Si Skyripper(???), and other mods.

[digi2t]

Hi all,

First all, I'll state for the record that I'm still a noob at this stuff, but (slowly) I'm learning. Since there are so many people here that have shared their knowledge with me, I believe it only fair that I at least share my test results. I may be weak on the "why and how", but I know what I hear (and like). Kind of like art... don't know s**t about it, but I know what I like.

I'd been fiddling with the idea of a Si Skyripper, so I started with some simple stuff. I kept everything PNP, and I think I've found a way to give a builder the option between Ge or Si for this build. Foremost, I didn't want to mess too much with the layout (I'm using a vero version), so this way I could just swap between Ge or Si trannies. Call me lucky, but after auditioning a bunch of different Si trannies, I happened upon a great combination for Q2 and Q4. 2N2904A in Q2, and a 2N2905A in Q4. Also, I found that with these 2 trannies, all I had to do is re-bias via the trimmers. I also went with a 1N34A diode. All these are pretty common, but mostly, I wanted parts that are a bit easier to find, and more stable / consistant than Ge trannies. As for the diode, definatately stay with a Ge here. I've tried all sorts of Si diodes, but none come even close to giving you a lot of the octave jumping, sputtering, and zipper effects that are available with the Ge diode. Although Ge diodes are pretty consistant, to my ears (and amp setup),  the AC128 I had in here was better than the 1N34A. But, for the sake of this experiment, I went with a 1N34A.

Before I continue, let me say that any hFe numbers I give are from my DMM. It's not a cheap piece of kit, but I know that it doesn't take leakage into account. Between it, and my ears, it works for me. So take the numbers with a grain of salt.

As for the trannies, I had been running with a pair of 2SB172's (Q2 @ 95, and Q4 @ 160) and an AC128 as the diode. These work fantastically well, BUT... I went through a crapload of different trannies to get to this set. Lets face it, Ge is Ge, no matter how you slice it or dice it. And, if you want solid Ge sets for your FF project, you have to pay the price (one way or the other!). Now, as for the Si diodes, my goal here is A) off the shelf consistansy, B) availability, C) price, and of course try to stay close to the Ge sound.

Now, I know some of you will get in a huff about the Ge tranny mojo, sound, whatever. Well, frankly, I don't care. As far as I'm concerned, it all lies between the ears. One mans trash, another man's treasure, so to speak. Yes, the Si trannies do sound a bit edgier, but nothing that the tone knob, or an EQ can't handle. To me the this Si pair sounds as good (not "the same") as the Ge set.

As for the hFe numbers, the 2N2904A is listed at between 40 and 120, and the 3-pack that I bought (1.40$), all read 70, 71, and 75 on my DMM. The 2N2905A is listed at between 100 and 300, and the 2-pack (0.98$) had one at 160, and the other at 185. I auditioned all of them, in different combinations (re-biasing each time), and settled on the 70 in Q2, and 185 in Q4. Honestly, there wasn't a whole lot of difference between them all. I had to choose 2, so I flipped for it. Bias is critical to this pedal!! Make sure Q2 is at -7v, and Q4 is at -1v / -4.5v depending on Switch 4 position.

Sidebar. I did also try a 2N2907A in Q4 as well. Didn't like the sound. Very hard edge. Very "ice-picky" to the ears. If that's your thing though... be my guest.

Anyway, sounds pretty good with the Si trannies, and I can still go and get all the Skyripper tricks that I could get before. Trannies are easy to find, cheap, and consistant. And, sound pretty good... to me. As for the 1N34A diode... it seems to work just as well with the Si trannies, as the AC128 does with the Ge trannies.

Just two more mods. I did these with the Ge trannies, but they're still there with the Si trannies and don't make a difference.

First mod, I added a 220pF(221) cap between B and C on Q2, and a 470pF(471) cap between B and C on Q4. I added these for the Ge trannies, since I run through a SS amp, and long cable runs, the caps took care of reducing what little hiss the pedal was producing to near zero. I left these in with the Si trannies, but I couldn't tell you if they'll make a difference with or without. Hiss-wise, I don't hear a difference between the Si and Ge trannies, so I'm not messing with them.

Second mod, 4 position rotary switch, with 4 different caps on the input side of the "Range" circuit. I kept the 5nF original cap, but added a 10nF, 33nF, and a 100nF cap as well. Gives a nice boost range, from singing treble, to very woolly bottom end. What ever blows your skirt up.

In the end, the builder will make the ultimate decision as to what sounds good for him or her. Just wanted to let everyone know that this puppy will happily run with Si trannies.

Who knows, maybe I'll make a switching system now to switch between the Si and Ge trannies... like don't have anything better to do!

Cheers,
Dino

[joegagan]

great report, dino. as far as i recall, this is the first Si skyripper i have heard of.

gus smalley was kind, years ago he drew up a proposed Si skyripper and sent it to me, i lost the schem in a move, but i would still like to try it.

i used to be more of a germanium purist, but in the last 8 yrs i've heard, built and played quite a few silicon fuzzes that were damn satisfying.