The EB2 Triple Bypass (Choke Bypass Modification)

[chromium]

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 happy with the outcome - especially since no irreversible modification to the bass was required.

To recap the whole bypass thing, the goal of this mod was to enable a means of selectively severing a section of the circuit (the choke/filter) - in order to eliminate a residual filtering effect.  This filtering colors the sound of the mudbucker by removing most of its mid and high frequencies, leaving only that resounding muffled, "low-hovering-cloud" sound.  At the other end of the spectrum, you have the nasal "baritone" sound, which is devoid of bass.  No real happy medium to be found.

As it turns out, the choke/filter circuit is always present and doing something in the overall scheme of things, even when the baritone mode is supposedly disengaged.  In bypassing the choke circuit, the full-range output of the pickup can be realized, yielding a tone with more midrange punch - not unlike the SG-style EBs.  This tone works wonders in live settings, helps the bass cut through the mix, and still retains that EB "girth".

In order to implement this mod, and still retain the two original settings (I like them for recording), 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 the hole it was situated in was drilled out to 1/2".  Given the fact that I would not have to touch a drill to install it, I opted for the 3-position Les Paul pickup selector switch.  Unlike the dual-pickup selector, this switch will give you DPDT capability.

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:



I realize this is an EB-2D schematic, but the choke circuit should be similar in EB-2 and Rivoli applications - so don't let that throw you off.  Someone had drawn up a modded EB-2 schematic - wish I would have saved it.



One note about the switch that I had chosen.  The switching behavior had to be 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, if you end up using this type of switch:




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:



If you're not into that approach, a regular DPDT toggle switch should fit the bill as is.  I just prefered the look and feel of the LP toggle - so I made it work.


That's really all there is to the mod.  I made a quick recording that highlights each sound, so you can hear the differences.  The first eight bars feature the original "unchoked, but filtered" 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 the original unchoked, and the choke-bypass modes at low volume, but you can hear the mids open up, and the boominess start to subside.  The effect is more exaggerated at volume.  (EDIT:  I just realized I had both pickups on the whole time.  I meant to switch it to neck only for a good baseline (and bassline  ) comparison - oh well.)

     


When I did this mod, I just gutted the bass, and built an entirely new harness with push-back wire, CTS pots, etc...  Here are some photos of the whole process:

Guts and glory:



My new, modified wiring harness:



Completed bass with toggle switch:



Hope this helps!

[drbassman]

Here's the original EB-2 schematic. 

[uwe]

I had that mod done by my luthier a while ago - difference is night and day.

[drbassman]

I had that mod done by my luthier a while ago - difference is night and day.

Was yours an EB-2D or EB-2?

[uwe]

EB-2, which was original. But I think I also have the mudmod (or a mod of the mudmod) on my EB-2D thanks to the fiddlings of a preowner: On my EB-2D, the baritone button does not affect the mudbucker which sounds unchoked in all its might, but it does affect ever so slightly the bridge mini which sounds flatter choked and a little louder, but generally "more there" with a slight presence shine if unchoked. I'm sure that this wasn't how it originally worked, but I like the way it is now. It's a subtle difference, but deactivating the choke makes the bass sound more assertive in two pup or bridge pup only mode.

Uwe

[chromium]

Dr. B - I revised that EB-2 schematic that you posted to reflect the mod.  This is what it would end up looking like:




Also, I located the sound samples that I did way back when I performed this mod.  These use only the neck pickup on my bass, and give a better representation of what the end result on an EB-2 would be:

[ramone57]

thanks, Joe.   gonna have to give this a shot some day....

[uwe]

Those sound samples are enlightening, I coul hardly hear the choked mud on my puny monitors here but the unchoked mud came out nicely - love the Long Cool Woman riff (Hollies do CCR)!

[chromium]

I forgot to mention it, but one other trick that came up in that old "mudbucker unleashed" thread was a relatively simple mod to permanently enable the "bypass" mode - without having to first excavate all the electronics from the bass.

All you have to do is locate this wire under the bridge, dig out a bit of the wax that surrounds it, and desolder the wire from the top of the inductor (the big square silver thing).  Knowing that I'd later reconnect it and perform the full mod, I just slipped a little piece of heat shrink tubing over the bare end of the wire (to insulate it), and set it right back down where I found it.  You can even melt a little bit of parafin wax in there to cover things up again, if you plan to leave it this way.




I did this trick initially to try out the mod, before committing to the larger rewiring effort.  The only downside with this is that you lose both of the original sounds in the process - the unchoked and baritone modes.  Some folks might not miss them at all, but I think they can be used to good effect in recording, and live if you're going for that choked Yardbirds sound.

I like eb2's idea of the push-pull pot.  There are probably lots of cool variations possible.  If you come up with something neat, post it here - I'd like to see it too.  Might give me an exceuse to pursue the Rivoli I've always wanted!   

[Granny Gremlin]

Excellent write-up.

[drbassman]

Dr. B - I revised that EB-2 schematic that you posted to reflect the mod.  This is what it would end up looking like:




Also, I located the sound samples that I did way back when I performed this mod.  These use only the neck pickup on my bass, and give a better representation of what the end result on an EB-2 would be:

Thanks so much for the diagram and clips.  Very cool sounding!  That's a typical 3-way switch you have in there, right?  I was also wondering about the wires on the inductor.  Mine was removed and I'm not sure what is what.  Any advice you could give?

[chromium]

That's a typical 3-way switch you have in there, right?  I was also wondering about the wires on the inductor.  Mine was removed and I'm not sure what is what.  Any advice you could give?

It's a typical 3-way toggle - but make sure you get the one for the triple-pickup SGs and Les Pauls.  They use a different switch in the two pickup models, and it does not provide the switching capability needed for this circuit.

Just to clarify on your inductor question - does it look as though you missing the inductor altogether?  or are you just wondering how to go about reinstalling it?  I'm asking because finding an exact replacement for a missing inductor will be difficult, but finding a equivalent will probably be possible.

[drbassman]

Here's what I have:

[chromium]

Dr B - are there three wires coming off of that inductor?  Is that short black wire soldered to the casing on the other side?  I think that is the one that you can see in my pic above.

Honestly, I only remember dealing with two wires on mine, but its been a long time - and I didn't have it completely out of the bass.  I'll look at the one in my EB-3 later on and see I can refresh my memory as to where that third wire would go...

[drbassman]

I checked it out and here's the scoop.  All 3 wires come out of the inductor.  The black wire is supposed to be turned up and soldered to the top of the inductor and has just broken loose.  That leaves the green and purple wires.