filament choke for 3cx3000A7 / 3cx10,000 /3cx15,000 transformer needs to be isolated
This choke is used on the primary side of the filament transformer for all large Vacuum tubes. The filament transformer needs to be isolated from the chassis and the choke is place on the primary side.
Note on the YC-156 and tubes that dont draw a lot of current this choke can be used to directly feed the vacuum tube. see picture.