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Extreme radius solutions

A

-Asdfgh-

Apr 13, 2010
15,415
27,612
UK
#1
I'm developing skills with my new router before I start my 34" EUB build, which gives me a chance to look at electronics.

The main pickup will be an under saddle piezo, going direct out to a preamp externally. This is to minimise internal elements that might get in the way of retracting the end pin.

I'd also like to add a magnetic pickup, again going direct to avoid internal components. Assuming I can keep string spacing appropriate, I have a spare P pickup, as well as J. Am I likely to be able to effectively angle a P pickup halves to get a good signal. Or would extremely staggered pole pieces on a J pickup work?

If not I'll have to think about either dropping the magnetic, or a longer term goal of a four coil pickup.
 
#5
I've spent some time experimenting with this exact problem, when I was developing out my Series IV Scroll Basses, which have a 4" radius fingerboard. Standard P-style pickup coils only work down to about a 6" radius. Smaller than that, and the corners of the coils will stick up between the strings and get in the way. Unless you lower them down so much that they don't work effectively.

And, if you are thinking of taking four P-coils and turning them sideways, one for each string, that doesn't work either. They are too wide for any normal string spacing. Even J-style coils are too wide.

I ended up developing my own special quad-coil pickups, to solve this problem. It's four separate pickups, which are long, tall and narrow. They run along the length of the strings, and are individually adjustable in height at each end. They work great set up at 4" radius and 3/4" string spacing (at the bridge). With different base brackets or longer screws on the center coils, they should work on a radius as small as 3".

Each coil has (3) 1/4" x 3/4" A5 rod magnets. So, the whole set of four coils is about twice as powerful magnetically as a typical P-pickup set. The coils are 5000 turns of #41 wire each, tall and narrow and tight around the magnets. The four coils are wired in series alternating, and the magnets of the coils are also alternating. The quad set is a full humbucker. The total DCR is 7.5K.

IMG_3275B.jpg
IMG_0937B.jpg


I generally don't sell my pickups separately, and I'm not interested in getting into the aftermarket custom pickup business. I'm just showing you how I solved the problem.

This is an old design that I developed in 2006, and don't use any more. I've stopped building my Series IV models, and the upcoming Series V uses a completely different pickup system.

I do have four leftover pickup sets similar to these, a custom variant that I made for another project. I mentioned them on JIO's EUB thread over on Luthier's Corner. But I'll only sell them to other Luthiers.
 
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#7
MPU said:
View attachment 2964051 You could also make coils like these and make a correct radius base for them.
Click to expand...

I've never tried to make a pickup! More information on the design would help - magnets, numbers of winds? I'd trust my wife doing this sort of delicate work more than myself. Making an enclosure might be tricky, getting a suitable curve. Ideally I would build a sapele box, and groove it to accept sapele veneer as a top, assuming I can set the groove near enough to the edge.

I've ordered a three pole cigar box guitar pickup, as if the base size could be reduced, then four of those, oriented along the string might work, and I'm curious. That would be there poles along a string and might sound awful, or the cheap pickup might just be inherently horrible.
 
#8
There's nothing at all wrong with having multiple magnet poles along the string. That's what mine are, three 1/4" magnets in a row, along the string. Having a big field aperture along the string and a narrower aperture across the string works very well. The big aperture along the string evens out the harmonics. You can hear all the harmonics, at any fret position, at the same level. The narrower aperture across the string tends to make it more percussive; more of a pulse on the beginning of the attack curve.

Your idea of using four of those cigar box pickups is good, but it depends on the width of the coils. It's hard to find any coils narrower than 3/4". If you are using standard 3/4" string spacing at the bridge, the strings will be closer than 3/4" at the pickup location. That's the basic problem you are facing. On my quad pickups, I made the finished coil units 5/8" wide, to allow a slight gap between them. Because of the thickness of the brass shell walls, that meant the coils themselves needed to be about 1/2" wide, and with 1/4" magnets in the middle. I had to make up my own special bobbins and geometry to fit them into that narrow space. It took some thinking and work.
 
#9
-Asdfgh- said:
I've never tried to make a pickup! More information on the design would help - magnets, numbers of winds? I'd trust my wife doing this sort of delicate work more than myself. Making an enclosure might be tricky, getting a suitable curve. Ideally I would build a sapele box, and groove it to accept sapele veneer as a top, assuming I can set the groove near enough to the edge.

I've ordered a three pole cigar box guitar pickup, as if the base size could be reduced, then four of those, oriented along the string might work, and I'm curious. That would be there poles along a string and might sound awful, or the cheap pickup might just be inherently horrible.
Click to expand...
One AlnicoV magnet per coil, bobbin plates are Forbon. 10 000 rounds of AWG 42 wire per coil. Please see
Multicoil pickup pictorial
only difference is steel poles instead of magnets. I may have a few extra coil I can sell if you're interested.
 
#11
Bruce Johnson said:
There's nothing at all wrong with having multiple magnet poles along the string. That's what mine are, three 1/4" magnets in a row, along the string. Having a big field aperture along the string and a narrower aperture across the string works very well. The big aperture along the string evens out the harmonics. You can hear all the harmonics, at any fret position, at the same level. The narrower aperture across the string tends to make it more percussive; more of a pulse on the beginning of the attack curve.

Your idea of using four of those cigar box pickups is good, but it depends on the width of the coils. It's hard to find any coils narrower than 3/4". If you are using standard 3/4" string spacing at the bridge, the strings will be closer than 3/4" at the pickup location. That's the basic problem you are facing. On my quad pickups, I made the finished coil units 5/8" wide, to allow a slight gap between them. Because of the thickness of the brass shell walls, that meant the coils themselves needed to be about 1/2" wide, and with 1/4" magnets in the middle. I had to make up my own special bobbins and geometry to fit them into that narrow space. It took some thinking and work.
Click to expand...

The specs said the coil width is 17mm, but I don't know to what extent I might be able to remove the base mounting plate. If I put some metal flats back on my ABG then I can test the quality of this $10 pickup by slipping it into the sound hole. It's a long lead time, though, on getting it.
 
#12
My super cheap cigar box test pickup still hasn't arrived, but changing poles on a cheap Strat pickup led me to thinking whether extreme stagger would work, which I might test on the cigar box pickup.

checking on the EUB neck, the poles need around a 6mm variation in height. Unless I find a pickup with particularly flat coils this is problematic, as the longest alnico magnets I can get are 20mm long, and a typical coil depth seems around 16mm, so that only gives me 4mm. Maybe that's a close enough match, but I can test on my ABG using the cigar box guitar pickup in the sound hole.

using a ceramic magnet and steel slugs gives me more options on length, but having tried replacing Strat poles with Alnico, the sound is better.

Another option might be using two short Alnico magnets stacked for the middle stagger, but I'd need to test that.

A final option, after gaining access to a coil winder might be to make my own more flat pickup and standard alnico magnets as I can source 12mm to 18.5mm from a single source, and suitable bobbins and wire. Not something I've ever attempted before, though. That's assuming the cigar box trial hints at the wild stagger working, and a flatter pickup does too.
 
#13
Just so you know, the longer the magnet is, the more powerful it is. So, if you use longer magnets in the center to get the arch, the magnetic field around them will be more powerful, and those two strings will probably be louder. If the magnets are all the same length, but staggered in vertical position, then you have a better chance of it all being balanced.
 
#14
Bruce Johnson said:
Just so you know, the longer the magnet is, the more powerful it is. So, if you use longer magnets in the center to get the arch, the magnetic field around them will be more powerful, and those two strings will probably be louder. If the magnets are all the same length, but staggered in vertical position, then you have a better chance of it all being balanced.
Click to expand...

I wasn't sure how much the coil would sense the magnetic field variations if only half in the coils without revising electromagnetism from my degree mumble years ago. With strength I thought I'd start with the radius then adjust the poles as required to get even string response, although the easiest purchase of bobbins seems to be fibre, so harder to adjust, so thank you for the useful tip, which has saved me a lot of frustration. Maybe a 20mm magnet, with 23mm being nominally required to follow the curve might be close enough, given magnet strengths.

I suppose an alternative is the ceramic approach, since that's magnetism by proxy, and might be more even, but having changed the poles in a pick-up, which leaves everything else controlled, I prefer alnico V. I've read that unmagnetised alnico slugs with a ceramic magnet are a reasonable compromise.

I suppose I could also rebuild one of the spare ceramic jazz pickups I have, rather than winding my own, but I bought the cheap winding machine to potentially do a four coil pickup, and my wife has some projects that will use it anyway, once I rehabilitate the workshop.

the longitudinal option might still be the best option, but I wanted to see if something more compact was possible first
 
#15
The cigar box guitar pickup has turned out to be not viable. I was expecting ceramic, but it is actually alnico on a fibre bobbin. The bobbin means that extreme stagger experiments will not be possible.

I had also hoped that I would be able to trim the base and pick up the coil ends to make a pickup from four of these, but the wires seem to be embedded in the fibre base. I am not sure what I might find if I trimmed it.

I will try a stagger test with a spare J pickup. My coil winder has also arrived.
 

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