"Michael A. Terrell" wrote in message
m...

A large, powdered iron toroid works even better. Like an old 3A
variac. I needed a modulation transformer for a AM mobile radio
back in
the '70s. I wound a second winding on a surplus 88 mH telco toroid,
and
had excessive audio bandwidth.

However a coiled sheetmetal 3 Amp Variac core may not work at all. The
response of one I measured fell to nothing above 600Hz.