Any valve/tube device that uses the Push-Pull circuitry needs to be set up properly to avoid bad distortion, especially at low frequencies, and under the appropriate load. Bad balancing will generate large popping sounds superimposed with the signal when compression occurs.
This is why the FAIRCHILD 670 mkII has a Balance control and the ability to compare currents on both sides of the circuitry.
The Fairchild metering allows you to measure the current that passes through both legs of the output transformer's primary windings, it is a bit like a tube tester, 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 :
So you should expect the meter to 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.
IMPORTANT :
1) If your FAIRCHILD doesn't indicate 0 Vu after it has been ON for over 15 minutes it is much better to check that it is biased properly and to perform a meter calibration, instead of "jumping in" and adjust the ZERO which will instantly mess-up the DC side chain matching between Left and Right in Link mode.
People do not realise that when you adjust the "ZERO" on any FAIRCHILDs, you are actually changing the Biasing of the tubes, you are not changing the Vu calibration by design !?
As your tubes are ageing, they will have less and less current/emission going through them and you will need to re-calibrate the meters more often than the bias, so just let me know and I will send you the correct procedure to sort it out, it only takes a few minutes to do it.