Keeley SD1 5-star GE mod: What is the 2n5484 for?

[jimlam]

Hi guys,

I was looking into a friends Keeley 5-star GE mod and noticed that there was a 2n5484 transistor with one leg to d7, middle to c2 and another to c9.

I never read anything about a transistor so was wondering what the purpose was to include the transistor.

Anyone can share what they know about this?

Also d2 was removed. How does this affect the sound?

Thanks.

Jim

[bajaman]

Hi
I believe the 2N5484 jfet is to provide an additional fet switch at the input to the distortion stage - it helps to eliminate the small amount of distortion that leaks through otherwise when the effect is bypassed.

The diode is removed to further enhance the assymetrical clipping around the op amp
Cheers
steve

[jschwalls]

you looked into an actual Keeley modded SD-1 and D2 was removed ?

i've never seen that listed anywhere either.

[jimlam]

Yup. I looked into an actual keeley modded SD1. D2 was removed. And I cant remember off hand but he changed out c1 c2 and c3 as well. Which I think is not that commonly reported.

[BubbaKahuna]

Here ya go (from another post here - the search function is your friend):

Sd-1
***** SD-1 5-Star Ge Mod ****
Board Number        New part                   Old part          Reason   

C10 -->        Metal Stack Film 1uF, remove electrolytic, increased fidelity,

C7--> Metal Stack Film 1uF,          remove electrolytic   increased fidelity,

R6 --> 2.4k ohm metal film,                     4.7k,           increased distortion

C3--> 0.1uf metal stack film,              0.047uf,          fidelity and bass response

C2--> 0.1uf metal stack film,            0.018                     fidelity and bass response,

R2--> 620k ohm metal film,             470k ohm,       increased input impedance, better bypass,

Add 47pF cap across diodes in clipping section to smooth out distortion, just like TS9,

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.

I've only upped the resistance of R2 on mine and socketed the IC for later tweaking, there's plenty of room for the dual leaf socket with the IC in it. In fact there's stil room to stack the IC if I want, which is the 'new' mod on Keeley's website. He's even dumping the whole Burr Brown mod and recommending stacking two stock chips for better sound. That's especially cool considering the price difference of the Burr Brown chips. Two stock OpAmps cost a fraction of what one BB does, double bonus if it sounds better!

I can already tell the difference from just the R2 swap & I like it a lot. I couldn't find a 620k in all my stuff so I desoldered one end of the stock 470k and pulled it up. I then soldered in one end of a 150k in the open hole and chained the two resistors together in series to reach the 620k called for in the instructions. Plenty of clearance for this in there for this one too.

I have a newer version of this pedal so my D3 & the 470 ohm resistor in the power section are already jumpered across with plain leads from the factory. I think it's interesting to see an aftermarket mod adopted by a manufacturer like that. 

[MetalGod]

you looked into an actual Keeley modded SD-1 and D2 was removed ?

i've never seen that listed anywhere either.

D2 is the 5.1v zener (in my MIJ SD-1 anyway) to drop the voltage down for the led.  It might not be needed depending on what led is swapped in there.

[chris1001]

If you do remove the zener diode, expect your 3v LED to illuminate slightly when the effect is in bypass mode. I think Keeley must be using 5v LEDs or above. Suprised that keeley dont do the 808 output buffer mod, also looks like they now leave C6 in situ.