12 volt conversion effecting 6 volt horn

I run 6 volt horns on 12 volt in my 49 Buick and my 52 Willys CJ3A. Nice and loud and no problems.

 

This is what I do and works well. Most ignition resistors work well also if ohms are right.

 

I have a resistor switch with a knob to speed up or slow down on my exhaust fan in the shop. Could I use one of those on a horn? Turn the knob up and down and WWWWoooooooooooowwwwwwwwwwWWWWW?

 

By the time I think to honk the horn, the offending party is long gone so it doesn't matter.