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Question relating to electric guitar pickups and capacitance .

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Sigurthr

Sun Feb 07 2016, 03:32AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Interesting! I didn't think the Q was high enough to hear audible changes from the interwinding capacitances. I assumed the per-guitar tonal differences were mostly from geometric differences and material density/hardness differences in the body, and would massively overpower any electrically induced resonances.

It makes sense that the tone circuit is a low pass filter.

Regarding your original question then, it would sound as if the unused taps if left open, would not have contributing capacitances, as parasitic interwinding capacitance is in parallel with the inductor, and if the lead of the inductor is left open then the distributed capacitance in those open windings is also left open.

Signification

Sun Feb 07 2016, 04:38AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

WRT all of the facts brought up here, inductance, 43AWG wire resistance "Q", etc. this brings back a memory of a high inductance coil experiment done at MIT that demonstrates the slow rise in current through a 30H @ 4 Ohm inductor (7.5s TC) in showing how long it takes a 2 Ohm incandescent lamp in series with a 2 ohm resister to reach full brightness. the delay was impressive!
------
Question: How do you make a 30H inductor (I don't recall if it was adjustable) with only 4 Ohm ESR?

The instructor commented to tell how later, but I never ran across it--been bothering me ever since!!!

Sigurthr

Sun Feb 07 2016, 09:41AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

30H with 4R ESR? Planar (strips of) copper.

Ash Small

Sun Feb 07 2016, 12:37PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sigurther, the wood/neck/body of the guitar can also affect the tone. There are a lot of factors to consider.

I'm interested in the pickups in this thread, though. Early guitar pickups were derived from steel guitar pickups which had inductances in the region of 10H, while it's quite common for modern pickups to have an inductance in the region of 2-3H.

Pickup manufacturers don't tend to quote the inductance though, generally. They tend to quote DC resistance, which they refer to as impedance.

Early electric guitars also tended to use weaker magnets, alnico 2 or 3, where as modern pickups tend to use alnico 5.

The obvious thing a lot of people say is 'why not use neo's?'....which have ten times the magnetism of alnico 5.

It has been tried, but as was pointed out above, stronger magnets cause more 'string pull' which damps the strings and kills sustain.

The deeper you get into it, the more interesting it gets.

EDIT: Stray capacitance plays a big part too, early pickups tended to be hand wound, with looser turns. These tended to have less capacitance than tightly wound ones wound by machine. Early pickups tended to use wire wirh Formvar insulation, which was thicker than enamel, with a lower dielectric constant, again leading to less capacitance.

Over the past 65 years pickups haven't necessarily become better, but they have become cheaper to produce

radiotech

Thu Feb 11 2016, 11:32AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Pickup manufacturers don't tend to quote the inductance though, generally. They tend to quote DC resistance, which they refer to as impedance.

Inductance of an iron core coil needs agreement on how it is to be measured.

Ash Small

Fri Feb 26 2016, 02:23AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Well, I bought a cheap kit on eBay and personalised it a bit, but this is the 'test rig' for my pickup ideas.

It came with a neck pickup as well, but I just want the one bridge pickup for evaluation purposes. The brushed stainless pickguard is to cover the hole for the neck pickup. The control plate cover is to cover the hole where the volume and tone pots, and three way switch normally go.

I figure I can add filters, etc. outside the guitar for evaluation purposes, before considering building them into the guitar itself. My amps all have volume controls

For those who are familiar with the history of solid bodied electric guitars, it's of the Fender Telecaster family, but, due to the single bridge pickup, and the fact that at the moment it has a truss rod in the neck, it resembles an Esquire rather than a single pickup Broadcaster.

For those of you who still have no idea about different electric guitars, it resembles the guitar used on the early Johnny Cash records, played by Luther Perkins.

It is 'hotwired' straight from the bridge pickup to the output jack. I plan to add external filters, etc. as required.

I built it as a 'test bed' for pickups, etc.

EDIT: No strings on it yet, by the way.The strings that came with the kit aren't what I want to use, but I may fit them while I'm waiting to order some others.

Ash Small

Wed Mar 16 2016, 01:56AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I'm still playing around with this, getting it set up as best I can with the bridge pickup that came with the kit.

I've added some brushed stainless steel to it, though.

Just working on a 'box of tricks' to plug it into, I've modified some three pole, four way rotary sawitches to single pole, 12 way, and I'm just finalising details of values of series/pararallel capacitors and resistors, for various combinations on passive filters, and re-thinking the whole pickup inductance thing.

I'm just playing it through an early solid state, Yamaha built Marshall clone. It's starting to sound quite good, I just need to learn to play

The idea behind this is to learn something about inductance.

Just bought these, building a cabinet for them at the moment, there are various online guides, but the Celestion handbook thing is the one I'm using mostly.

Link to the amp I'm building here:

radiotech

Fri Mar 18 2016, 07:47PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

The output transformer has no DC current in primary. Does the plate choke maintain the inductance with the
DC current through it ? C3 seems to a standard polarized capacitor which may somewhat rectify the current and change
the sound from that had with a non polarized type.

Most output transformers are designed to perform with a DC component because of the gap.

BigBad

Sat Mar 19 2016, 12:40AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Ash Small wrote ...

Sigurther, it's pretty much exactly what you describe as 'wierd resonances'. That's basically all that differentiates the tone or sound of one guitar compared to another.
...
Any LCR circuit will resonate at some frequencies, and tend to block others.

Any guitar pickup has a resonant peak, frequencies around this resonant peak are enhanced, while frequencies above this tend to be blocked. Frequencies below this point just sound like you would expect them to.
...
Stray capacitance or added capacitors will lower the resonant peak.

LOL, short answer: no.

Long answer: yes, the extra taps have capacitance that could make a difference even if they're not carrying current because they're repeatedly raised and lowered in voltage, but no it makes no difference, because that capacitance is minute. You'd only have to worry about the extra capacitance if the pickup was running at its self resonant frequency, which likely to be in the megahertz range; and enormously beyond the audible range, and is unlikely to be driven by anything.

If the pickup is not running at that frequency, then the extra few tens of picofarads is irrelevant.

hen918

Sat Mar 19 2016, 09:56AM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

BigBad wrote ...

...
... You'd only have to worry about the extra capacitance if the pickup was running at its self resonant frequency, which likely to be in the megahertz range; and enormously beyond the audible range, and is unlikely to be driven by anything.

If the pickup is not running at that frequency, then the extra few tens of picofarads is irrelevant.

10H 20pF

1/(2Ï€âˆš(10*20*10^-12))ÂÂÂ=11254Hz
That's Audible.

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