You may have noticed that as you roll off volume you lose the treble end of your sound faster than the low end and you end up with a dark, muddy tone. There is a physics-related explanation, involving relative resistance and capacitance between potentiometers and pickups, but to be honest we don’t need to go into that here to get a practical understanding of the problem. Suffice to say that, as they are brought into play to reduce gain, volume potentiometers work raise resistance, creating what’s called a ‘low pass filter’ – low pitch is allowed to pass through and high pitch is filtered out of the signal to a higher degree.

This is more of an issue with single coil pickups than humbuckers and some people don’t ever notice it because they have their guitar turned up to ten all the time (that's me!). However, if it’s an issue for you, the way to resolve it is to employ a high pass filter which bypasses the volume potentiometer in the form of a treble bleed mod. As the name suggests, a high pass filter is the opposite of a low pass filter and allow through only high end pitch. This is achieved by placing the filter between the ‘in’ and ‘out’ lugs of the volume pot (the ones that aren’t connected to the back of the pot). It’s an easy mod to achieve as nothing else needs to be de-soldered.

There is a potential drawback with treble bleed, in that the guitars tone may become too bright when the volume is rolled off. But there are different varieties of treble bleeds so you can experiment with different schemes and values. There are three different treble bleed mod options (from left to right):

1. A capacitor on its own, usually of a value between 500 – 1000 picofarads

2. A capacitor of between 1 – 2 nanofarads in parallel with a resistor of a value between 100 – 350k

3. A capacitor in series with a resistor (values below)

The capacitor on its own might create too much brightness for you as you roll off volume. Adding the resistor in parallel, the Seymour Duncan scheme, will attenuate the amount of treble pitch being passed through. Option 3, sometimes called the Kinman treble bleed (after its creator Chris Kinman), employs a resistor and a capacitor in series and is in my opinion the most useful solution (Fender agree and use it in their guitars). However, you might want to use option 1, a capacitor on its own, in a bridge pickup where you want to maintain the bite of the sound as you roll off to a greater extent. PRS use a cap only in their guitars. It’s all very subjective and in the ears of the beholder, so use whichever one you perceive to be the best for your guitar.

Values of the caps and pots are important, but within reason there’s no harm in experimenting. A lower value capacitor allows more highs through and a higher value resistor prevents them from going through. Chris Kinman recommends a balance of a 1.2nF polyester capacitor and a 130k resistor and recommends that you should match a low value cap with a low value resistor (or high with high).

Despite reports to the contrary, you will find that different types of capacitors impart different qualities to tone. However, you don’t need to spend a fortune on Russian military paper-in-oil caps or ‘orange drops’ to get your sound (although in some guitars they will be the best tone caps to use). Inexpensive, low voltage film caps of any kind will usually work well (I like polystyrene). Physically larger sized caps will get in the way, make it harder to wire up the pot, and can create ground issues if they make accidental connections in an overcrowded control cavity.

Use a hot iron and heat sink (a crocodile clip or similar) when you solder in the treble bleed. It will help prevent the heat damaging the capacitor.

I hope this information is helpful in achieving your ideal sound.

In the last blog we looked at why you get unwanted hum from single coil pickups. Now, we are going to look at two coil pickups or ‘humbuckers’ as they are commonly known and also why magnetic polarity and wind direction are so important when installing pickups.

The reason why humbuckers effectively tackle 50 (or 60) cycle hum and electromechanical interference that if you run a signal through a coil in one direction then reverse it in relation to the magnet and wind in the next one, the waves of the AC current’s oscillating 50 cycles (i.e the hum) go unseen by the pickup. The two coils are wound in different directions; looking from the top) one is wound clockwise and the other counter-clockwise. In addition, the magnets inside the coils have opposite polarities. One coil has the north side of its magnet at the top and the other has the south side of its magnet at the top. These two factors are the ‘polarity’ and ‘phase’ of pickups. Phase and polarity are why, even though all three Stratocaster pickups may look the same, you will probably run into problems with phase and hum if you put them in a different position to the one indicated on the pickup.

If you have a Stratocaster or another guitar with two single coils you can get the humbucker effect in the switch ‘mix’ position, but only if both the polarity and wind direction are opposite in the two mixed pickups. A stock factory fender Strat's middle pickup should always be opposite in polarity and wind to the bridge and neck pickup. If the polarity and wind were the same in both pickups you will have an in-phase arrangement but you will not not get hum cancelling. But, if only one of either the polarity or the winding is different, you will get phase cancellation - what’s called an ‘out-of-phase’ signal (see the diagram above). This is where most of each coil’s signal cancels out the other coil’s signal because the waves are working in opposite directions. Because no two pickups are identical, some of the signal will get through, but it won't be a very full sound and it will be quieter. You only get both in-phase and hum cancelling if both the phase and winding are opposite in the two coils.

An out-of-phase sound is 'thin' - quieter and lacking in middle range. Some guitarists find this sound useful and some even prefer to have their pickups wired this way. Peter Green of Fleetwood Mac deliberately used out-of-phase humbuckers in his Les Paul. However, most people prefer the beefier in-phase sound and would want to avoid a permanent out-of-phase effect with their humbucker or when mixing two single coils.There was a period in the eighties where many Japanese companies' guitars came equipped with in/out phase switches and it is not a difficult mod to achieve if your humbucker has separate wires for each coil.

Using the following polarity and winding direction combinations will give you the following results:

You will note that only opposite winding and opposite magnet polarity gives you an in-phase tone and hum-cancelling properties.

Some pickup manufacturers will tell you whether their pickups are reverse polarity and wind or ‘standard’ (although what is standard in terms of polarity is not agreed by manufacturers). Seymour Duncan, for example, produce standard and ‘reverse wound, reverse polarity’ (RWRP) pickups so that if you want to replace your pickups you can do so and avoid issues with phase and hum. This is certainly something to be aware of when you are considering replacing your pickups. We have sometimes had issues with a client’s replacement bridge pickup being out-of-phase with the middle and neck single coils, producing a thin mix position. Because there is no agreed standard for coil wind direction and magnetic polarity, it’s not easy to know what the wind and polarity of your pickup is and you will need specialist equipment (a centre position microammeter) to properly test polarity.

Ideally, on a three pickup guitar the middle pickup will have reverse wind and polarity, so that when it is

mixed with either the neck or bridge pickup, hum is cancelled and the guitar is in phase. The same

should apply to a guitar with two single coils, like a Telecaster. In our experience, this often isn’t the case, especially with less expensive guitars, like the Ibanez RG pictured here, where the two single coils are identical and therefore not hum cancelling. At a guess, this is because it would cost more to produce two differently wound pickups and maybe it’s just easier and more reliable to pull out all the pickups from the same box when assembling a guitar on a production line.

One thing worth bearing in mind when coil splitting a humbucker is that, when split, it is preferable to use the coil which is RWRP to any other pickup you may want to mix it with. For example, if you have an H-S-S format strat, you need the split humbucker’s live coil to be opposite to the middle position single coil and this may not be the one which the manufacturer defauls to in their wiring recommendation. The same would apply in a guitar with two split humbuckers of differrent type. You will need some equipment to work out the polarity and wind, or you can take the experimental road and swap the wires around until it’s right!

In this blog we’ll look at what causes pickups to hum and how to deal with it.

We’ve had a couple of guitars in the workshop recently which have had hum issues, so it’s worth recapping how and why this happens. How pickups convert the vibration of strings into sound via an amplifier is another blog, so let’s take that as a given and look at the reasons why hum occurs and how it can be tamed.

For reasons we will come on to in the next blog, hum is normally only an issue with ‘single coil’ pickups. By single coil, we mean a pickup which contains one magnet with one length of wire wound around it. Single coil pickups are typically found, for example, in classic Stratocasters and Telecasters. ‘Humbuckers’, which contain two separately wound magnets, were invented to overcome pickup hum and first appeared in Gibson guitars in the 1950s.

Unwanted pickup noise is normally produced by two things: 50 cycle AC power (or 60 cycle, if you are in the US); and electromagnetic interference (EMI). You can only hear it when the pickup is idle and not generating its own sound. Usually, a guitarist will notice the hum and EMI gets worse in a rehearsal studio or at a gig. This is because at these venues, there are a lot of devices plugged in to the power network, reinforcing the 50 cycle oscillation, and usually also producing their own EMI. Lighting and computers are also a known problem. Sometimes, in extreme cases, your guitar will pick up the local cab drivers or other radio transmissions. A rehearsal studio I use has an electrified railway track above it, which produces an uncomfortable amount of noise through the amps when it rains!

The way to tackle hum is to first ensure everything that should be grounded in the guitar is joined properly (i.e. strings, via the bridge, and all the electronic cases in the control cavity), but only joined once – creating ‘ground loops’ can in itself produce hum. There are various ways to test this with multimeters, which we won’t go into here, but there are plenty of online resources on this. Once you have eliminated poor grounding you can look at what surrounds the pickups and their controls. Single coil pickups and their controls should ideally be ‘shielded’.

By shielding we mean creating a metal ‘compartment’ around the controls (and, if you really want to, around the pickups too). You will often see partial shielding in the form of an aluminium sheet applied to the back of Stratocaster-style guitars’ scratchplates. This is only partial EMI shielding - it isn’t very effective because the cavity is generally not shielded, but it’s better than nothing.

You can do proper shielding with either: metallic shielding paint; or copper foil. Paint is easily applied and you can get it into the wire cavities as well. However, it is not as good a conductor as copper and can lose its properties over time. We see a lot of guitars which are ostensibly painted with conductive black or grey paint which, when tested with a multimeter, conduct nothing. That is why we prefer adhesive copper sheeting and tape, which is conductive on both sides and lasts forever. If applied properly, copper will create a ‘Faraday cage’ around a control cavity which will block EMI. Copper shielding also eliminates the need to solder ground wires between potentiometers, because your pots will be grounded via the copper (as long as the pots are tightened sufficiently!).

Another manufacturer tactic is to use shielded cable. This is most often seen in Gibson guitars. The centre of the wire carries an insulated ‘hot’ signal and the outer metallic braid carries the ground. The braid can be soldered to the back of potentiometers, meaning the whole wire path is prevented from acting as an EMI antenna. In our opinion this is a bit OTT, and you are limited in this by the wiring that comes with your pickup (unless you want to get really serious and change the wires on the pickup yourself).

It’s rare to be able to eliminate hum completely, but shielding will definitely make a big contribution. Some pickups seem more or less prone than others. Shielding Fender or Squier Jaguars seems to make a tremendous difference, although possibly this is also because those pickups have a bit of built in shielding.

Whatever you try to do, you will be lucky to completely eliminate hum and EMI with single coil pickups, but shielding will definitely help tame the beast. In our next blog we will look at how to use the mix position to turn two single coils into a humbucker and eliminate hum and how polarity effects pickup sound.

You can get your guitar shielded at String King Guitar Works. Check the Services page for pricing.