Resistors

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Resistors
Resistors "resist" the flow of electricity. Resistance is measured in "ohms". A resisitor with a higher ohm rating resists current flow more; it will let less current flow through it. Most resistors don't have their values printed on them; they use a standard color code (described below).
Resisitors also have power ratings. These represent the largest amount of power a resistor can dissipate (shed) without problems. The electricity that a resistor stops from flowing through it is converted to heat. Too much heat will destroy a resistor. (This is true of any electronic component, but is of particular concern in resistors.) The amount of electrical energy a resistor dissipates is measured in watts. Resistors come in standard sizes; you can identify most resistors' wattage rating by their sizes.
Resistors are almost always tubular.
Resistors That Have Been Around A While
The most common resistors (almost all you will see in older musical amplifiers) are brown cylinders with colored bands around them, with a lead sticking straight out from each end of the cylinder. These are "carbon composition" resisitors; the middle of the resistor is a carbon compound which does all the work (the rest is just to hold things together and carry away heat). Carbon resistors commonly come in power ratings of 1/4, 1/2, 1 and 2 watts. 1/4 watt resistors are rare in tube amps, but common in transistor amps.
You may occasionally see a wire wound resistor. These are tubular also, but with a bump at each end. Except for the smallest ones, they are usually hollow. These may use color codes or may have their values printed directly on their sides. Wirewound resistors are used when exact values are needed or when larger amounts of heat must be dealt with (more than 2 watts). These can vary from very small (the size of a 1/4 watt carbon comp resistor) to very large (think big test tube).
Occasionally you will see rectangular resistors. These will usually be larger, power resistors - 5 watts or more.
Newer Resistor Types
Newer resistor types include carbon film resistors, metal film resistors, and metal oxide power resistors. You won't find these in older tube amps! (Purists maintain the amps don't sound right with anything besides carbon comps, or wirewounds where necessary. But feel free to experiment.)
Carbon film resistors are produced by vacuum depositing a thin film of carbon on a ceramic form. They are the least expensive resistors, have 5% tolerance, high stability and low noise, and are moisture stable.
Metal film resistors are conceptually similar - they are produced by vacuum depositing an extremely thin layer of metal alloy on a substrate. They are precise (1% tolerance), extremely quiet, and have a high temperature range.
Metal oxide power resistors are a less expensive alternative to wire wounds in smaller power ratings (typically 1 and 2 watt). They have a much higher operating range than wire wounds (-55C to +240C vs -55C to 155C).
Color Code
The resistor color code consists of 3, 4, or 5 colored bands wrapped around the resistor. These should be closer to one end of the resistor than the other; start reading with the band closest to the end. The first two bands represent the first two digits of the value. The next band represents a multiplier. If present, the fourth band represents the resistor's tolerance - how close to its actual value it's guaranteed to be. Carbon comp resistors will be within 20%, 10% or 5% of their nominal value. (One reason wirewound resistors have values printed on them is because they often have much tighter tolerances, such as 1%.)
The fifth band is rare (esp. on older resistors used in instrument amps). Unfortunately there have been two separate systems which used 5 bands: in the older system the 5th band represents the percentage the resistance may change per 1000 hours of operation. In the newer system, the 3rd band becomes the third significant digit, the 4th band becomes the multiplier, and the 5th band denotes tolerance. Hopefully you won't have to deal with any of these. (I have seen 5 band resistors with bands which appeared to be centered along the resistor's length, making it difficult (in some case impossible) to know which way to read the value. In these cases you will have to measure the resistor's value with an ohmeter.)
[Note - some carbon comp resistors look like wirewound resistors and use a different code that is trickier to decipher. If you run across one of these, you'll need to look it up.]
Color code
----------
Color    Digit    Multiplier      Tolerance
Black      0      1
Brown      1      10
Red        2      100                +/-2%
Orange     3      1,000
Yellow     4      10,000
Green      5      100,000
Blue       6      1,000,000
Violet     7      10,000,000
Gray       8      100,000,000
White      9      1,000,000,000
Gold              .1                 +/-5%
Silver            .01               +/-10%
[none]            -                 +/-20%
A resistor whose bands are "red red red" would be 22 x 100, or 2.2k ohms. "Blue green yellow" would be 65 x 10k, or 650k ohms. "Brown black red gold" would be 10 x 100, or 1k ohms, at 5% tolerance.
I've never seen a red tolerance band.
color code hints:

The multipliers can also be thought of as 10 raised to the Nth power, where N is the significant digit value for that color. For instance, red in either of the first bands would mean "2". In the third band, it's 10^2, or 100.
The code goes from dark to light at the ends, with a rainbow in the middle: black and brown, the spectrum from red to volet, then gray and white.
A 5% tolerance on a 1K resistor isn't too big a deal - the resistor can be from 950 to 1,050 ohms. That's normally close enough for guitar amp circuits. A 20% tolerance on the same resistor would mean that its value could be anywhere from 800 to 1,200 ohms - quite a spread! One way to get more consistency between channels, or even between amps, is to tighten up resistor value ranges. The easiest way to do this is to simply replace all 10% and 20% resistors with 5% resistors. If you're really picky, you can buy a bunch of resistors and test them with a good quality ohmeter, and use only the ones closest to their designated values, or at least use matched sets in matching circuits.
While this will seem excessive to some, others will find it worthwhile. One place almost any amp will benefit from this is at the inputs. If all inputs on a channel use 20% resistors whose actual values are at opposite ends of the spread, one input will be louder and less brilliant than the other. If you want this, great - but not everyone does.
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