SP-210 Audio

 (from TM 11-866)

20. AF AMPLIFIER.

a. The AF amplifier has three stages using one VT-65 triode (V13)
and three VT-66 pentodes (V14, V15, and V16). The grid of the first
tube (V13) is connected to the moving arm of the AUDIO GAIN control
(R26) through a blocking capacitor (C24). Its plate is coupled to the
grid of the second AF amplifier (V14) by means of a capacitor C25,
plate resistor R28, and grid leak R29. The second amplifier tube (V14),
while a pentode, is operated as a triode by connecting its plate and
screen together. It drives the output tubes (V15, V16) through a
push-pull input transformer (T7). The output tubes (V15, V16) are also
triode-connected and are operated as class AB2 amplifiers, which means
that grid current flows during some part of the input cycle. For a
power output up to approximately 3 watts no grid current flows, and
harmonic distortion is negligible. Above 3 watts, and up to 10 watts
(maximum output) grid current steadily increases causing a
corresponding increase in distortion.

b. The output transformer (T8) has two secondary windings;
a 600-ohm secondary (4-5) for power output, and a secondary for
headphones (6-7) designed to deliver about 3% of the output power
into an 8000-ohm resistive load when the 600 ohm secondary is
connected to a matching load. The turns ratio and resistance of
the headphone winding are such that the power delivered to any
load between 8000 ohms and 80 ohms varies less than 6 db, and
the power input into a 250-ohm load is but 2 db greater than that
to a 4000-ohm load.
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