and repost these photographs or link directly to them from my server.

• Replacing the Jack
• The Kubicki Preamp
• Frequency Response Measurements
• Preamp Modifications
• 9v Preamp

First purchased new in 1988/89 I ran low on money and sold it to a friend in '94. In 2007 I asked if I could buy it back but he wasn't ready to part with it. Then in December of last year he said I could have it back. As it happened work sent me to So Cal in January. He drove down from northern CA through the night and showed up at my door of the hotel at 6:15 AM. It was a surreal moment.

Aside from a few dings and scratches it was just as I remembered it. While the laminated neck always felt unusually stable I'd never fully understood why it just felt so right in my hands. In all the time I'd owned it previously it never entered my consciousness that it was a medium scale bass.

Replacing the Jack

In the 21 years it had been out of my possession the 1/4" jack had gone intermittent. I did a search and found a website that mentioned a Switchcraft number but a search for that part came up blank. I contacted Mike Haney who informed me that the part was readily available and that the part number was Switchcraft 152B, not 125B as the linked website claimed. The "design flaw" referenced on that website regarding the jack is similarly incorrect - there is simply no way to "jostle" the bass as to cause the jack's contacts to short to the PCB.

The circuit board contains the preamp but also the three controls including the rotary switch (two of the controls are concentric and have two complete potentiometers each). These are soldered to the printed circuit board and are bolted onto the body. The fact that there are two dual pots on the board gives extraordinary stability to the assembly. If the controls become loosened from the body perhaps the circuit board might move. But in more than 28 years since it was built the controls were showing no signs of loosening.

By removing the nut inside of the body the old jack can be pulled through the hole. The mounting lugs have been clipped of excess and the wires soldered directly onto these stubs.

The new jack comes untrimmed, of course. Trim them and take note of their position relative to the orignal jack.

Make sure the nut and lock washer stay inside of the control cavity.

Clip, strip and tin the wires. Pass them back through the hole in the body.

Solder the wires back to their respective trimmed lugs.

Make sure the new jack's lugs don't short against the PCB :)

The Kubicki Preamp

While I've always loved the playability of the instrument, in all the time I'd owned it I never really loved the sound I got from it. There always seemed like a degree of 'baked-in' contour regardless of how the tone controls were set. While the second active position is always bound to initially impress its practical use is limited. I thought I'd take a look at the circuit, see how its frequency response looked and whether I could modify it to fit my needs.

You'll need a 5/16" Allen key to remove the knobs and a 1/2" nut driver to remove the nuts from the potentiometers.

That old PCB pet peeve of mine - IDs within the component's footprint (corrected on the 9v version) - meant that I had to lift the majority of resistors to read them. In the process of tracing the circuit I noticed that C6 is missing so this is not an error on my drawing. Keep in mind that the circuit was traced quickly and hasn't been confirmed as correct. But if you'd like to download a PDF version you can do so here.

Here are some measurements I took after tracing the circuit with descriptions:

There were lots more measurements taken but these should provide a baseline for the existing preamp that the bass came with when I purchased it new.

It was around this time that I noticed that the description of the "9v" preamp used during the Fender Custom Shop period is different than the functions on my preamp. Mine was made in 1988 and is not an FCS. I got curious about the 9v preamp and emailed Carla at Kubicki. What I received was unexpected. She gave me some specific component substitutions that would change the characteristics of the 18v preamp.

C5 - Change to .01uf Mylar or Polyester 35v or > +/- 20% Tolerance
C7 - Remove and leave blank
C14 - Change to .0027uf . Mylar or Polyester 35v or > +/- 20% Tolerance
R20 - Change to 10k All resistors are 1/4 watt 5% carbon film
R25 - Change to 10k
R30 - Change to 100k

Note that the midrange dip present in the stock configuration is now gone. The frequency response is now much closer to flat.

The Q of the bass boost is reduced as is the level. I find the changes make the instrument's sound much better than before without the midrange dip in the first position. Similarly the second active position is much easier to tame without the pronounced peak at 55Hz or so.

Carla also sent a table showing the differences between the 9v and 18v preamps and granted permission to reproduce it here: