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.

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!

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:

We can remove all the knobs, and unscrew the 3PDT to have something like this:

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.

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.

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.

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.

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.

Verify that you can indeed put the switch and screw it in the enclosure:

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.

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).

Lets put everything back in the enclosure, and we are done!

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! 

Here is the modded Soul Food! 

 

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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