Ampex 351 - Page 78

8-5
POWER SUPPLY
Vacuum tube 3V7, connected as a conven 
tional full wave rectifier, supplies plate power
for all tubes in the electronic assembly, and it
also supplies the record indicator light. Selen 
ium rectifier CR1, connected as a conventional
full wave rectifier provides d-c filament volt 
age for 1V1,1V2 and 2V3.
The center tap of the 2V3 tube filament pro 
vides a ground for the d-c filaments. This tube
must be in its socket for proper operation. A-c
power input is connected at 5J8 POWER recep 
tacle and is controlled by switch 4S5 POWER.
The power is fed through fuse 5F1 and im 
pressed across the primary of power trans 
former 6T4 and also through fuse 5F2 to the
tape transport.
There are four secondary windings on the
power transformer — three for filament supply
and one for high voltage. One filament wind 
ing serves rectifier tube 3V7, one center-tapped
winding provides 12.6 volt d-c filament voltage
after rectification, one winding supplies .12.6
and 6.3 volt a-c voltage, and the other center 
tapped winding furnishes high voltage. An rc
network consisting of the four section capaci 
tor 3C16 and resistors 3R54, 3R55 and 3R56
provides filtering action. Relay contact 3K1B
shorts resistor 3R54 in the record mode to pro 
vide a nearly constant B+ supply in any mode
of operation.
Through record relay 3K1C, B+ is applied
to the bias oscillator and the last stage of the
record amplifier. Whenever the PLAY button
on the tape transport is pressed, 115 volt d-c
is available at pin 3 of 5J7, and when RECORD
button 4S6 is pressed, the 115 volt d-c is applied
to the record relay coil. As long as 115 volt d-c
is available at pin 3 of 5J7, contact 3K1A holds
89-0144
Issue A
NOTE
Catalog number 30750 and 30950
electronics provide the signal to
MONITOR JACK 5J4 from the plate
of 2V5.
One secondary of output transformer 6T3
delivers signal to 5J5 output jack, and to LINE
TERM switch 5S4 for selecting resistor 5R48
to obtain a nominal 600 ohm termination when
necessary. The output signal is also delivered
to VU meter 4M1 through resistor network
6R51, 6R53, and 6R52 which determines the
amount of signal required to obtain a reading
of 0 on the meter. The output signal also ap 
pears across 4J6 PHONES jack for aural moni 
toring. .5
Transformer strapping and cabling connec 
tions for various outputs are discussed in SEC 
TION 2 INSTALLATION.
BIAS AND ERASE OSCILLATOR
A dual triode tube 1V6, connected as a push-
pull oscillator, provides a high frequency bias
and erase signal. Both halves of the tube are
resistance coupled triode amplifiers, with the
output of each plate coupled to the grid of the
other triode section. Any signal on the grid
of either tube will be amplified in the plate
circuit and coupled to the grid of the other tube.
The signal then will appear at the plate of the
second tube and be coupled back to the grid
of the first tube in phase with the original
signal. Frequency of oscillation is approxi 
mately 100 kc.
The oscillator output is fed through variable
capacitor 5C33 ERASE ADJUST where erase
current adjustments are made. From 5C33 it
follows another path through variable capacitor
5C13 BIAS ADJUST where bias current adjust 
ments take place. The bias signal is then mixed
Plate voltage is supplied to all reproduce
stages when the POWER switch is in the ON
position. The signal can be monitored from the
output of the feedback winding by using phone
jack 5J4 MONITOR AMPLIFIER. This same
output winding provides negative feedback to
the cathode of 2V4A. This position can be use 
ful for feeding such devices as low gain am 
plifiers, sensing strips, et cetera.
with the record signal and delivered to the
record head.
NOISE BALANCE control, potentiometer
1R63, in the oscillator grid circuits is adjusted
to correct for any asymmetry in wave form,
which would cause random noise during repro 
duction and distortion while recording.
Plate voltage is supplied through relay con 
tact K1C only when the equipment is in the
record mode.