• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Heater wires through conduit

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There are two fundamental issues, in my mind. One is electric fields, which are due to the voltage magnitude, and can be tamed by either your conduit system (aluminum, steel) or simple shielded twisted pair. STP seems much more practical a solution for this.

Second issue is magnetic fields, which are due to the current through the wires. For some tubes, the amperes of AC heating current can be significant. Traditional copper or aluminum shields are ineffective to shield magnetic fields, so you would have to resort to a magnetic shield, such as steel, to effectively constrain the magnetic field to within its boundaries.

Typical alternate solution is to tightly twist the wires, effectively minimizing magnetic field strength outside the wires. Practical issues seem to favor this approach as being sufficient.
You'll will find that hum is usually the result of your heaters living so close to your cathodes... a few thousanths of an inch.

Techniques abound for balancing the capacitively coupled energy into the cathode and kill the evil hum. It all depends on the particular circuit/topology employed as to the best solution.

Tightly twisted wiring is ALWAYS a good place to start.

Elaborate and unnecessary. How do I know this, hm?

Twist your heater wires together fairly tightly, and don't bundle them with the other wiring. Keep them an inch or two away, and cross as orthogonally (at right angles) as you can.  Reference the heater circuit to a voltage (ground is fine) that is within the tubes' heater-cathode rating, especially if the heater secondary is on a general power transformer, core being shared with other stuff.

I use AC heaters always, and my hum + noise is between 20 - 40µV, or two to four one hundredths of a millivolt, and is dominated even in a fairly simple circuit by tube rush.

One geek's opinion (I got a belly button, too!).


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