RESISTORS: Basics for Guitar Pedal Builders

Everything listed below is about guitar pedal builds and is not intended cover all possible uses of resistors.  It's an incomplete and general reference intended for someone just starting out building pedals.   So if I have missed particular properties or information, it's because I either don't understand them or it's irrelevant for this purpose.   

RESISTOR BASICS FOR PEDAL BUILDERS

All fixed resistors are made from a combination of conducting and non-conducting materials (insulators).  

The ratio of the two materials determines the resistance level, measured in ohms.  

In other words, if a resistor has a high amount of conducting material compared to non-conducting material, it's a lower-value resistor.  Electrons easily flow through the resistor, as resistance along their path is low.  

If the opposite is true, and there's a lot of non-conducting material in the resistor compared to conductors, then it's a higher-value resistor.  Electrons now have a harder job of passing through the resistor; therefore, resistance is high.

What do resistors actually do?   As mentioned above, they provide resistance to the flow of electrons, which controls current (sometimes called intensity).  Current is measured in Amperes, usually shown as I in equations.  So thinking of current as intensity when I is used as the symbol can be handy.  

In very broad terms, resistors are used to control the intensity of the current going to other components in the circuit.  Current relates to voltage and power as shown in Ohms Law, so as well as controlling current, resistors can also control voltage to components.  Resistors are also used to set impedance, divide voltages and form filters with their friend, the capacitor.  I'm sure they do more, but we'll keep things simple here.

CARBON COMPOSITE RESISTORS

The magical resistors make everything sound vintage right?  Well, they at least look the part, and that's the only reason I use them in a build.   

Pros:  Look vintage/period correct.

Cons:  Expensive, can be hard to find, tend to be large compared to other resistors, technically higher in thermal and current noise than other resistors.  Tolerances are usually 10% or 20% and are not very stable.   

Here's an interesting link with a summary of studies on carbon composition resistors over the decades.  It will not be good news if you like carbon composite resistors.  Incredibly inconsistent and noisey is my one takeaway.

CARBON FILM RESISTORS

Carbon film appeared after carbon composition resistors and is still widely used today.  Nothing wrong with carbon film at all.

Pros:  Look vintage/period correct, very cheap and accessible.  Tolerances are good

Cons:  Technically higher in thermal noise than more modern resistors (but again, you probably won't hear this in your build)

1W Carbon Film Resistors

METAL OXIDE RESISTORS

Metal oxide came after carbon film, so the next technological progression.  

Pros:  Technically better than carbon comp or film.  Some amp builders like these for their ability to handle high power and surges.

Cons:  Not as readily available as metal and carbon film.  Perhaps a little more noise than metal film.

1W Metal Oxide Resistors

METAL FILM RESISTORS

Metal film is an improvement on metal oxide, so technically a better resistor than any other standard resistor available and is considered the standard by most people.

Pros:  Cheap and accessible, technically very low noise, and high tolerances - usually 5% or better.

Cons:  Can be easier to damage with power surges.

RESISTOR VALUES

At first, the available values might seem very random, but there is a system to this.  The standard is the E24 series - and there are many different series in the E standard, but let's not go there.

Resistors are spaced in values so that one resistor in series, does not overlap with the next in the series, when tolerances are considered.

For every value resistor that you can find, there are others available in multiples of 10, 100, 1,000 etc.  example:

33ohm        3.3k        33k        330k        3.3M

So when trying to trace a circuit, you know you are probably not reading a resistor correctly if you come up with an odd value, not in the usual series.  Custom resistors are available but rarely used in guitar effects (likely due to cost and being pointless).

RESISTOR MARKINGS

Resistor values and tolerance are designated by colour codes, which are the coloured bands on the resistors.  

READING RESISTOR VALUES

The coloured stripe closest to one end is the first stripe.   Read left to right from the first stripe.  The last stripe is always tolerance.

Some resistors are easier to read than others, especially four-band resistors using a gold or silver band for tolerance.  It's really obvious where to start and there are only three colours to consider.

I won't list the colours etc, here, as while I remember most of them now, I still use an online calculator occasionally to double-check.  There's plenty out there, just google it and bookmark a favourite.

RESISTOR TOLERANCE

Resistors come in various tolerances, ranging from plus/minus 20%, 10%, 5% and 1%.  Tolerance is how far from the stated resistance value the resistor might be.  It doesn’t mean they will be 20% off; it just means they might be.

In a guitar pedal, 5% tolerance is fine.   Even 10% is not going to be a big deal in many cases.

Smaller tolerances can be found, but they're expensive and unnecessary for anything related to guitar effects. 

Most resistors, especially metal film will measure quite close to their actual value.  Vintage carbon composite resistors, on the other hand, can be a bit off - they can drift in value over time.   

RESISTOR POWER RATINGS

Resistors come in a range of power ratings, measured in Watts - 0.125 W, 0.25 W, 0.5 W, 1W, 2W etc, and once we see larger values, they start getting into wire wound and specialised resistors, which are not used in small signal circuits.

Quarter to half watt are commonly used for pedal building.  Don't buy big resistors for mojo value, as they generally don't fit a board designed for regular-sized resistors.  But suppose you are doing a mojo build with fancy axial or tropical fish capacitors. In that case, normal quarter-watt metal film resistors can sometimes look out of place (at least in my opinion).   

Unlike resistance & tolerance, power ratings are not marked standard resistors.  If you have an unknown resistor, you must guess based on size.  

Exceed the power rating, and you will have a dead resistor (it will overheat and burn).  Kind of odd that they never list power on the resistors, as the result of mistakes is a dead resistor / possible circuit damage.