This is from my memory, mainly based on references, and some experience.

There were several, separate aural power reductions.

Before NTSC color TV was adopted, the rules allowed aural power between 50% and 100%
of the peak visual power. The FCC color rules were effective in December 1953, in those rules
or soon thereafter, the aural power was reduced to between 50% and 70%.

By about 1956, as part of rule changes to make UHF TV more viable, UHF stations could
use aural powers between 10% and 70%, VHF stations still had a 50% minimum.

The change you were thinking of occurred about 1965. The maximum allowable aural power
for all TV stations was reduced to 20% of peak visual. Both VHF and UHF stations were required
to hold their aural powers between 10% and 20%. The 20% aural power maximum made proper
fine tuning of a color set easier and simplified the design of color receivers. On many sets, color
or black and white, adjacent channel interference was reduced, but that was not the reason for
the change.

There was one more change. In 1985 as part of a relaxation of technical rules, the minimum
10% power limit was eliminated.


In the mid 1960's, RCA supplied kits to reduce the aural power of many of their transmitters.

Many stations running the TT-50AH high band transmitters bypassed the aural final amplifiers.
The aural output from the 10kW driver could produce 7 kW.

The RCA 25 kW amplifiers (7 parallel 5762 tubes), used in the TT-25BL, and several other models
from the 1950's, were capable of 100% aural power. Since this was not usually needed, a station
could remove filament power from two of the aural final tubes and still be able to operate at 70%
aural power. The two tubes had to stay in the amplifier so that it would tune properly.

Donald Wilson
K6DSW