Vintage EL84-SE Amplifier III
In the middle of the picture a piece of mu-metal shields the ground conductors that part from
the star ground underneath towards the input RCA sockets. The plate bends U-shaped back to the
solder stripe below where it is soldered to ground for fixing it.
The two ground wires run parallel to the transformer windings. Without the shield, the
transformer would induce noise between the power supply and the signal ground. This noise would
manifest as a loud 50Hz hum. Mu-metal is a good shield against magnetic fields.
Lesson learnt: Avoid running ground wires parallel to transformer windings!