Wiring Color Code (Tube Amplifier)

The question of a wiring color code for vacuum tube circuitry is one we get quite often. As far as we know, there is standard color code for this. Below is the color code that Bruce Heran uses for his valve audio projects. These color codes can be used in conjunction with Bruce's tips on the Design and Construction of Vacuum Tube Amplifiers.

Tube Amplifier Wiring Color Code
Black or black with stripe = AC mains input
Green on the AC mains is chassis ground (if your electric code permits)
Red is high voltage DC, usually the B+
Red/Orange = intermediate B+
Bright orange = positive side of heaters if DC operated
Bright green = negative side of heaters if DC operated
Pale green = heaters if AC operated
Blue = signal level / inputs (where not shielded)
White = negative feed back
Brown = cathodes
Purple = control signals

The typical wiring found on a lot of power and audio output transformers. Not all transformer makers use these color codes, but it is good guidance if you have no clue about the transformer.

Power Transformers
Black and Black with stripe = AC mains
Red = high voltage secondaries
Red with black stripe or solid black (coming out of the same hole in the transformer) = high voltage center tap
Brown = usually standard values of heater voltage 6, 12
Brown/with stripe = standard values with center tap
Green = also used for standard heaters
Green with strip = also used for standard heaters
Blue = non-standard heater value (like 8 or 10 etc)
Blue with stripe = center tap
If there is a second heater it is often yellow and yellow
If there is a third heater or a bias winding it is often orange and orange

Audio Output Transformers
Red = B+ input
Blue = plate number 1
Blue/White = screen number one if a U/L transformer
Brown = plate number 2
Brown/White = screen number 2
White = speaker ground
Orange = 4 ohms
Yellow = 8 ohms
Green = 16 ohms