How to Restore Your Model A, Volume 8, pp.110-112, has a reprint of an article by Bob Hess on "Converting to 12 Volts." Part of the discussion is about modifications needed so one can reliably use their existing 6 volt horn. I would recommend that anyone planning a 6 to 12 volt conversion should look at this article.
As the reprint is from a 1991 original article, revised 2002, I found one additional update that is still needed.
Because of the heavy current draw of a horn, the author indicates a small resistor, sometimes used for dropping the voltage, does not work when converting a 6V horn for use with higher voltage. He recommended an Ohmite Divideohm part 210-100-1 RO. That part number no longer is current. Since it took a bit of searching to find the current part number and a source where I could buy it, I am posting the updated number for anyone who can use it.
The new Ohmite part number is D100K1ROE.
I found it at
www.galco.com
the other side shows how it is adjusted, moving the slider bar in the middle:
The author also recommended adding a relay to the horn circuit as a means of getting a stronger current to the horn. I built a small test circuit, shown below, to be able to test and adjust the sound of my horn.
My horn still needs some adjusting as I put it together from several different, older (mostly aftermarket) horns and replacement parts. The little 2X3 board is just a simple mounting arrangement of the components of the circuit. Once I get it all adjusted, I will permanently mount the relay and resistor on the floor pan under the seat.