The way that Pertronix/Aldon Ignitor electronic ignition is wired on positive earth cars is completely different to negative earth and the usual tip for getting the RVI tach to work correctly cannot be used.

This drawing is taken from a T-Type web page and shows the 12v supply from the ignition switch connected to the Ignitor instead of the coil SW terminal, and the coil CB terminal being connected directly to earth instead of the points. What this drawing does not show is the earth connection of the Ignitor which is picked up from the distributor 'points' plate. I can imagine this being so they can use the same internal electronics module for both versions. T-types have a rev-counter in place of a tach of course so there is no mention of the RVI tach problem. On the MGB the 'Ignition Switch Wire' comes via the RVI tach loop so there is no opportunity to power the module from an alternative 12v source as there is when the car is negative earth. Aldon at least suggests that the tach might have to be converted to RVC to work correctly, note that an original RVC tach would be negative earth.. But there might be another way and that is to insert the existing RVI tach in the earth connection to the coil instead of the 12v connection.

Below is the pickup you should find on the back of a positive earth tach. 'A3' is the ignition switch terminal and 'SW' the coil terminal, with the arrows showing the direction of current flow from +ve earth to -ve 12v. Remove the white wire and insert the new black wire such that the 'A3' end comes from the coil CB terminal and the 'SW' end goes to earth.

Note that this is a suggestion, I have not tested it!