[Heathkit] 401/300 need part

[yash at aol.com]

Iam running a sb-401/300 combo. Want to add a sb-650 heath digital 
readout to the setup. Reading the manual they say a required choke is 
needed to be added to the 401 or the system will not work as it should. 
They give no value for the choke in the manual ...anyone have a idea 
what I need? checked the part cross listings for heath ,could't find 
any thing. Would be greatful for any help.
thanks
dale  wt4t