Number of turns vs length of winding (resistance)

[wrnchbndr]

Let’s say I want a pickup constructed like a P90 but for a mandolin. Legend has it that an original P90 has 10,000 turns of wire. My mandolin pickup is going to be shorter and this is going to use quite a lot less wire and have a lower resistance. For a similar type of response, would 10,000 turns still be appropriate? Take this a step further and say that I want to make a pickup for a three string cigar box guitar. Does the same rule apply or does the situation change?
My thoughts are that it is the number of cross sections of wire that matter.

 

[hamerfan]

Re: Number of turns vs length of winding (resistance)

If you know the dimensions of the bobbin, you can calculate all parameters here:

http://www.jdguitarworks.com/coil/coil.html

 

[AlexR]

Re: Number of turns vs length of winding (resistance)

Wire has an ohms per foot measurement. Not sure where to access this, but the info was part of a thread many moons ago here......all I remembered was that each common gauge for pickups differed by 17% or so from the next one.
So you need to figure out what each turn will take length wise, and then find your target reading approximately......and the two metrics will allow you to figure out turn count.

Existing pickup K readings should be easy enough to find.....So you just need to know what sort of pickup you want to wind in a ballpark sort of way.

 

[wrnchbndr]

Re: Number of turns vs length of winding (resistance)

You’ve sort of touched on the point. With a pickup that is designed for a shorter span of strings, do the comparative resistance values become irrelevant and number of turns becomes more of a guide to the designer?
Let’s say I want a magnetic pickup for a single string- just a single pole piece. Should the design look to achieve the typical 5-6k of resistance or would 8 to 10 thousand turns of wire do the trick?

 

[DavidRavenMoon]

Re: Number of turns vs length of winding (resistance)

You wind by turn count. If I wind 5,000 turns on a 6 string bobbin, I’ll still wind 5,000 turns on a 7 string bobbin.

It will raise the resistance, but output is the number of turns and magnetic strength.

This is why you have to switch to thinner wire to wind hot pickups because the bobbins run out of room.


Sent from my iPhone using Tapatalk

 

[wrnchbndr]

Re: Number of turns vs length of winding (resistance)

I’m using 42. I can’t imagine using anything thinner despite my homemade winder being a lot better than many I’ve seen. I thought that # of turns was the actual goal and it’s nice to hear someone else confirm it. I haven’t yet completed building the mandolin and I’ve only held the pickup up to a guitar to verify an output. It is basically a 4-string P90 but with a Bakelite base and ebony making up the bobbin. The instrument is a prototype before I build more of them. The hand-made pickup is part of the theme and I’m hoping it performs well.