Aldrin (Lunar Module) to Screw Driver transformation

Chuck D. Bones

Well-known member
The Skreddy Lunar Module and Skreddy Screw Driver are built to essentially the same schematic, with some different component values and transistor part numbers. This mod transforms an all silicon transistor design to a hybrid MOSFET-Silicon-Germanium design. The advantages of the MOSFET 1st stage are higher input impedance, smoother 1st stage overdrive and the ability to drive the 2nd stage harder. See the attached file for a table of parts changes. Only the parts that change have been listed. The pot values all remain the same. There are two optional parts that get soldered onto the bottom of the board: D1 & C101. D1 provides static protection for Q1's gate and is highly recommended. C101 adds a high-freq power supply bypass. D1 is soldered in parallel with R4 and will clear the pots. C101 is soldered between the + power terminal and the input jack ground terminal (right next to the + terminal) and will clear the pots. There is one more difference that I did not install: The Screw Driver has a 33K in parallel with C14 that alters the Volume pot's taper slightly. C14 is 4.7nF in the Screw Driver, but with the 33K left out, I kept C14 at 2.2nF. I actually used a 2N5210 for Q2, mainly because I have a pile of them. Any high-gain, low-noise NPN will work, including 2N5088, 2N5089, MPS-A18, etc. I always change the LED resistor value because I use violet LEDs in my builds. There is a pic of my Screw Driver in the Build Reports forum.

- Chuck
 

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HamishR

Active member
Cool! I really rate both pedals so will look at this more closely. I tend to steer away from Mosfet designs because every pedal I have built with Mosfets has sounded somehow harsh to me. They sound great at first but soon get wearing on the ears. To test this I once built an OCD with BPJ trannies as clippers rather than Mosfets and even that made for a smoother OD to my ears - although it didn't sound like an OCD much any more! I don't mind the OCD until I play it for more than 10 minutes. Then it gets really irritating. Same with the Box of Rock etc. Maybe it's just me?
 

Chuck D. Bones

Well-known member
The Screw Driver is not at all harsh. The MOSFET front-end is biased to have plenty of headroom. You have to turn the Pre-Gain all the way up and hit it with Humbuckers to get it to clip. This is one of my favorite Dirt Pedals.
 

K Pedals

Well-known member
The Skreddy Lunar Module and Skreddy Screw Driver are built to essentially the same schematic, with some different component values and transistor part numbers. This mod transforms an all silicon transistor design to a hybrid MOSFET-Silicon-Germanium design. The advantages of the MOSFET 1st stage are higher input impedance, smoother 1st stage overdrive and the ability to drive the 2nd stage harder. See the attached file for a table of parts changes. Only the parts that change have been listed. The pot values all remain the same. There are two optional parts that get soldered onto the bottom of the board: D1 & C101. D1 provides static protection for Q1's gate and is highly recommended. C101 adds a high-freq power supply bypass. D1 is soldered in parallel with R4 and will clear the pots. C101 is soldered between the + power terminal and the input jack ground terminal (right next to the + terminal) and will clear the pots. There is one more difference that I did not install: The Screw Driver has a 33K in parallel with C14 that alters the Volume pot's taper slightly. C14 is 4.7nF in the Screw Driver, but with the 33K left out, I kept C14 at 2.2nF. I actually used a 2N5210 for Q2, mainly because I have a pile of them. Any high-gain, low-noise NPN will work, including 2N5088, 2N5089, MPS-A18, etc. I always change the LED resistor value because I use violet LEDs in my builds. There is a pic of my Screw Driver in the Build Reports forum.

- Chuck
Awesome!!!
 

HamishR

Active member
I LOVE my Skreddy Hybrid Drive. It's a 3-knob pedal but is obviously based on the Lunar Module/Screwdriver circuit. Oddly enough it uses a 2N5210. And a BC109C and an AC127. I made a list of the SMD resistors but there are no codes on most of the SMD caps so I can't trace the circuit. I would love to build my own version of it so I could add a tiny bit of low end to it.

I also just got a Hybrid Fuzz Drive which can sound quite similar to the Hybrid Drive but it's a touch more compressed. Both are wonderful, amazing pedals. I like them even more than the Screwdriver and Lunar Module (which I also have had). That Skreddy fella knows his stuff! I suspect you may even be able to use the PCB for the Lunar Module to build these pedals but can't say for sure.
 

Mir9

New member
HamishR, can you share more about the insides of the Hybrid Fuzz Drive? I built the Screwdriver and Lunar Module (both great) and was always curious about that one.
 

Chuck D. Bones

Well-known member
HamishR,
Can you take some hi-res photos of the board so we can see what you're seeing? I've found a couple of gutshots of the 5-knob Hybrid Fuzz Driver; all of the parts are thru-hole. The transistors appear to be 2x BC109 and AC127. Neither of the two pix are from the right angle to read the capacitors. I haven't compared with pix of the Lunar Module or Screwdriver yet, but it's entirely possible that Marc Ahlfs is using the same PCB for all three. I've been scouring the 'net looking for the Hybrid Fuzz schematic, but so far, no luck.
 

HamishR

Active member
I'll try to take some pics tomorrow. I did notice that the PCB inside the Hybrid Drive has the "Major" crossed out on Major Drive. So he obviously used the same PCB as the Major. I just opened the Hybrid Fuzz Drive and it's a different PCB which has the input and output sockets on it. All the parts are on the inside, so I'll have to pull it to bits to see what's going on.
 

Chuck D. Bones

Well-known member
Here's what I've managed to collect so far. This post in TGP by Marc (Mr. Skreddy) Ahlfs explains the evolution from Screw Driver to Lunar Module to Hybrid Fuzz Driver. Besides giving us some good clues as to what parts changed, it provides insight into the pedal development process by one of the most inspired and inspiring designers. If you're not familiar with Mr. Ahlf's work, check out the P19 (Danube). But I digress... Here are a couple of gutshots of the Hybrid Fuzz Driver I found on the web. The pix are not of the same unit, you can tell by the wire routing and a couple of the components are in different packages. These appears to be built on a Screw Driver Mini Deluxe board, based on the control pot labeling. There is almost enough information here. I have a draft schematic, but some of the part values are unknown. Many of the component values and some of the ref designators are not visible in these photos. What we need are some pix from various angles that show the markings on all of the R's & C's, all of the silkscreened ref designators and ideally, photos of the backside of the board. I've seen gutshots of what appears to be a later model. The connectors are mounted on a daughterboard and the main board marking are different. The parts placement looks to be the same. Those photos are too blurry to share here.
Have a great Labor Day!

Skreddy Hybrid fuzz driver CCA 1.jpgSkreddy Hybrid fuzz driver CCA 2.jpg
 

HamishR

Active member
My Hybrid Fuzz Drive is quite different. It still has through-hole components but the whole thing is on one large board with board-mounted sockets, so I expect it's a later version. I agree re Mr Ahlfs - I have played most of his Muffs and they are all amazing. My faves are the P19 and Pig Mine.
Anyway, here are some pics of the Hybrid Fuzz Drive. Very difficult to get a shot of the board as you will see:
 

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HamishR

Active member
Now the Hybrid Drive - this is a great pedal! I think I prefer this over the HFD. It's a bit punchier, but you can get the two to sound really close. Both are incredible but the HD with a Les Paul is something else. As you can see it's difficult to tell what the caps are and I'm not about to unsolder them.HD001 copy.jpgHD001 copy.jpgHD002 copy.jpgHD003 copy.jpgHD004 copy.jpg
 

Chuck D. Bones

Well-known member
Curse you anonymous surface-mount capacitors!

This is good stuff. At least we can read all of the thru-hole caps and the SMT resistors. This will keep me busy for a while. Thanks for doing this.
 

HamishR

Active member
BTW the pots are Gain: C1K, Volume: A100K, Tone: B100K. And going by how they are labelled on the board Q1=2N5210, Q2=AC127, Q3=BC109C.
 

Chuck D. Bones

Well-known member
Thanks! Can you see if the leads on Q2 and/or Q3 are crossed/swapped? Going by the layout, I'm pretty sure they are.
 

HamishR

Active member
No the leads on the BC109C and the AC127 seem to fit the board without twisting. They are both triangular in layout though, ie not in a straight line. That might help.
 

HamishR

Active member
I've actually spent quite a bit of time with the Screwdriver and Lunar Module circuits, trying different transistors and pots, etc. I have very little technical knowledge because my brain doesn't work that way! But I do know how to build and experiment.

What I would really like is a Skreddy Hybrid Drive with the bass/fat control of the Screwdriver. The other thing I would like to try is using a silicon transistor instead of the Ge in a Screwdriver or Hybrid Drive. I love how well the HD cleans up with the volume pot on my guitar but don't really need that. If I could use a low gain Si tranny instead I would be all for it.

I really like how the Screwdriver sounds with the pre-gain up high and regular gain down low. Same with the Lunar Module. Very little "fuzz" and beautiful gutsy overdrive. So any suggestions would be gratefully accepted!
 

Chuck D. Bones

Well-known member
Adding the SHARPNESS control back in to the Hybrid Drive is simple enough. Only problem with building a Hybrid Drive is we don't know all of the capacitor values. You say you like the sounds of the Screw Driver and Lunar Module with the PREGAIN up high and FUZZ down low. I've played my SD that way and I get what you're saying. Is that not good enough and if not, what do you want to be different? Where do you set the SHARPNESS control?
 
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