[Boatanchors] shielded audio pair - twist and shout.

[BLIMPY]

with a balanced line audio input to push pull grids.. you want a shielded 
PAIR... not a single shielded conductor. ( which comes in hi-z and low z 
versions anyway).

If you dont have this handy.. you can  and should twist a pair of wires 
together.. and run that inside salvaged RG-58 shield.

scootch ( important technical term)  the braid down over the coax like an 
expanding chinese handcuff ( if you dont know chinese handcuffs you are too 
young to work on boat anchors) .

somebody will want to submit some formula with circular mils per whatzit to 
arrive at the correct size wire.. but just get some handy stranded hook up 
wire... twist your self as many feet as you need ( plus a bit)... and have 
at it.

In really Low-Z application like carbon mic cords... this will often work 
without any shielding at all.  Really depends on ambient RF in your shack.

BC-610  isnt famously well  by-passed for RF anyway..
and HT-4 couldn't be any better. So, the sheilding is cheap insurance.

When in doubt... consult the manual... see what they did on the original.

Novel Idea, eh ?