Post by ms on Apr 22, 2020 18:19:34 GMT -5
It is not easy to make a humbucking pickup that fits into a strat rout but has all its good properties, or, especially, improves on them. These properties are the following:
First, it must be narrow enough to fit; it can be deeper since there is extra room below the standard sized strat Fender pickup. Second, it must sample as a single coil does, that is, a single sampling region is short along the length of the string. Third, its inductance must be in the range of a strat pickup so that the resonant frequency is in the strat range with a typical cable. Fourth, the Q of the electrical circuit must be high enough to achieve the typical strat tone. And of course it must be humbucking, as well as relatively easy to construct.
A stacked humbucker is a good choice, although most, if not all, available such pickups fall short in one or several of the properties listed above. A stacked humbucker and its relationship to a sidewinder were discussed here:
guitarnuts2.proboards.com/thread/8863/modeling-sidewinders-stacked-hbs-shield.
A sidewinder design using ferrite for cores and neo for the permanent magnets was discussed here:
guitarnuts2.proboards.com/thread/8538/ferrite-neo-sidewinders-why-testing. It is suitable for humbucker and p90 sized routs, but is too wide for single coil strat . This discussion shows how to do this with a stacked design, accompanied by verifying measurements. Just like with the sidewinder, here we also have two important factors. First, the “shield” increases the inductance and sensitivity, but reduces the mutual coupling between the two coils. Second, the mutual coupling is negative, reducing the inductance of the series combination. In a stacked design these two properties tend to oppose each other, and a good design is thus a compromise between the two.
The basic approach is to use two standard humbucker bobbins with cylindrical ferrite pole pieces and rectangular ferrite pieces in between the top and bottom coil for the so called shield. The ferrite pole pieces have a slightly bigger diameter than standard, and so it is necessary to drill out the holes to 13/64”. You can start with either a slug or screw bobbins. The permanent magnetic field to magnetize the string is made by small neo magnets on top of the pole pieces. They are located to maximize the spread of the field across the string to maintain good sensitivity when bending the strings. The pole pieces are from Amidon corporation FB-73-1801. The 73 material is for low rf frequencies, but works well in the audio range. It has a permeability of nearly 2000. The cross pieces that go between the coils are Amidon DC-43-9A. The 43 material is for somewhat higher frequencies, but has a permeability of about 800, and so is good for this application in the audio range.
The permanent magnets are 1/16” in diameter by 1/32” thick. This gives 300 or 400 Gauss at the string location. The first picture shows the two prototype pickups temporarily mounted in a guitar. A plastic strip is glued to the bottom of the bottom coil; it extends out to receive the sawed off humbuclker screws that mount the pickup.
The next image shows how a cross piece (one of six parts of the shield) is located on the bottom of the top coil.
It does not take so many turns to do the job; and winding a humbucker bobbin with not so many turns is one of the easier tasks encountered in pickup winding. Here are some of the basic measurements:
coils for neck pickup with 3750 turns of #41 wire:
no cores (air core):
top coil: 0.5679 H, 2.325 KOhms (120Hz AC resistance)
bot coil: 0.5664 H, 2.324 KOhms
(This is a better match than I usually achieve.)
Cores in place, but pickup not assembled:
top coil: 0.9899 H, 2.335 KOhms
bot coil: 0.9870 H, 2.349 KOhms
Assembled pickup (that is with ferrite shield pieces in place between the coils):
individual inductances:
top: 1.4776 H
bot: 1.4645 H
series connection out of phase (as connected for normal use):
2.05 H
series connection in phase (as connected for test):
4.666 H
The signal from the string picked up by the lower coil is 10 db below that picked up by the upper coil. The impedance of this neck pickup is shown in the last attachment. The capacitance of the series combination of the two coils is very low, and so I have added 520 pf in parallel to lower the resonance a frequency expected with a typical cable. The Q 3 KHz (5.8) is plenty high enough to achieve strat sound.
I have measured the output level of the pickup by a very primitive technique. I look at the highest peak to peak output while playing an A minor barre chord (fifth fret). The value is a it more than .5 volts peak to peak maximum value for a short time.
The bridge pickup is very similar but uses 4000 turns of #42 wire. It has an inductance of 2.5 H. The output level by the same test is about the same as the neck pickup.
First, it must be narrow enough to fit; it can be deeper since there is extra room below the standard sized strat Fender pickup. Second, it must sample as a single coil does, that is, a single sampling region is short along the length of the string. Third, its inductance must be in the range of a strat pickup so that the resonant frequency is in the strat range with a typical cable. Fourth, the Q of the electrical circuit must be high enough to achieve the typical strat tone. And of course it must be humbucking, as well as relatively easy to construct.
A stacked humbucker is a good choice, although most, if not all, available such pickups fall short in one or several of the properties listed above. A stacked humbucker and its relationship to a sidewinder were discussed here:
guitarnuts2.proboards.com/thread/8863/modeling-sidewinders-stacked-hbs-shield.
A sidewinder design using ferrite for cores and neo for the permanent magnets was discussed here:
guitarnuts2.proboards.com/thread/8538/ferrite-neo-sidewinders-why-testing. It is suitable for humbucker and p90 sized routs, but is too wide for single coil strat . This discussion shows how to do this with a stacked design, accompanied by verifying measurements. Just like with the sidewinder, here we also have two important factors. First, the “shield” increases the inductance and sensitivity, but reduces the mutual coupling between the two coils. Second, the mutual coupling is negative, reducing the inductance of the series combination. In a stacked design these two properties tend to oppose each other, and a good design is thus a compromise between the two.
The basic approach is to use two standard humbucker bobbins with cylindrical ferrite pole pieces and rectangular ferrite pieces in between the top and bottom coil for the so called shield. The ferrite pole pieces have a slightly bigger diameter than standard, and so it is necessary to drill out the holes to 13/64”. You can start with either a slug or screw bobbins. The permanent magnetic field to magnetize the string is made by small neo magnets on top of the pole pieces. They are located to maximize the spread of the field across the string to maintain good sensitivity when bending the strings. The pole pieces are from Amidon corporation FB-73-1801. The 73 material is for low rf frequencies, but works well in the audio range. It has a permeability of nearly 2000. The cross pieces that go between the coils are Amidon DC-43-9A. The 43 material is for somewhat higher frequencies, but has a permeability of about 800, and so is good for this application in the audio range.
The permanent magnets are 1/16” in diameter by 1/32” thick. This gives 300 or 400 Gauss at the string location. The first picture shows the two prototype pickups temporarily mounted in a guitar. A plastic strip is glued to the bottom of the bottom coil; it extends out to receive the sawed off humbuclker screws that mount the pickup.
The next image shows how a cross piece (one of six parts of the shield) is located on the bottom of the top coil.
It does not take so many turns to do the job; and winding a humbucker bobbin with not so many turns is one of the easier tasks encountered in pickup winding. Here are some of the basic measurements:
coils for neck pickup with 3750 turns of #41 wire:
no cores (air core):
top coil: 0.5679 H, 2.325 KOhms (120Hz AC resistance)
bot coil: 0.5664 H, 2.324 KOhms
(This is a better match than I usually achieve.)
Cores in place, but pickup not assembled:
top coil: 0.9899 H, 2.335 KOhms
bot coil: 0.9870 H, 2.349 KOhms
Assembled pickup (that is with ferrite shield pieces in place between the coils):
individual inductances:
top: 1.4776 H
bot: 1.4645 H
series connection out of phase (as connected for normal use):
2.05 H
series connection in phase (as connected for test):
4.666 H
The signal from the string picked up by the lower coil is 10 db below that picked up by the upper coil. The impedance of this neck pickup is shown in the last attachment. The capacitance of the series combination of the two coils is very low, and so I have added 520 pf in parallel to lower the resonance a frequency expected with a typical cable. The Q 3 KHz (5.8) is plenty high enough to achieve strat sound.
I have measured the output level of the pickup by a very primitive technique. I look at the highest peak to peak output while playing an A minor barre chord (fifth fret). The value is a it more than .5 volts peak to peak maximum value for a short time.
The bridge pickup is very similar but uses 4000 turns of #42 wire. It has an inductance of 2.5 H. The output level by the same test is about the same as the neck pickup.
