Seven & Eight string humbuckers, sometimes called “extended range” humcbukers seen a very daunting prospect when you first start out winding pickups, but, in reality, they’re not that different to their 6 string cousins – if anything, you get much less variation on the extended range stuff in terms of string spacing and such.
Nowadays, 7 and 8 string humbuckers are looking a lot more traditional as the guitars become a little more main stream.
With seven string humbuckers, there aren’t actually that many quirks to really worry about to be honest – it really is just a larger humbuckers in every sense of the word.
You have to use a longer magnet (a 6 string would normally max out at 62mm) – anything longer then that should work in a 7 string – ideally, somewhere around 68mm.
Magnet width and height, remain the same (somewhere between 12.5 and 13mm wide, and height, somewhere between 3 and 5mm)
Beyond that – it’s the obvious stuff really – you need 7 string bobbins, bases, 14 pole pieces – the only real exception, is the spacers and the wire – same as a 6 string in that regard.
AS a little tip when winding/designing 7 string humbuckers – if you plan on making a “7 string version of your 6 string” and copy the number of winds like for like, the 7 string will be much hotter (each wind will be longer) – it sounds a little counter intuitive, but it’s worth shaving off around 10% of the total wind number to keep things roughly the same – just avoids any new design getting a too far from the original.
8 strings have a few quirks to them, that are worth knowing.
Beyond the obvious, that you need 8 string magets (about 83mm), bobbins, bases etc. – the way the poles sit against the magnet is a little different to a normal humbuckers.
Where, traditionally, a 6 or 7 string humbuckers would have the pole slugs in contact with the bar magnet on one side, and then the pole screws would be in the keeper bar on the other, so everythings touching everything – 8 strings are different.
If you’re using slugs on one side, screws on the other, you don’t use a keeper bar (assuming the magnets are 12.5-13mm wide anyway)
You cant make a 16 slug 8 string – the slugs foul the magnet, and you end up with a gap running down the middle of the humbuckers.
And, the weirdest one, is when you’re making 16 “screw” 8 strings – in that case, you use a keeper bar on one coil, and don’t install one on the other.
Basically, 8 string humbuckers always assume one out has a 5mm pole underneath, and one 3mm (where a 6 string is built assuming 5mm on both sides)
Strange design choice, but it does make some sense!
One area where they do differ too, especially is in the style of cover and mounting – up until very recently, 8 string humbuckers have usually been repurposed 6 string Bass pickups, using large “rail” poles hidden under a plastic cover.
Even the “traditional” style 8 strings don’t mount like you imagine, simply because of a lack of easily available mounting rings – its more common to see them mounting directly to the wood of the guitars body.
As a little tip, as with the 7 strings – don’t wind like for like to your favourite 6 string – the coils will be longer, and the pickup hotter as a result – worth shaving off between 15 and 20% of the final winds to get a similar sort of tone. (“Similar”! Its next to impossible to replicate a 6 string identically into a 7 or 8 string!)