The MGA's Lucas 12-volt positive-earth electrical system relies on three main items of control and protection hardware: the voltage regulator (known as the control box), which governs the charging output of the dynamo; the fuse box, which protects the accessory circuits from short-circuit overload; and the fuses themselves, which are the consumable protective elements in the fuse box. This node covers these items for pushrod 1500, 1600 and 1600 Mk II cars as well as Twin Cam and De Luxe variants.
Control box, voltage regulator
The control box is the Lucas RB106-2 on most MGAs including the Twin Cam, mounted on the left-hand side of the bulkhead on right-hand drive cars and on the right-hand side on left-hand drive cars. It houses the voltage regulator (which controls the dynamo's charging output to match the battery's state of charge), the cut-out relay (which disconnects the battery from the dynamo when the engine is stopped to prevent the battery discharging back through the dynamo windings), and the associated terminals. On the original factory specification, the control box has five screw terminals and is fitted with a lid covering the regulator mechanism. A production change on the 1600 introduced Lucar snap connectors in place of the original screw terminals for the dynamo harness, from engine number 16GA/6272, probably coinciding with the main harness modification at car/chassis 74489.
The 5-screw-terminal control box stocked by MGOC matches the original pre-Lucar specification and is the correct item for 1500 cars, Twin Cams, and earlier 1600 cars. Later 1600 and 1600 Mk II cars with Lucar-connector harnesses need the matching Lucar-terminal control box, identify which type is fitted before ordering. A further production change occurred in approximately January 1961, when the carbon composite resistance inside the control box was replaced by an improved wire-wound type, a change visible only if the control box is opened for inspection and is not a fitment concern, as both types carry the same external terminals.
Fuse box, Lucas SF6
The factory fuse box on all MGA variants is the Lucas SF6, a compact two-fuse unit with screw terminals. The SF6 carries two 35-amp fuses in circuit and has space for two spare fuses clipped into the underside of the lid, so a roadside fuse failure on an original-specification car is recoverable without opening the hood stowage compartment or the toolbox. The fuse box stocked by MGOC is the direct replacement for the original SF6 and suits every MGA variant regardless of chassis number. The SF6's two fuses protect different circuits: one covers the always-live accessories (horn, clock, courtesy lights), and the other covers the ignition-switched accessories (lights, wipers, indicators).
This is a minimal fusing arrangement by modern standards and is one reason why electrical faults on MGAs should be investigated rather than simply re-fused after a blown fuse.
Glass fuses, 35 amp
The original fuse specification for the SF6 is the 35-amp glass fuse, a cylindrical glass tube with metal end caps, the standard British Lucas-type fuse pattern of the period. MGOC stocks 35-amp glass fuses sold singly for routine replacement. The glass fuse differs from modern blade fuses in both physical size and in how the fusing element is mounted; the correct 35-amp fuse must be fitted in an SF6, a blade fuse or a higher-rated glass fuse will either fail to hold or will fail to protect the circuit properly.
Scope, relays
The node title includes "Relays" but the MGA's factory electrical system uses very few relays, the main items of relay-type hardware are the cut-out within the control box itself (not a separate item), and the flasher relay type DB10 fitted to 1500 and early Twin Cam cars only. A headlamp flasher switch relay, Lucas SB40-1, was available as an optional extra on all models from 1959 onwards, primarily for North American market cars. Additional relays fitted to MGAs today (for alternator conversion, auxiliary lighting, horn upgrade) are aftermarket items covered under Upgrades & Alternatives.
Ordering considerations
For control box replacement, confirm whether the car has the original screw-terminal harness (pre-16GA/6272, which is all 1500, Twin Cam, and 1600 cars built before the harness change) or the later Lucar-connector harness. For fuse box replacement, the Lucas SF6 suits every MGA. For fuse replacement, the 35-amp glass fuse is the factory specification; keep two or three spares in the car at all times, as this is the one item in the electrical system that is consumable under normal use.