I am planning to convert my 6 volt system over to a 12 volt system. Are all the alternators from Model A vendors the same? Are there comparable alternators available from NAPA or other auto stores? If so, what one best fits? Would it be best to purchase one from a Model A Vendor along with the bracket?
Any pointers would be appreciated.
Thanks.
I am running my roadster on 12 volts with the original generator and it works great!!!
First if you think that using one of the alternators from any of the vendors will make it easy to find a replacement on the road or at the local parts house, think again! This was mentioned here a few days ago, the one wire hook up alternators, six or twelve volt have been converted and are not sold at your local Pep Boys or NAPA. Most if not all one wire converted alternators require the engine to be reved pretty high before they will begin to charge, not only this, some will never show much more than a 2 or 3 amp charge and don't always keep the battery charged, if you don't drive them pretty regular. It has also been mentioned that the converted alternators have a slight current draw and if not driven on a very regular basis the battery won't have enough power to turn the starter and fire the distributor. I have alternators on a couple of my cars and I prefer the generator any day. The only advantage of the alternator would be if you needed to run lots of accessories and the more powerful quartz halogen bulbs.
I don't run any accessories on our roadster and I use the less powerful halogen bulbs from Australia. these bulbs are easy to obtain and are much better than the original type 50 candle power bulbs. I wanted brighter headlamps and faster cranking so that my wife, daughter, and grandaughter would have no problems cranking and driving our roadster. Bulbs, batteries and battery cables are readly available for the 12 volt system and much less expensive.
Changing over to 12 volts with the original generator is VERT VERY easy!!! Other than the common sense things like bulbs, you will need the 12 volt battery, a resistor for the original coil or use a 3.0 ohm coil and a Fun Projects voltage regulator. This regulator takes the place of the cut out and looks like the replacement cut out that has been sold for years by the model A vendors. This regulator can be had in 6, 8, and 12 volt in positive or negative ground. I have used this regulator 12 volt positive ground on our roadster for five years with no problems.
The original wiring harnesses will work great with this setup. Original type six volt wires are larger gauge than modern 12 volt wiring and will carry the extra voltage with no problems. If you leave it original positive ground no wire connections will need to be changed. if you need to run the negative ground regulator you will need to reverse the connections at the ammeter and battery. you will also need to re polarize the generator if you wish to run negative ground. To repolarize is simple, you will just need to use a wire to jump across the posts of the cutout for a couple of seconds, BEFORE installing the regulator.
Here is a couple of things that i did for extended reliability and these are optional. I used the modern starter drive and the generator cooler band, both from Snyders. For electronic ignition type performance ( I think better) I used the Pertronix flame thrower 40.000 volt epoxy filled 3.0 ohm coil. this coil looks just like any other replacement coil and gives instant starts no matter how bad the girls mess up when trying to get the A CRANKED. This coil gives a good boost in power and the spark plugs run clean.
This set up has been very dependable and we have had no problems in 5 years. If you run an alternator you will experience some of the quirks that I have mentioned and you will get a lot of negative comments about the alternator not being original. The generator is pretty simple and if they can't see the battery there will be no other visible changes to the car. All wiring will remain unchanged. Good luck, whatever your choice.
Now, I need to get busy and make a 6 volt compressor clutch! 
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