Color coding of tube amplifier wiring
last updated: 22.12.2012

There is no official standard for this anywhere, so this here is just a good proposal. Also when building, you are going to see that you can't always do it 100% right as the number of available colors is not enough. Besides there is no need for a unique color scheme, since we usually ALREADY know we are working on DC heated filaments, just we don't know what is plus or minus. Also don't make an issue about two different colors connected together, this is normal.

Mains

General recommendations for wiring:

  1. Keep apart the safety ground and circuit ground. The idea is, safety comes first. For this reason, safety ground wires should not be cascaded. This means, each safety wire must still work, if another one has a loose contact. A typical mistake is described like this: The safety ground from the AC inlet is grounded to a near by metal part called "A". Then, to that part (A) some other safety ground wires are attached, and one of them goes to metal part "B". Then, on Part B several safety wires are connected. The risk is now, if the wire from A to B is bad, this can become unnoticed, and everything connected to B is not grounded anymore. If you would ground everything on Part A, this risk is eliminated, though it costs some more wire. Additional the chassis connection from Part A and B (and others) should be with a wire. All if this is just to prevent a non working safety ground in case of human errors.
  2. Safety ground may never be connected with a screw, because a plain screws gets often loose over time. If you have no other choice, you should attach a (copper) bolt to the chassis, and attach it to the chassis in a way that can never get loose. On that bolt you can attach some lugs with a nut. If you look into your washing machine, you'll find this pattern, it's the safest. It sort of assumes wires get bad, and anything with a thread is no good safety connection.
  3. For safety, do not stick a safety wire into a solder lug, and then solder it, but wire it around the lug first, and then solder. This prevents cold flow of the connection, in case of mechanical force on the wire.
  4. Always switch the mains with a two wire switch.

Mains Coloring

In some countries the mains have one "hot" wire, which has the full mains voltage, and one "cold" wire which has zero Volts to ground, or just a few volts. In other countries the mains is symmetrical, meaning each wire has half the mains voltage, and each is 180° out of phase with the other. This will greatly improve the safety aspects, but it is more expensive. When wiring the mains part of the amplifier, you assume, one end of the inlet is "hot" and the other is "cold". Even when you have a symmetrical mains. Ssometimes curious hard to cure hum problems can be solved by reversing the mains plug. To have this option, you must wire the amplifier clean and tidy.

Twist all mains wires, and make sure the current in each of the two is in opposite direction. This will greatly eliminate the radiated hum field.

Green-Yellow    
Safety ground
Brown  
Hot end before mains switch
Blue  
Cold end before mains switch
Black  
Hot end after mains switch
Blue  
Cold end after mains switch

 

Output transformer coloring

This wire is often seen, but exceptions exist

Red   Supply Voltage
Pink   Tube side
Black   Output Ground
Orange   Output 4 Ohms
Yellow   Output 8 Ohms
Green   Output 16 Ohms

 

Ground Path

Truly specialist know how to make wiring without wasting too much wire, sometimes make small sins, that do not cause hum, to save wire and reduce the number of solder joints. Sometimes you have an old radio or music amplifier, and it is absolutely hum-free, and yet simple wiring, no chokes, and remarkable low power supply capacitors like 2uF only. This marks true designer art of the old days. If YOU try it, you may be disappointed. To prevent problems here are some rules of thumb:

Do not fight hum with overly large capacitors. Sure voltage will be more stabile, the larger they are, but charge peaks are excessive, and will be terrified when you make those peaks visible on a scope. They are sharp, strong peaks and current shape is aggressive. So you solve one problem, and you get another. Better is to increase the choke value. That will also make the output voltage more stabile AND reduce current peaks. Even so, try to make the capacitors smaller, and the chokes accordingly larger. The silence of such a power supply will surprise you now. Also the rectifier tube will thank you with longest life.

Use a single star grounding to the chassis, located at a position where signal is low. Alternatively, use or a fat (2mm) ground bar, which at first you wire independent of the chassis, and the amplifier should be able to work without a chassis ground. Since this bus bar carries the signal, and resistance must be low, the best is silver wire. We sell silver wire per meter. The price is not so exceptional, and compared to what you pay for good speaker connectors and other vital parts, this is a useful investment. So the ground bar is the artificial"chassis" to which you connect everything. Then after this is done, and it can work without a connection to the metal housing, you connect the metal bar to the metal housing with ONE wire only. This, wire is to even out any potential, but will not have a current flow. You should check this with an ohms meter, before you connect that wire, resistance must be endlessly high. This prevents a ground loop systematically.

Amplifier coloring

Red   Supply Voltage. Also to choke.
Orange   Lowered Supply Voltage. For instance via an RC filter to the pre-amp
Black  

Signal ground

Red   DC operated heater, plus. Twist red with black.
Black   DC operated heaters, minus . Twist red with black.
Brown   AC operated heaters. Always twist.
Blue   Any Negative Voltage to ground.
Green   Signal Grid (G1)
Orange   Other Grids (G2 and G3)
Brown   Cathode
Purple   Control signal, such as wires to bias potmeters, or switch wires for some functions.
White   All tone signals, including feedback Signal.
Gray   Optional

Fusing

Fuse the mains in the hot end (brown wire) before the switch. If the mains transformer is the main power user, this fuse is calculated to the mains transformer. Any smaller power users, such as switched AC to DC modules for heater supply, should be separately fused. Perhaps all together as a group. For simplicity you can take solderable fuses here, since likely the modules will not fail, and if they do they will not short. Do not connect a fuse directly to choke wires, or directly from the tube plate to the transformer. If you want to protect a tube, and you see no other way as to fuse it, , there are methods, but this is too much to explain here. Be aware you may never interrupt the current through a choke or output transformer, as the result will be leakage inside the windings. The last will be audible as crackle noise, or high frequency loss or power loss on one channel.