Ampex 351 - Page 77

NOTE
O utput
8-4
Bias and negative feedback is
unbypassed resistor 1R21 and
The signal now is coupled to the grid of the
third stage by capacitor 1C5, bias and negative
feedback is provided through unbypassed resis 
tor 1R16. Further amplification takes place in
this third stage and pre-emphasis circuitry for
HIGH and LOW tape speeds is provided at
capacitors 1C46, 1C7 and 1R17 which provide
the necessary high frequency rise. At the low
end of the frequency spectrum, an effective 3
db gain is furnished by the resistor/capacitor
combination lR18and 1C8.
NOTE
Number 30750 and 30950
TO OUTPUT OF 2ND
RECORD 4MPLIF ER STAGE
89-0144
Issue A
1
OUTPUT
When reading meter indications with
the METER AND OUTPUT SWITCH
in the record position, only the first
two stages of the record amplifier and
the last three stages of the reproduce
amplifier are connected in the circuit,
omitting record pre-emphasis and re 
produce equalization circuitry so that
meter indications will reflect only the
flat action of each amplifier.
REPRODUCE
HEAD
Plate voltage for the first three stages is
supplied whenever POWER switch 4S5 is in
the ON position. For plate voltage to be applied
to the final stage, the equipment must be in
the record mode, at which time relay contacts
3K1C complete the necessary circuitry.
In the fourth stage, coupled to the third stage
by capacitor 1C9, the signal is applied to the
grid of 1V2,
supplied by
1R22.
Block diagram, reproduce circuit
eter 2R30 is selected when EQUALIZATION
SWITCH 4S2 on the face panel of the elec 
tronic assembly is in the LOW position; PO 
TENTIOMETER 2R31 is selected by the HIGH
position. For the 71/2 and 15 ips tape speed
pair, potentiometer 2R31 serves both speeds.
The signal now is delivered to amplifier stage
2V4, the tube receiving the signal through
coupling capacitor 2C17, PLAYBACK LEVEL
potentiometer 4R36 and switch 4S3a. The out 
put of 2V4a is coupled through 2C19 to one
grid of the push-pull output stage, and a portion
of this output is coupled through capacitor
2C18 to phase inverter 2V4b. Both signals, now
180 degrees out of phase with each other, are
fed through coupling capacitors to the respec 
tive grids of push-pull amplifier 2V5 and then
to the primary of center tapped output trans 
former 6T3.
Catalog
electronics used an LC network (1L1
and 1C11 or 1C12) which supplied a
high frequency boost by its resonant
characteristics.
REPRODUCE AMPLIFIER
The reproduce section of the electronic as 
sembly is a resistance coupled audio amplifier.
Three dual triodes arc used to provide three
stages of amplification, phase inversion and
a push-pull output amplifier.
Signals on the moving magnetic tape induce
voltages in the reproduce head. When high
impedance heads are used, this induced voltage
appears across resistor 2R25 and then on the
grid of 2V3. Bias on this first stage is derived
from the voltage divider network consisting of
resistors 2R26 and 2R28. Capacitor 3C16a and
resistor 3R32 form a plate decoupling network.
The amplifier output of this first stage is
coupled to the second stage grid through capa 
citor 2C14. Capacitor 3C16b and resistor 3R35
form a plate decoupling network. Reproduce
equalization is achieved by means of capacitor
2C15 and resistors 2R29, 2R30 and 2R31. For
the 3% and 7'/2 ips tape speed pair, potentiom-
The fourth stage is a constant current circuit
which minimizes level variations which would
normally occur between different frequencies
as they were fed to the reactive load presented
by the record head. The output of this fourth
stage is mixed with the signal from the bias
and erase oscillator before being delivered to
the record head.