SP-210 IF Amplifier

{From TM 11-866)


a. The intermediate frequency amplifier has three stages con-
sisting of three coupling transformers (T2, T3, and T4) and three
pentode amplifier tubes (V5, V6 and V7) of the remote cut-off, or
super-control type. The first two transformers (T2, T3) are identical,
and have tuned primaries as well as tuned secondaries. The secondary
coils are fixed in position, while the primary coils are mounted on
slide rods permitting them to be moved back and forth with respect
to the secondaries, thus changing the degree of inductive coupling
between them. When the coils are farthest apart the coupling is at
its lowest value and the transformers exhibit their maximum selec-
tivity or minimum band width. Conversely, when pushed close
together the coupling is greatly increased and minimum selectivity
or maximum band width results. At any adjustment between these
two extremes, an intermediate degree of selectivity is obtained. This
variation in coupling is accomplished by a combination of cams and
levers operated by the BAND WIDTH control (E9) on the front
panel. The third transformer (T4) has a tuned primary and a closely
coupled untuned secondary wound directly over it (L32). This
transformer couples the 3rd IF amplifier (V7) to the diode 2nd
detector (V8).

b. The coils (L28, L29, L30, and L31) in transformers T2,
T3 are wound with 7/41 Litz in three pies on ceramic cores and are
tuned by means or air-dielectric variable capacitors (C36, C37, C38,
and C39). Grid coils L29, L31 are tapped for connection to the
control grids of amplifier tubes V6 and V7. These taps are located
at approximately one tenth of the total turns up from the low-
potential ends of the coils. Consequently, variations in grid input
capacitance and conductance due to changes in SENSITIVITY
control settings (or AVC) have no noticeable effect on the tuning of
these IF transformers.