Thanks @electricDruid !

Yes, "Foot Witch" is an unintentional mistake : it's an unseen typing error... But OK : the signification is
funny instead of
serious !

The inconvenient of the Serial Tone Killer is his weight :
circa 32kg in carrying order, that is to say with cables, FSW, cover.
At the start, I designed it with a toroidal power transformer, but had to return to the classic EI transformer which proves to be much more silent without much less hum and buzz radiations than the toroidal models that I tested, even encapsulated and shielded.
The "wings" added around the power transformer are magnetic shields to kill remaining 50Hz radiation along the main magnetic direction.
The original idea was to save weight by using a toroidal PT and a Neodymium loudspeaker. This ended to be an EI transformer (5.5kg) and an EVM12L from 1980 (9kg)...
A+!