Electro Harmonix Soul Food diodes mod (step by step)

So I decided to mod my EHX Soul Food. As you may know, the Soul Food is a pedal "heavily inspired" by the famous Klon Centaur. The Klon is a rare overdrive pedal, which sells around 1500 euros today! This stompbox, created by Bill Finnegan in the 90s, is an overdrive with 3 controls: gain, volume and treble. It has been used and abused by many guitarists, including some famous ones like Jeff Beck. The Centaur is also known for its good quality buffer, like the Pete Cornish pedals. Thus, the Soul food is a cheaper version of the Klon.

However, when looking closer to the pedal, we can see that the diodes ("essential" says Bill Finnegan on the PCB of the new version of the centaur, the KTR) are not the same as in the Klon. Indeed, the klon centaur's diodes are germanium, whereas the Soul Food uses Schottky silicon diodes! The diodes are hidden on the top part of the PCB, in order not to scare the klon maniacs I guess. Diodes are indeed essential to generate saturation generally speaking, and germanium and silicium diodes have different properties, so changing it seemed a good idea.
Klon KTR diodes
Today, lets see how the diodes can be changed by adding a simple switch to choose between the stock diodes and the original centaur germanium diodes.

Disclaimer:
I will not be held responsible if you break your Soul Food by doing this mod. The Soul food is fragile: potentiometers are cheap, everything is surface components which are sensitive to heat ... etc. The modification is not difficult nor risky in theory, but be careful and cautious in order not to damage it!

Something else: as we will see later, this mod is after all quite anecdotal. There are no big differences between the stock version diodes and the centaur diodes, and I guess it would be quite difficult to distinguish between the diodes in a blind test... So it is more like a test to prove that sometimes, importance of some components just results from the musical "hype" around it! Learning how to do this will allow you to do mods like this on other pedals like tubescreamer, where the effect of diode selecting is much more pronounced. But do not expect great changes on the overall sound of your soul food!

I noticed that East River Drive form EHX has almost the same design than the soul food, so you probably could do the same mod on the East River Drive following this tutorial. This would also be a much more useful mod on the East River Drive, much more sensitive to diode clipping.


What do you need to do it?

To do this mod, you will need a few things: a Soul Food of course, germanium diodes and a DPDT switch. The original Klon diodes are still unknown, however the DIY community did determined that the diodes were really closed to D9E russian germanium diodes. If you want the closest diodes to the original, get these ones on ebay. Anyway, germanium diodes have really close properties between them, so classical germanium diodes like 1N34A, BAT41, OA1160...ect. will perfectly do the job!
Diodes mod soul food EHX
You also need everything to solder: soldering iron, solder, tweezers. A third hand can be useful too.
Ready? Lets go!

First step: dismounting the pedal
Diodes are on the top part of the PCB. So to have access to it, we have to entirely dismount the pedal. To do that, we need to remove the knobs first. Beware ! Knobs are really difficult to remove, and the pots are really fragile (60 dollars pedal, here you go...). Be gentle, or you can easily break a pot. A good working way to do this is to use two spoons as levers both side of the knob. Place the spoon under the knob, one by each side, and press both gently at the same time to remove the knob:
knobs soul food
We can remove all the knobs, and unscrew the 3PDT to have something like this:
EHX removing knobs
We can then remove the backplate, with a screwdriver or a driller. We can see the circuit, all with SMD, some film and electrolytic capacitors. We can also see the switch allowing true bypass or buffered mode.
Soul food EHX mod
We can remove the circuit from the box, by pulling gently the jack inputs on both sides.



Second step: removing the original diodes and assemble the DPDT switch

Once the circuit pulled out, we can see the diodes on the top part of the circuit:
To remove them, do not try to dessolder them. There is solder on both sides of the PCB, and these commercial solders are really dry / hard to melt. Moreover, fragile SMD components are on the other side of the board (heat sensitive !)... So my advice would be to simply cut the legs of the diode with pliers or a cutter. Try to keep as much metal legs possible, so we can add some length later by soldering legs of other components to it later. I did that way, and by twisting a bit the diodes, I took them off quite easily.
diodes EHX soul food
We can then solder the germanium diodes on the DPDT. You have to solder them in an opposite polarity, on the top (or bottom) part of the DPDT.
Then, we can do the same with the original diodes from the Soul Food. First, we can improve the length of the legs by adding some cut resistor legs. I just soldered it on the diode, using a third hand.
We can then solder it on the bottom of the DPDT like the germanium diodes.
Soul food diodes
We can then insert the DPDT in the enclosure.


Third step: drilling the enclosure

We have to insert the DPDT in the enclosure. There is not much room for it, so we will have to sacrifice the battery compartment. You can also put it next to the switch, but then when you activate the pedal you can just move the setting with your foot... I decided to place it vertically on the bottom side of the enclosure. For me it is better if you want to squeeze your pedals on your pedalboard than a side switch.
With a hammer and a nail, we can mark the spot where the hole will be drilled.
diodes soul food mod
We can see the mark of the nail. It prevents us from skidding with the driller. We start with a small diameter, and increase progressively the size until the DPDT fits in.
Soul food mod switch
Verify that you can indeed put the switch and screw it in the enclosure:
diode mod soul food
We are nearly finished now, last step!


Last step: soldering the DPDT to the PCB and mount the pedal back

So we are going to solder 2 wires, one for each lug of the DPDT (in and out). To have the good length of wire, try to see approximately how much lenghth will be needed in the enclosure. Do not make it too short, it is better to have a little more than a little less in those cases! Do not forget to put a bit of solder on the naked wires before soldering them to make it easier.
Soul food mod
Then, we can solder those wires to the PCB, where the diodes were. Just put one wire to each side of the diode D3 for instance (or D4, they are connected on both sides anyway).
Soul food EHX mod
Lets put everything back in the enclosure, and we are done!
soul food diodes mod
For a more professional looking, I just used letter stamps to engrave which side of the DPDT was which diode: "K" for Klon diodes (D9E), and "SF" for Soul Food (original diodes). A bit of China ink and voilà! Finished! 
EHX mod soul food
Here is the modded Soul Food! diodes soul food mod

 

How does it sound?

I have to say that I am quite disappointed... Differences between the 2 types of diodes are really minimal! Maybe the germanium diodes sounds a bit better at higher gain settings ("maybe"), a bit brighter... But nothing that really stands out really. I am not sure that the difference could be heard in a blind test. I tried to change the germanium diodes for classis 1n4148 diodes, and it was the same... Then, I tried with a soft clipping with 4 diodes (2 on each side), and the changes were minor.... Changind these diodes seems to have very very little influence on the overall tone and gain of the pedal!
Desperate, I tried to remove the diodes! And very surprisingly, there was almost no influence on the sound! 
Most of the saturation comes from the distorting OP-amp and not the diodes... So maybe the "these are essential" is finally quite a joke from Bill Finnegan!

So my advice would be to stay with the stock version!
But, if you are curious, you can do this mod to ear it yourself!
I am really surprised that some builders (JHS...) still do this mod, I do not get the use of it, apart from being closer to a real klon circuit.... Musically speaking, there are almost no differences!


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Any questions? Suggestions? Post a comment!
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15 Comment
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Almost all of the vividness emanates from your distorting OP-amp but not your diodes... Consequently probably your "these are generally essential" can be last but not least a good tall tale via Invoice Finnegan.
guitarscamp.com/best-electric-guitar

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Yes! I finally realized that when analyzing the circuit. Bill Finnegan surely has a good sense of humor! :)

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This improvements were being trivial, Changind most of these diodes has very very very little have an effect on within the entire develop in addition to attain on the pedal, also read more information.

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Apparently it is not trivial for everyone as people are still buying JHS modded soul food! I think trying it by yourself is a good idea, and allows you to enter slowly but surely into the DIY world.

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Being honest, it´s not tthhhhaaaatttt change. For me, Klon diode sounds just a little fatter than SF: the sensation was about listening a SF with the Treble Knob around 10 O´Clock, and a little more "compressed"; the tone was more compact. But SF is a very good pedal. Inspired on Klon: Yup. But with it´s peculiarity and own voice! That´s my vision! ;-)

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Honestly, I could not hear any change between the two settings (stock diodes and Klon diodes). However, my Klon clone that I made is clearly better sounding, especially in the high gain settings! I guess that the components choosen for the Soul Food (cheap CMS) might affect the sound

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thanks for the test, I love the internet, how would we ever get to the bottom of stuff like this. I was up for changing the diodes, but after your very detailed post I will not bother. when your in a full band setting with dust falling off of the light fittings, noisy pub/club etc not sitting on your own in a sound proof room etc on your own with a bunch of pedals all sounding similar doing a test who will ever bloody know....pedal snobbery is complete pants, well done mate for exposing this pedals snobbery.

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This comment has been removed by the author. - Hapus
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In joyful with the mod for now.
The Ge. Diodes seem softer and more round, addititionally there is an volume fall as a result of the forwarding voltage that is lower.
To be honest I must play with it out loud in a gigabyte before I decide to mod one.
I’m not certain if I would like to go to all the problem again.
The mod and outlet boards in this build are more complex than hard wiring or a straight swap.
Read more

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The Klon myth continues no matter.

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Glad you posted this! I was up for changing my soul food until I read this article! I'm looking at getting a Wampler Tumnus instead. ($180 around the time I'm posting this). I don't know how close to a Klon it is but it's much cheaper! :)

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That is the truth! I used to have a giant pedal board and just played a 2-night funk gig at a casino. Got disgusted with twiddling with settings and brought a small board with 3 overdrive pedals and a small multi fx for modulation and delays. Blissful night not tap-dancing all over a big board. The overdrives stack well also for different sounds. Only used a modded Electro-Harmonix LPB-1 boost, Cayline Pure Sky overdrive, and a modded Boss SD-1. Not tons of distortion. No high dollars. Tore the roof off the place. Most pedals are too compressed for live situations anyway. The louder the band gets, the less some of these compressed pedals come through, and the cleaner your sound needs to be. Low-dollar board, high-dollar playing is where it's at. Cheers!

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Do you have a sound sample for me, so i can hear the difference for myself?

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My dad owns a JHS modded Soul Food, and it really sounds a lot better than mine...

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Interesting article, I appreciate the deep dive and deconstruction. I suspect that the answer to the enigma of the Klon centaur v. KTR v. Soul Food with JHS mod is emotion, and not technical. Do I strain to hear a distribution between the germanium diode toggle setting and the standard setting? Yes. Am I compelled to insist on a germanium diode nonetheless? Yes again. I find mysel obsessed with the legend of the centaur. I have purchased both the EHX soul food with the JHS mod and the Klon KTR, and I'm guessing my search will not end until I acquire that mythical creature, the centaur.

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