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Step 7The Plan
Here's where we decide how to implement the changes.
Wiring the N-68X iso transformer
Primary--
The N-68X can be used with either 120V or 240V AC systems.
US 120V
For 120V, place the two primary coils in parallel.
Tie these colors together, and connect to the mains (through the switch, etc.):
-- Black and Red/Black
-- Yellow/Black and Green/Black
Euro 240V
For 220-240V, wire the N-68X primary coils in series:
220V / 240V mains-- Black and Black/Green .
Connect Yellow/Black and Red/Black together.
Secondary--
Use only the two Red secondary wires. The white wire is the shield. Connect it to the chassis (or earth ground) if it's mounted there, or if you experience any noise.
Re-routing the switch
The original ON/OFF switch is mounted on the chassis panel. To keep the switching truly functional, we'll have to route it differently.
We could leave the switch as-is, but then the primary of the isolation transformer would be in a permanently ON condition. Only unplugging the cord would cut the power to the trannie. The switch would still operate the amp, but there would still be some current draw. That's wasteful and "bad form."
To use the original switch, a simple two-conductor wire can be attached, and run down to make/break the incoming AC connection to the isolation transformer.
Connect the earth ground
With the three-prong cord addition, a true earth ground is available.
Attach a wire from the center prong (should be Green, but verify) of the plug and connect it to the chassis.
Optionally, the transformer casing can also be grounded.
Power -- connecting the isolated AC
OK, here's where things get a little "iffy."
The Simple Way:
The transformer's secondary can be connected directly where the old power connections attach. In this case
Wire 1 ) to the rectifier plate, and the series filaments
Wire 2 ) to the chassis ground
The order of the secondary wires doesn't matter--the AC from the transformer is isolated, so there's no Hot or Neutral side. They are both Red for a reason...
The Correct way:
Read the next Step--it deals in depth with half-wave rectification...
Wiring the N-68X iso transformer
Primary--
The N-68X can be used with either 120V or 240V AC systems.
US 120V
For 120V, place the two primary coils in parallel.
Tie these colors together, and connect to the mains (through the switch, etc.):
-- Black and Red/Black
-- Yellow/Black and Green/Black
Euro 240V
For 220-240V, wire the N-68X primary coils in series:
220V / 240V mains-- Black and Black/Green .
Connect Yellow/Black and Red/Black together.
Secondary--
Use only the two Red secondary wires. The white wire is the shield. Connect it to the chassis (or earth ground) if it's mounted there, or if you experience any noise.
Re-routing the switch
The original ON/OFF switch is mounted on the chassis panel. To keep the switching truly functional, we'll have to route it differently.
We could leave the switch as-is, but then the primary of the isolation transformer would be in a permanently ON condition. Only unplugging the cord would cut the power to the trannie. The switch would still operate the amp, but there would still be some current draw. That's wasteful and "bad form."
To use the original switch, a simple two-conductor wire can be attached, and run down to make/break the incoming AC connection to the isolation transformer.
Connect the earth ground
With the three-prong cord addition, a true earth ground is available.
Attach a wire from the center prong (should be Green, but verify) of the plug and connect it to the chassis.
Optionally, the transformer casing can also be grounded.
Power -- connecting the isolated AC
OK, here's where things get a little "iffy."
The Simple Way:
The transformer's secondary can be connected directly where the old power connections attach. In this case
Wire 1 ) to the rectifier plate, and the series filaments
Wire 2 ) to the chassis ground
The order of the secondary wires doesn't matter--the AC from the transformer is isolated, so there's no Hot or Neutral side. They are both Red for a reason...
The Correct way:
Read the next Step--it deals in depth with half-wave rectification...
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The onboard transformer is expecting +/ - 115V (plus and minus 115V). You're only supplying only +115V, that would cut the voltage in half.
Carefully, check the voltages on the secondary of the chassis power transformer. Remember, this stuff can kill you...
Here's an example from Triad.
There is one big disadvantage, however--the amp would not be correctly grounded unless a three-prong cord is installed. An amp without proper grounding is still dangerous.
Three-prong cords modifications don't seem to degrade the value of most vintage amps, unless you're looking at really old ones (1930's to early 40's.) or really rare amps--but I wouldn't use one without the mod.
But people retrofit cords on vintage Fender amps all the time.
Is it absolutely necessary? Hmmm. I've played non-isolated amps now and then, just to test them. I'm still alive, but I have received shocks in the process...