Sketch Me Up!

125 Enclosure Model

Here’s the table of contents for the whole DIY pedal building series:

  1. Intro to DIY Pedal Building
  2. Beginner’s Course in Sketchup
  3. Drilling a 125B Guitar Effects Pedal Enclosure
  4. Pedal Enclosure Finishing: Surface Prep, Priming, Painting
  5. Using GIMP to Create Pedal Artwork
  6. Printing and Applying Waterslide Decal to Pedal Enclosure

This is part 2 of my new series on building guitar effects pedals.

This part of the project ended up being far more grandiose than I originally intended.  I started out planning to just show how to model this diecast aluminum 125B guitar pedal enclosure, to make sure my PCB and components would fit properly inside.   But by the time I was done with it, it was essentially a complete beginner’s how-to course for Google Sketchup.  (Note, Sketchup is now part of Trimble instead of Google).

To give you an idea of how long this took for me to complete, I started filming the intro and the Sketchup tutorial videos in June of 2011.  Over the last year, in the time-spaces between the rest of life, I’ve been gradually editing, revising, extending, and composing music for the project.

In this tutorial, I start out with the absolute basics, and gradually pick up the pace, progressing to more and more advanced topics.  I divided the video up into 5 sections, but kept them all together in one 55-minute video.  In my previous multi-part videos, I had to split them into multiple YouTube clips, due to the YouTube length restrictions, but now that they’ve relaxed those for my account- I hope it’ll be better having it all in one video.

Here is the Sketchup file that I used in the tutorial if you’re interested: Download sketchup file.

Each section assumes a familiarity with the previous section topics.  Here’s the table of contents:

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A Worthy Tribute

Epiphone Les Paul Tribute PlusEpiphone Les Paul Tribute PlusEpiphone Les Paul Tribute PlusEpiphone Les Paul Tribute PlusEpiphone Les Paul Tribute Plus

My new Epi Les Paul Tribute Plus arrived yesterday for my birthday, and it’s a beauty!  These things are hard to find- none of the local stores have inventory, and every online retailer is backordered.  I managed to get Guitar Center’s last one in the country(!) – from the store in Tonawanda, New York.  The store manager there was great- sent me some pics, played it for me to confirm it was all good, gave me a great July 4th discount, and shipped it over for free.   Thanks Chip!

Cosmetically, it’s really nice.  Clean, well-defined flamed maple top and cream bindings.  The cherryburst finish looks near-perfect- with rich warm coloring- not the bright overexposed yellow in the pictures at Guitar Center’s website.  The Grover locking tuners feel fantastically smooth, and the switch, knobs and jack all seem good.  And after some quick adjustments, it plays pretty well (but still needs some fine tuning).

Push/Pull for Series/ParallelThe ’57 Classic pickups sound really dynamic, rich and beautiful – they’re warm when played gently, and crank when spanked. Love em.  There’s quite a range of sounds with the push/pull series/parallel switching on the tone knobs.  When a tone knob is pulled, the humbucker’s two coils are wired in parallel giving a lighter, brighter, thinner sound, somewhat reminiscent of a single coil (though different).    It’s completely different from the ultra-thick and heavy series-humbucker sound (knob pushed in).  This is a really versatile setup: a total of 8 different sounds using the 3 switches, not to mention the variations you can get by adjusting the volume knobs in the middle switch position.

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Potentiometer Tapers Graphed

Here’s a great reference image, which clearly shows the differences between the typical A (audio/log), C (reverse audio/log), B (linear), and W (s curve) tapers.   Less common are the K, D, and G tapers.

Pot Resistance Tapers

Source: Keiko corporation website

Tone Pot as a Low Pass Filter

A reader recently asked me a question about the low pass filter in a guitar tone circuit:

Will a 250k tone pot with a .02uF capacitor sound the same as a 500k pot with a .01uF capacitor (all else being equal)?

This is an interesting thought experiment, and the answer is simultaneously obvious and non-intuitive.

imageAt first glance, you might be tempted to look at this standard low-pass filter schematic (borrowed from the LPF wiki), imageand the associated formula for cutoff frequency as 1/2piRC, and conclude that the two circuits would behave identically (since 250k*.02uF is the same as 500k*.01uF).  However, the problem there is that the R in the formula is not the tone pot!   That R is really the internal resistance of the guitar, or the resistance of the pickup.

Circuit with toneIn the tone circuit, the pot actually sits above the capacitor C, but below the branch to the output Vout, as shown at left.   So, the formula for the cutoff frequency is more complex.  In this analysis by a guitarist/mathematician named Bill, he suggests a formula for the cutoff frequency as follows:

How’s that for insanely non-intuitive?!  Bill points out that the lower square root term only works with tone resistances less than about 20k (since otherwise the value would go negative producing imaginary numbers in the square root), thus explaining the often limited useful range of tone pots, and why log taper pots are more useful for tone than linear.  Nevertheless, this seems to be an over-idealized formula, since in practice, I do see more variation in the tone pot even at higher resistances.   This formula doesn’t seem to capture the full complexity of the reactive network made up of pickup inductor, and overall circuit resistance and capacitance (including cable capacitance).

Ok, so math is clearly the wrong way to think about this!!  Too complicated!   Back to the original question.  Let’s think of it more simply.  Imagine you turn both pots down to zero- you’re basically eliminating the variable resistance pot and wiring the cap directly to ground.  Of course, the larger capacitance .02uF will sound darker than the .01uF.  So they’re obviously not equivalent circuits.  

Next up, experiment!  Grab a couple pots, caps and some alligator leads and try it out!  You’ll find that they do indeed sound quite different.  The larger capacitance with the smaller pot resistance sounds darker, no matter how you slice it, when compared to the 500k pot and .01uF cap.  Even with both pots up full, the larger capacitance with the smaller pot sounds a bit darker. 

This all begs the question, why do guitar manufacturers often pair a 250k tone pot with a .047uF cap,  versus the .022uF cap with 500k pots?   The former will produce a darker sound both because of the larger capacitance but also because of the increased load on the pickup from the smaller resistance.   “Double whammy” as Bill points out at the end of his paper.

Push/Pull Pots

Bourns PDB183-GTR01-504A2, Gibson PPAT520, AllParts EP 4286-000 long shaftLet’s take a look at push/pull potentiometers.  Shown here are three specimens- Bourns PDB183-GTR01-504A2, Gibson PPAT520, and AllParts EP 4286-000.  These are all 500k audio taper pots with knurled split shafts.  The EP4286 has a longer 3/4” shaft, while the others have the standard 3/8” or “short” shaft.  This Bourns has the highest torque of the bunch– it’s the hardest to turn, while the Gibson is the easiest, and the AllParts has just a touch of mechanical graininess to the feel.

A push/pull pot is just a potentiometer sitting on top of a DPDT switch.  The switch actuator is the shaft of the pot itself, which drives down right into the switch.  When you pull the shaft up, you are moving the switch actuator to connect the top half of the switch, and when you push the shaft down, you are moving the switch actuator down to connect the bottom half of the switch.  There’s no built-in electrical connection between the pot and the switch – if you want the switch to control the pot’s behavior, you need to connect up some wires (as in the example described at the end of this article).

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