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.
Source: Keiko corporation website
Here’s another excellent article on guitar pickups, courtesy of Pete Biltoft at Vintage Vibe Guitars. Thanks Pete for the permission to post this here!
August 2011
DC resistance measurements are widely used as a gauge of the “output” of passive magnetic pickups. This use of DC resistance is both technically incorrect and often misleading; to find out why, read on…
In its most basic form a single coil magnetic pickup consists of a permanent magnet or set of magnets typically housed in an electrically insulating coil form (commonly referred to as a bobbin) and a coil of electrically insulated wire wound onto the bobbin (see Figure 1).
Figure 1. Simplified drawing of an unwound and a wound single coil pickup bobbin.
In operation, the vibrating strings of the instrument (which contain iron and nickel, both magnetic materials) cause the magnetic field of the pickup magnet(s) to fluctuate.
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.
At first glance, you might be tempted to look at this standard low-pass filter schematic (borrowed from the LPF wiki), 
and 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.
In 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.
I’m home- still recovering from jetlag, and readjusting to reality.
I’m doing my best to catch up on all the email/youtube questions and comments that I received while away. Please be patient if you haven’t heard back from me yet.
I have several videos left on the burner from before we left. These just need final editing and post-production, so stay tuned!
Hi everyone,
I’m in China right now visiting family.
Internet access is intermittent and slow/glitchy from where I’m staying in Urumqi, and it looks like the great Chinese firewall blocks all access to youtube and facebook, so I won’t be able to reply to youtube video comments or private messages until I return in a couple weeks. Thanks for your patience!
I did receive everyone’s facebook birthday wishes via email update, but am unable to reply. Thanks everyone!
I’ll look forward to sharing my travel guitar experience when I return 
John
I’ve received some questions about resources for learning and building tube amps. These are the books on my list to read:
The Tube Amp Book, by Aspen Pittman
Design and Construction of Tube Guitar Amplifiers, by Robert Megantz
Designing Tube Preamps for Guitar and Bass, by Merlin Blencowe
How to Service Your Own Tube Amp: A Complete Guide for the Curious Musician, by Tom Mitchell (also available at stewmac for a reasonable price)
Following last year’s All About Pickup Magnets, here’s another excellent article on guitar pickup magnets, courtesy of Pete Biltoft at Vintage Vibe Guitars. Thanks Pete for the permission to post this here!
In this article, Pete consolidates his own expert research on pickups, along with a bunch of information from the Wikipedia magnet entries and Magnet Kingdom, to give us an overview of magnets in general, as well an in-depth look at AlNiCo magnets for guitar pickups.
June 2011
Howdy!
A recent question from a customer made me think that it was time to go back and review the properties of AlNiCo magnets as used in guitar and bass pickups.
Historical documents suggest ancient Greeks living in the prefecture of Magnesia in Thessaly (modern Manisa, Turkey), first discovered and observed the properties of naturally occurring magnetic materials.
The earliest known surviving descriptions of magnets and their properties are from Greece, India, and China around 2500 years ago. The properties of lodestones and their affinity for iron were written of by Pliny the Elder in his encyclopedia, Naturalis Historia.
Figure 1. Naturally occurring lodestone attracting paperclips.
First a few magnet basics:
Ok, I admit it. I was seduced by the low low price. This Ktone travel guitar, found cheap on ebay, is apparently a knockoff of the Hofner Shorty. The Shorty gets reasonably good reviews, so I took a chance on this one. After a few minutes with the Ktone, it became very clear that the flaws in workmanship and detail far outweigh the price savings.
Turns out, sometimes you get exactly what you pay for… Take a look:
So, I’m now turning my attention to the significantly more expensive, but undoubtedly waaaay better Traveler Guitar Escape EG-1 Vintage. I had a chance to play it at a local GC, and this one looks to be a winner.
Here’s some pictures of the Ktone guitar, which start off promising and then go downhill fast:
I’ve often heard people talk about how their headphones or guitar speakers really “opened up” or became more “dynamic and alive” after breaking them in.
I recently had the opportunity to borrow a virgin, unplayed Celestion G12M Greenback speaker, which is the same speaker as in my Vox AC15HW1 hand-wired.
So, I thought it would be interesting to do a swap-in comparison with my current G12M, which has probably about 100 hours of playtime on it, at both bedroom and gigging levels.
Honestly, I was not blown away. I thought the differences would be really obvious, but they’re pretty subtle.
Have a listen:
And here’s a little pictorial of swapping the speaker. After removing its screws, the speaker is still stuck in there. There’s a thin layer of cork between the speaker and the cabinet, which either has adhesive applied to it, or has just kinda bonded with the cabinet interior. It needs a little coaxing to get out of there. I’m using a 15" utility bar to gently pry free the speaker.
As you can hear in my recent video review of the Vox SSC33, the Vox CoAxe pickups sound amazing. They’re dynamic, noiseless in all modes, and most importantly offer up a wide range of incredible sounds.
With the two blades sandwiching the pole pieces, you can see right away that these aren’t your typical humbucker, single-coil or P90…
Curious for more details on these mysterious creations, I got in touch with the man behind the magic- the inventor of the CoAxe pickup: Eric Kirkland, Chief Designer at Vox Guitar Development (G-Rok), in Novato, California. Read on…
Why the name CoAxe?
Eric: The name “CoAxe”, of course, refers to the orientation of the coils. Stacked humbuckers (the so-called “stacked single coil” pickups) are also coaxial, but our pickups are co-planar as well. Maybe “Concentric” would have been a more descriptive name, but it just didn’t sound cool enough.
Tell us about those blades, poles and coils!
Eric: The arrangement of the coils is significant, as is the position of the blades between the coils. The inner sensing coil, with its load of six poles, works like any single coil. Since the load of the outer noise canceling coil consists of both the six poles and the two blades, less wire is required to produce a noise signal equivalent to the noise in the inner coil. Less wire means less impedance, so the Clean and Crunch modes can be both noise-free and sparkly. (Exposed to typical ambient EMI, our pickups have less noise than a covered Gibson PAF type humbucker – and more output.)
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