CHAPTER 1
THEORY
1. Scope
width, and reduce noise and distortion.
The negative feedback is adjustable to
a. T h i s m a n u a l c o v e r s f i e l d a n d d e p o t
control gain and frequency response. The
maintenance for Amplifier, Radio Fre-
i n p u t signal. is applied to the grid of first
quency AM-1881/U. It includes instruc-
rf amplifier V1 where it is amplified and
tions appropriate to fourth and fifth
coupled to the grid of V2. The signal is
echelons for troubleshooting, testing, ad-
further amplified by V2 and coupled to
justing, and repairing the equipment, and
t h e grid of V3. Cathode follower V3 pre-
replacing maintenance parts. It also lists
sents a relatively low impedance to ex-
tools, materials, and test equipment for
ternal loads.
f o u r t h and fifth echelon maintenance. De-
b.
Negative
Feedback
Network.
Nega-
tailed functions of the equipment are
t i v e feedback from the output of V3 is fed
c o v e r e d in the theory section.
to the cathode of V1 through a resistor
voltage divider network. The gain of the
Note. There are no maintenance functions
amplifier is controlled by a two-position
assigned to third echelon.
G A I N switch (20 DB or 40 DB) that con-
b. T h e c o m p l e t e t e c h n i c a l m a n u a l f o r
trols the amount of feedback to V1. Fre-
this equipment includes three other pub-
quency response of the amplifier is also
lications:
controlled by the negative feedback net-
T M 11-6625-353-12, TM 11-6625 -353-
work.
20P, and TM 11-6625-353-35P.
c. Power Supply. The power supply is
c. F o r w a r d c o m m e n t s c o n c e r n i n g t h i s
electronically regulated to stabilize op-
m a n u a l to the Commanding Officer, U. S.
e r a t i o n during changes in line voltage and
Army Signal Materiel Support Agency,
l o a d conditions. It consists of power trans-
A T T N : SIGMS-PA2d, Fort Monmouth, N. J.
former T1, power rectifier V4, series
regulator V5, regulator control V6, and
Note. For applicable forms and records, see
p a r a g r a p h 2, TM 11-6625-353-12.
voltage reference stage V7. The series
regulator stage output is taken from the
cathode and supplies the regulated output
2. Block Diagram
direct-current (de) voltage. The gaseous
The AM-1881/U is a general-purpose
OA2 voltage reference tube supplies a
w i d e frequency range (5 cycles per second
stable reference voltage to V6 against
( c p s ) to 2 megacycles (mc)) amplifier. The
which a sample of the output voltage is
signal path is shown in the block diagram
compared. The regulator control tube am-
plifies any change in the difference be-
l o w . For complete circuit details, refer to
tween the sample output voltage and the
the overall schematic diagram, figure 12.
reference voltage, and applies it to the
a. First and Second Amplifiers and
series regulator control grid. This action
Cathode Follower. The AM-1881/U am-
h o l d s the output voltage constant at +210
plifier section consists of two stages of
v o l t s dc.
voltage amplification (V1 and V2) con-
3. Stage Analysis
n e c t e d in cascade, a cathode follower out-
a. First Amplifier (fig.2).
put stage (V3), and a negative feedback
The first
network. Pentode tubes are used in all
amplifier, V1, is a type-5654 pentode
t h r e e stages. Amplifiers V1 and V2 provide
voltage amplifier tube. The input signal
high gain and wide-frequency bandwidth
i s applied to the control grid and the nega-
w i t h low-noise level. Negative feedback to
t i v e feedback signal is applied to the cath-
V 1 is used to stabilize gain, increase band-
o d e . Capacitor C1 blocks any dc component
2
