Wire-wound resistors are used when a lot of power needs to be dissipated in your amp and usually these are around the output valves for bias resistors. As the name suggests the resistors are made from restitive wire that is wrapped around a piece of material to make a resistor. They are used especially in amps with an output of 40 watts or more. There is a picture below of wire-wound resistors . Depending on the manufacturer and the age they can be many different colours.
It is important that they are run regularly and that the rating used ensures they get hot enough. One reason is that the cement on the earlier ones allowed moisture to be absorbed and if left unused for a long time the wire inside them would corrode ! This causes the resistor value to change or completely fail.
These resistors do get hot enough to burn if you touch them. In modern amps they mount them on the printed circuit boards and they can get so hot that they burn the boards turning them from a nice green colour to dark brown. This is not really a problem until the metal tracks on the board get so cooked they peel away fro the printed circuit board and you have an open circuit board failure. Old amps that used turret board have these resistors hanging in the air so they dont damage the turret board. This is another reason to have a hand built non printed board amp !!
Published with permission using the picture and information from and Dave Porter and Peter Chadwick