Ge. Diode in SD-1

[phillip]

Which of the three diodes in the SD-1 would be the ideal one to replace with a Germanium diode?  I'm thinking that the single diode would be the best, and leave the two Silicon diodes in series alone.

Phillip

[Peter Snowberg]

Not having seen the schematic, I'm going to guess this is a situation with 2 diodes in parallel with a reversed third one....

I would 1st try replacing one of the Si units in the double side for asymmetrical clipping, and then to test out something much more severe, put the Ge in reverse parallel with 2 Si diodes. You might also want to try the less severe combo of two Si diodes on one side and a Ge+Si combo in the other. Let your ears be the guide.

-Peter

[Mark Hammer]

Anytime you switch from the same number of SI to Ge diodes, you will be reducing the potential signal ceiling by about half.  So, where the present SD-1 willl yield a maximum output from the clipping stage of about 500mv or so for one half-cycle (and around a volt for the half cycle that goes through 2 such diodes), switching to a Ge diode will drop that ceiling to around 200-250mv depending on the individual diode.  It may only be one half cycle, but it will still reduce your overall peak-to-peak output by a quarter volt.  That may or may not be something you can adequately compensate for with the pedal volume control.

This will have a number of consequences.  First off, you will be able to generate additional harmonics at lower volume, or rather pick attack, because the GE's clipping threshold is that much lower.  Second, because of the substantial difference in clipping threshold between positive and negatve half-cycles, there will be greater asymmetry in clipping across a broader range of input levels than is currently the case.  That may be a better thing or a worse thing depending on what you like and how you play.  Consider sticking a 10-50k pot in series with that Ge diode to be able to vary the degree of asymmetry.  Jack Orman's delightful paper on warp circuits goes into more detail and theory on this matter.

[WGTP]

I recomment that you put a socket there instead of the GE and then you can have fun subing different combos of stuff.  2 GE's equal an SI threshold wise (approx.)  3 SI equal an LED (approx.)  

Also try a Jfet or Mosfet in there.   You can spend the rest of your life right there.

[Ansil]

try a 1n4001  they are supposed to give the 2nd harmonic more umph...  acording to the tubescreamer mod page over at keeleys.

[phillip]

Does anyone know which one Keeley replaces in his SD-1 mods?

Phillip

[Ansil]

my guess would be the singular one

[LH]

I believe Keeley replaces D6.

[Marcos - Munky]

Hope this helps. He used a Ge transistor as a diode.

***** SD-1 5-Star Ge Mod ****
Keeley Custom Shop designed this mod to enhance the tone and functionality of the SD-1.
Freely copy and distribute. We have the right to change our modifications at any time without notification. It is fun to watch what big name people use the mods.
Board Number New part Old part Reason Where to get the part
C10 --> Metal Stack Film 1uF, remove electrolytic, increased fidelity, digikey
C7--> Metal Stack Film 1uF, remove electrolytic, increased fidelity, digikey
R6 --> 2.4k ohm metal film, 4.7k, increased distortion, digikey
C3--> 0.1uf metal stack film, 0.047uf, fidelity and bass response, digikey
C2--> 0.1uf metal stack film, 0.018 fidelity and bass response, digikey
R2--> 620k ohm metal film, 470k ohm, increased input impedance, better bypass, digikey

Add 47pF cap across diodes in clipping section to smooth out distortion, just like TS9, radio shack
On some older units, short out D3 and the 470 ohm resistor in the power supply section to increase the voltage to the circuit. This increase the headroom and gain. Newer units have it shorted already.
Here is the tricky part of our mod, take a germanium (Ge) transistor and use the Base-Emitter junction (legs) of the device and use it in place of D6. If you use a PNP transistor, the Base will be the negative or cathode side, the Emitter will be the anode. Here is a diode drawing Cathode---|<----Anode. This increases the amount of distortion produced by the pedal, it also sound really nice. Try this! Don't leave it out! It's fun. Get your Ge transistors from Steve Daniels www.smallbearelec.com

If you want to drill out the LED and install a 5mm big job, change R30 to a 1.2k-3.3k ohm depending on brightness and battery drain you want, replacing the stock 3.9k ohm.

[phillip]

And now we know!  I have a strip of Ge. diodes so I figure that one of those will work just as well, and look a little neater too  I don't think there are any advantages to using a Ge. transistor as a diode as opposed to using a regular Ge. diode, but I do have a few leaky PNP and NPN Ge. transistors that would also work well.

Phillip

[LH]

I believe Keeley said that he uses Ge transistors because he has lots of them and they are "fun".