FAIRCHILD metering explained :

Any Tube device that is arranged as a Push-Pull circuitry needs to be set properly to avoid bad distortion, especially at low frequencies and under load... Bad balancing will generate large popping sounds superimposed with the signal when compression is used. This is why the FAIRCHILD 670 has a Balance control :


A Fairchild meter is a bit like a “built in tube tester”, as well as an output transformer tester, it allows to measure the current that passes through :
a)The “Push” side of the output stage using the “BAL” position to the Left,
b)Through both sides of the output transformer in the “ZERO” position,
c)And on the “Pull” side of the output stage on the “BAL” position to the Right.

Indeed, this metering is very useful, not only it indicates gain reduction in the ZERO position, but it also tells you how healthy your tubes are :
You should observe the meter go down very slowly through the years, as your tubes start to age, and of course you can compensate that by adjusting the ZERO preset with a screwdriver until your tubes no longer have enough emission and need changing...