Any Tube device that is arranged as a Push-Pull circuitry needs to be set up 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 :
The Fairchild metering allows you to measure the current that passes through both legs of the output transformer's primary windings, it is also bit like a tube tester, so you can measure :
a) The “Push” side current of the output stage by flicking the “BAL” switch to the Left,
b) The overall current that is flowing through the centre tap of the output transformer in the “ZERO” position,
c) The “Pull” side current of the output stage by flicking the “BAL” switch to the Right,
Indeed, this metering is very useful, not only does it indicate gain reduction in the ZERO position, but it also measures how healthy your tubes are :
You should expect 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.
