Do I need a ballast resistor?

November 1, 2015 | By Staff


In the April issue I became concerned about your advice regarding ballast resistor wire. I’m concerned because I converted from two 6-volt batteries to a single 12-volt battery. I also replaced the distributor with an upgraded racing Mallory type along with a 40,000- volt coil. Finally I installed a Pertronix pointless device for my 19741 ⁄2 MGB.

My question is will this work or will it burn up? Perhaps this is a question for the Moss jobber tech line. They are the ones who sold me the parts. No special instructions came with the parts and I installed them right out of the box. Your publication exceeds all expectations. I read them over and over—hence the months-long delay in asking my question.


Just to be sure, I checked with the technical representatives at Moss Motors, and they confirmed what I believed to be true, which is that you do not need a ballast resistor with the ignition setup you have installed.

On most old cars ballast resistors are there to reduce the voltage going to the points so they will not burn out. The maximum that most breaker points can handle is about nine volts, so a ballast resistor is used to step down the voltage going to them from 12, which is too high. But in the case of some MGBs, it was to protect the original coil.

There would be a further possible consideration if you kept the original twin six-volt batteries though, because the amperage from that setup is roughly twice that of a single-battery 12-volt system, which could require cutting the amperage going to other things too. But the techies at Moss assure me that with your 12-volt battery, high-performance coil, and Pertronix ignition no ballast resistor is required on your ’74 MGB. If you need further clarification here is how to contact them:

Moss Motors