"Is there a commercially available solid state voltage regulator that can be used with a generator?"

Having been asked this question once too often, one day I got over zealous and did some serious Googling, and to my surprise, I actually found something that might be useful (maybe). This is Bosch part number 30019 electronic voltage regulator. This part is listed as a replacement for the electromechanical regulator for 1967-1973 VW Beetle, and some other VW models of similar vintage, which used a generator (not alternator) with negative earth electrical system (or possibly converted to negative earth).

Addendum June 2005:
For anyone who may have been looking at this page and considering using this regulator for your MG, forget it, because it won't work with the Lucas generator.

The Lucas generator has the field coil grounded internally, and the field current is controlled by modulating the field input voltage. The Bosh generator has the field coil connected internally to the power output (system voltage), and the field current is controlled by modulating the field ground connection.

Notice the two hex head screws on the centerline at top. These have insulating bushings to keep them electrically isolated from the top shell, but are electrically connected to the bottom mounting bracket (which is to be grounded). There is one screw post terminal on top (with a hex nut) that is electrically connected to the outer shell. This post is marked "D+", and according to the wiring diagram is the connection for the dynamo power output, which means the outer case will be electrically hot when operating. Obviously this device needs to be covered when operating to prevent a short from the outer shell to ground.

Before I go any farther, .... Clicking on the image below will bring up a large (60KB) high resolution image suitable for reading and/or printing. This device was manufactured in Mexico, and the instructions are printed in Spanish. I don't read Spanish well, but I did get it translated into English. In general it only says to make the connections securely as shown, and polarize the generator if necessary. I was hoping it said something about protection against shorting the case to ground, but not a word. I have called Bosch tech assistance for this question, and the answer is, "That's the way it is".

Addendum September 2011:
This is a note from Bill Moon in Spring City, Utah, USA, who owns a 1971 VW Type2. "I also was concerned that the D+ post is electrically connected to the case but I think they compensated for this by coating the case with an insulating coating. When I use an ohmmeter to measure continuity between the post and the case, I have to scrape the ohmmeter lead on the case very hard to get a reading. So we may be okay as far as the case being hot".

I suspect the deep drawn case is anodized aluminum. Anodizing the part creates an aluminum oxide protective coating that will resist corrosion. Aluminum oxide is also an electrical insulator, but it can be scratched through to the soft base metal, so don't count on it being non-conductive if you drop a tool on it.

I also looked to see if this device might fit inside the cover of the original Lucas regulator on the MGA, but no-go. At best it would take a lot of trimming on the case and brackets of the new device to squeeze it in there. Bottom line, not recommended

Click for large printable copy of Bosch instructions.