Capacitors: Values and Types

There a few topics more likely to cause arguments on guitar forums than capacitors (caps). So, let's have a look at what's really going on.

In a guitar signal path (note that I am not using the term 'circuit' - more on that later), capacitors are usually employed as a 'high pass filter'. This is useful in a guitar if you want to vary the tone coming from your pickups.. By sending part of your signal to ground via a potentiometer you can use caps to variably filter out the high end and this is generally what they are used for in guitars.

The value of the cap you use will determine how much of the high end of your tone is removed. Traditionally, somewhere in the range of 0.022 to 0.047 microfarads (22 - 47 nanofarads) are the usual values, but you can find 0.1 microfarad caps in some older guitars. These days many guitar players and techs are experimenting with much lower values. This is largely because many people find the larger value caps chop off too much high end and can in some cases sound more like a volume control. Using a lower value cap can gives a more subtle tone variation. If you want to experiment with this try something between 3.3 and 6.8 nanofarads.

This brings us on to the most contended area of the subject of capacitors - materials. Capacitors can be constructed of a few different types of materials: paper-in-oil (PIO); film (polypropylene, polysyrene or polyester); and ceramic are the most usual formats. There are two ends of the spectrum here. Some people will say that only old-style PIOs such as 'bumblebees' sound good in guitars. Alternatively, it is often argued that it makes no difference what type of material a capacitor is made of and that it is only the value that counts. This is a belief most strongly held by trained electronics experts. The truth of the matter lies somewhere in between.

While it is true that in an electrical circuit the material of a capacitor has no relevance in its ability to act as a capacitor in the traditional sense, and the electronics boffins can produce formula to demonstrate this, a capacitor in a guitar is part of a signal path, not an electrical circuit. In other words, it isn't doing at all the same job in a guitar. What is passing through the wires in your guitar is a current which is acting an analogue of the vibration of the strings, a conversion of very complex frequencies, overtones and tones into an electrical signal, which will be converted back into sound by your amplifier. The capacitor filters out some of those signal frequencies via the material of its construction, and the frequencies and tones that material allows though and removes from the signal path can vary. You can check this for yourself by setting up some wire and crocodile clips and connecting different types of capacitor of the same value to a pot. Provided the caps are genuinely of the same value (which can be checked beforehand with a meter) subtle differences in tone can be heard. Some people will argue that this is 'confirmation bias' (you expect to hear something and you do) but I have blind tested this with clients.

People who argue that only old capacitors taken out of radios or expensive Russian PIOs will make your guitar sound good are probably trying to sell you expensive, old capacitors. Sometimes a Russian PIO will sound great but sometimes a film cap costing a few pennies sounds better. Polypropylene Sprague Orange Drops (and there are plenty of fakes of those out there) sell for alot more than much cheaper film capacitors but they may not be any better and with some guitars sound worse (although they sometimes are just right for a particular guitar). Mullard 'mustard' caps are prized by some, but I have found them far to dark to use in a guitar.

Generally, I don't recommend any particular type of capacitor as each guitar responds differently to different materials and values, but a polystyrene film capacitor of 10 nanofarads is where I will start or what I will use if a client expresses no preference. I rarely go up to 47 nanofarads or beyond - there just isn't usually a call for that much high tone to be rolled off and it usually results in the tone pot acting more like a volume control. I will never use ceramic capacitors in a guitar. Some people will argue that is OK, because both Fender and Gibson use them in theirs. The reason the big manufacturers use them that is that ceramics are a cent each and even in bulk a film cap will be 10 to 100 times as costly. If you are making thousands upon thousands of guitars every year, the savings from using ceramic caps are significant. It's not an aesthetic decision, it's an economic one, and, despite using ceramics in most of their instrument, even Gibson say that the material makes a difference.

A lot of people will argue that when your tone potentiometer is set at 10, the cap is having no effect on tone. This is ignoring the physical fact that the potentiometer is connected to ground via its wiper all the way across the range from 1 to 10. It's true the graphite screen in the pot is very narrow at 10 but I will challenge anyone with a decent pair of ears not to notice the difference between a tone pot working at 10 and a pickup disconnected from a tone pot. This is why you can buy 'no load' potentiometers, if you want them, which are genuinely disconnected from the signal path at 10.

The voltage of a capacitor has no impact on its tone. Most of the ones found in guitars are around 200v and that's just fine. The voltage going through your guitar is around 0.25v maximum. A larger value cap will just take up more space in your control cavity. Of course, if all you have is a massive 600v cap, there isn't any reason not to use it as long as it physically fits. The format of the capacitor makes no diference either. You can use caps with radial or axial leads or 'box' capacitors.

So, that's my musings on tone capacitors in conventional use. In some circumstances, a capacitor will be used to brighten up a pickup by placing it in between the pickup and the controls but you will want to use a very high value for this and it's an esoteric solution for pickups which are too dark. Capacitors are also used more commonly in treble bleeds, and there is another blog on this website on this subject.

I hope you found this article useful. If you need to discuss anything cap-related to do with your guitar drop us a line.

What's a Treble Bleed Mod?

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.

Humbuckers, Polarity and Phase

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!