codes

RESISTORS

Resistors used in vintage valve equipment tend to fall into three categories, only two of which are colour coded. The third category is the wire-wound type of resistor, where values are normally printed on the vitreous paint finish of the component.

Resistors are measured in OHMS. The symbol for resistance is the Greek letter OMEGA, which looks like a small upturned horseshoe. The OHM is the unit of resistance but is is a very small unit for valve radio use and most resistors will be found to have high values i.e. thousands of ohms or even millions of ohms. Rather than print out lots of zeros, the symbol 'k' -kilo - is used to denote thousands and the symbol 'M' - mega - is used to denote millions. Examples: 56000 ohms is printed 56kohms. 1,200,000 ohms is printed 1.2Mohms.

Note that the omega symbol is not used in the text because of the limitations of the web.

When buying modern resistors, the multiplier may be inserted in place of the decimal point, hence 1.2Mohms becomes 1M2 ohms. Note that this code only refers to three or four banded resistors.

Carbon resistors are colour coded either (1) as body-tip-spot or (2) triple bands of colour. A fourth band may be used on either type for tolerance, which is a measure of how close the actual value is to the coded indication. No band equals 20% tolerance. Close tolerances only become important when the resistor value is quite low. Otherwise, valves are usually able to operate within the range of 20% above or below the coded value. For replacement purposes, fit similar tolerance replacements. Conversely, fitting a close-tolerance component when unnecessary will do no harm.

On banded resistors, read from the end toward the centre. On body-tip-spot types, read in that order. If, on the latter type, a colour seems to be missing (either the spot or the tip) it is because the 'missing' value is the same as the body. Example: Red body, red tip, brown spot = 220 ohms.

The resistor colour code chart

EXAMPLES

The standard resistor shown in the graphic is coded Red -Yellow - Orange.

Red = 2. Yellow = 4. Orange = 3 (the number of zeros). Therefore the resistor is 24000 (24k)

The carbon stick resistor shown in the graphic is coded Orange - White - Green.

Orange = 3. White = 9. Green = 5 (the number of zeros). Therefore the resistor has a value of 3900000 (3.9M)

Body = red, tip = green, spot = orange. Value (in ohms): 25,000 (25K). No silver or gold band means a 20% tolerance.

First band = green. Second band = blue. Third band = red. Fourth band = gold. Value (in ohms) 5600 (5.6k). Tolerance of 5%.

CAPACITORS

There are a number of methods of coding, depending upon the amount of information given. The coding is the same as for resistors. Colours are read from left to right, the first colour being nearest to one end. Some capacitors have an arrow denoting the direction the code is to be read.

ONE COLOUR: Tolerance only

TWO COLOURS: Tolerance and voltage rating

THREE COLOURS: Capacitance (in picoFarads)

FIVE COLOURS: The first three colours denote capacitance in picoFarads, the remaining two denote tolerance and voltage rating.

AMERICAN CAPACITORS

Slight variations on the above. The RMA three-dot code is used for capacitors having a voltage rating of 500V and a tolerance of 20%. The dots simply give the value in picoFarads.

The RMA six-dot code offers - top row: First, second and third significant figures. Bottom row: voltage rating, tolerance and DECIMAL MULTIPLIER.

American fixed ceramic capacitors have a broad band followed by four narrow bands or dots giving temperature coefficient, first significant figure, second significant figure, DECIMAL MULTIPLIER and tolerance.