[old_tv_nut]

A note on the use of a fixed AGC/gain voltage:

Changing the bias of the RF and IF amplifier tubes to adjust signal gain can affect the tuning of the stages. In the earliest sets, it is likely that the designers tried to minimize this or just lived with it - hence, the best procedure in alignment was to use a specified bias instead of a specified input signal level.

When designers later got this effect understood and under control, they designed IF amplifiers for black and white sets that deliberately changed response at low signal conditions to roll off the high video frequencies (low IF frequencies), thereby reducing noise ("snow") in the picture. In these sets it is even more important to use known bias and hence strong signal conditions that do not produce the roll-off when aligning the IF.

Finally, when color came along, designers had to minimize any response shift, which could have attenuated the color signal. Still, it was good practice to align the IF under specific fixed bias conditions, so any undesired variations would occur from there and not some random conditions that the technician happened to use.