The MGA's factory charging system is a Lucas C39PV-2 or C40 dynamo working through an RB106-2 control box. On the original cars this produced around 20 amps peak, though after losses through the regulator and aged wiring the net output at the battery is often closer to 10 to 15 amps. Adequate in 1955 for ignition, lights, wipers and a horn. For any MGA used today with modern lighting, electronic ignition, a period radio or other accessories, the dynamo struggles to keep the battery charged, particularly at low engine speeds, where dynamo output falls close to zero.
This node covers the Dynamator alternator conversion, a high-output modern alternator housed inside an authentic-looking dynamo casing, along with a negative-earth dynamo for owners who have already converted the car but want to retain a period-appearance charging unit.
Tudor Dynamator, alternator output, dynamo appearance
The Tudor Dynamator is a purpose-built unit that houses a 45 Amp alternator inside a casing externally similar to the original Lucas dynamo. The unit fits the original dynamo mounting brackets, adjusting link, pulley and fan belt, no modification to the engine required. A built-in regulator removes the need for a separate external control box, so the factory RB106-2 is redundant. It weighs about 3.5 kg (around 8 lb), less than half the weight of the original Lucas dynamo at nearly 15 lb, reducing stress on the mounting bracket and crankshaft drive.
It replaces Lucas C39, C40, C40L, C42 and C45 dynamos, covering the full range of MGA factory dynamo specifications (C39PV-2 on 1500 and Twin Cam, C40 on 1600 and Mk II from engine 16GA/6272) plus related BMC applications. Positive-earth or negative-earth versions are stocked, giving MGA owners a critical advantage over most alternator conversions: the car does not need to be converted to negative earth. For factory positive-earth MGAs, the positive-earth Dynamator slots in where the original dynamo came out, with no polarity changes needed for any other system (coil, fuel pump, radio, ammeter).
For cars already converted to negative earth, the negative-earth Dynamator is the correct choice.
Why the Dynamator is a practical upgrade
Higher output, 45 amps peak versus about 20 amps peak from a Lucas C40. The alternator delivers useful output at low engine speeds, so the battery continues to charge at idle and in town driving.
The original dynamo produces almost nothing below about 1,200 rpm engine speed. Built-in regulation, the Dynamator's internal regulator eliminates the unreliable mechanical RB106-2 control box, a known weak point on any Lucas-equipped classic car. The factory control box is typically old, with worn contacts and drifted calibration, and replacement units are not always the quality of the originals. Lower weight, the 3.5 kg Dynamator replaces a 15 lb original dynamo.
Visual authenticity, unlike a conventional alternator conversion requiring a modern-shaped alternator in a fabricated bracket (or the later MGB alternator), the Dynamator looks from the outside like a factory dynamo. For cars shown or used in period events, this preserves the engine-bay appearance while delivering modern electrical performance.
Negative-earth dynamo for retained-dynamo installations
The negative-earth dynamo (A + B + Midget fitment, new or reconditioned) is stocked for MGA owners who have converted the car to negative earth but want to retain a period dynamo appearance. This is a genuine Lucas-pattern dynamo, rewound for negative-earth operation, fitting the original dynamo mounting, pulley and belt. The output is still dynamo-specification (around 20 amps peak) rather than the alternator-level output of the Dynamator. For MGAs retaining factory positive earth with a working original dynamo, the main Dynamo & Alternator node (MGA Electrics > Dynamo & Alternator) covers genuine original-pattern positive-earth replacement dynamos.
Control box considerations
When fitting the Dynamator, the original Lucas RB106-2 control box becomes electrically redundant, the Dynamator's internal regulator handles the charging control. The control box can be left in place for cosmetic authenticity (bypassed electrically per the installation instructions) or removed entirely. On positive-earth installations, fitting an ignition warning light relay is normally recommended per the manufacturer's guidance.
Ordering considerations
For factory-specification positive-earth MGAs: the positive-earth Dynamator is the primary upgrade.