1966 Gibson EB-2D “Triple Bypass” Modification

For those brave souls who wish to go fishing in their F-holes, I present to you a case study in implementing the fabled EB2 choke-bypass modification.  This was my attempt at it, and I was extremely happy with the outcome – especially since no irreversible modification to the bass was required.

 

What is this “choke” business and why bypass it, you might ask?

The Gibson EB-2 (single pickup), EB-2D (dual pickup), EB-6 (six string bass), and original Epiphone Rivolis all contained a passive tone filter circuit – sometimes referred to as a “tone choke” or “varitone” on the Gibson basses and guitars with the rotary switch (e.g. EB-3).  On the EB-2 derivatives, this “baritone switch”, as it is sometimes known, implements two different filter topologies.

In one setting, a resistor and inductor (RL) work together to form a low pass filter.  This has the effect of passing only the low frequencies of the neck pickup, and rolling off the upper mids and highs – leaving only a resounding muffled, “low-hovering-cloud” of bass tone.

The other setting combines a resistor, capacitor, and inductor (RLC) to form a bandpass filter.  This has the effect of “choking out” the low and high frequencies (hence the term “choke switch”) – leaving only the upper mids.  This creates a plonky, nasal bass tone.

For additional detail and depth on the varitone circuits, I recommend checking out this excellent whitepaper:

http://www.gtvnyc.com/okguitars1/varitonepaper.pdf

Both of these two stock filter settings inhibit the full range output of the neck pickup – which is endearingly termed a “mudbucker”, due to the hazy, incoherent sound typically associated with these basses.  This moniker is probably due more to the filtering than the pickup itself, which is in fact capable of articulating a full range of frequency response.  This is the motivation for the choke bypass modification!

The goal of this mod was to enable a means of selectively bypassing the filter circuit, while still retaining the original two varitone settings.  This bypassed tone works wonders in live settings, helps the bass cut through the mix, and still retains that characteristic EB “girth”.

In order to implement this mod, I chose to replace my pushbutton (SPST) baritone switch with a 3-position (DPDT) toggle.  The stock switch had a 3/8″ diameter shaft, yet it was situated in a 1/2″ hole – leaving just enough room for the new toggle switch without drilling.  I opted for a 3-position Les Paul pickup selector switch.  Unlike the dual-pickup selector, this switch will give you the DPDT switching capability needed for this modification.

The original schematic from Gibson looked like this:

 

Note the red highlighted connection to ground is a mistake, and should not be present.  This was an error on the original schematic that I obtained from Gibson, and if implemented would result in no output whatsoever.

Here is the modified schematic, depicting the new switching capability:

 

Please note that the choke circuit implementation is similar in both the (more prevalent) single-pickup Gibson EB-2 and Epiphone Rivoli applications, and this modification will work there as well.

Here’s a schematic depicting what this mod would look like in those basses:

 

One note about the switch that I had chosen.  The switching behavior had to be slightly altered by bending one of the prongs using needlenose pliers – prong #4 to be exact.

I snapped a photo to illustrate exactly what to modify:

 

The pins are numbered #1-#6, starting at the top.  The goal is to make is to make it so that pin #4 never comes in contact with #5 – otherwise, the center position would enable the original unchoked mode as opposed to the bypass mode.

Once that change has been made, the switching behavior will work as follows:

 

For those not into that switch modification, a regular DPDT toggle switch should fit the bill as is.  I simply prefered the look and feel of the LP toggle, and so I made it work.

That’s really all there is to the mod!

I made a quick recording that highlights each sound.  The first eight bars feature the original “normal” mode, the next eight bars feature the new unchoked/unfiltered mode, and the final eight are the original “baritone” mode.  Note that it is kind of hard to appreciate the difference between these settings at low volume, but you can hear the frequency range of the neck pickup open up and even out a bit:

http://www.hillscloud.com/music/EB2(unchoke-bypass-baritone).mp3

 

Please note that I had both the neck and bridge pickups on during this recording.  I meant to switch off the bridge for a good baseline (and bassline  Smiley ) comparison between the settings, but old habits die hard!  Guess I’m just conditioned to rely on that bridge pickup for clarity, and always tend to have it on.

When I implemented this mod, I took the approach of gutting the bass and building an entirely new harness with push-back wire, CTS pots, etc…  This way, the original harness can be reinstalled in the future if desired.

Here are some photos of the process…

Guts and glory:

 

My new, modified wiring harness:

 

Completed bass with toggle switch:

 

 

Leave a Reply