• 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.

6AU6 replacement

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Screening is placing a grounded conductor in the way. That stops an electric field.
I've never heard of that. So just running a conductor that attatached to ground in between a heater line and a signal line will block the field?
Mains transformers often hum a bit. Mount on rubber washers, but ensure you have a ground connection to the transformer frame.
This is the first amp of this size I've built, everything else has been SE 6BM8 or 6BQ5 amps. So I'm not used to a transformer of this size. It hums and buzzes.
 
AC heater wires can cause hum problems from both electric and magnetic fields. Twisting reduces the magnetic field.

The electric field can be reduced by running the wires in the corner of the chassis. This is because, to a limited extent, putting a grounded conductor behind can reduce the field. (See 'method of images' method for determining the field from a charge near an equipotential plane.) A screen all round is much better of course.
 
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