CIRCUIT
DESCRIPTION
27
In performing this function, not all the electrons pass
through this grid and on to the anode but some strike
the screen grid wires and
pass
on into the screen
cir-
cuit to cause the
flow
of screen current. There are
other important
uses
of the screen grid. However, they
do
not apply in the Type 82 circuits.
9. Gas tubes, such
as
the OA4G, have the unique
characteristic of not starting conduction until the
start-
ing anode receives the required positive voltage. Once
ionization or conduction begins, however, the starting
anode
loses
all control, and current
flow
in the tube
can
be
cut
off
only
by
opening the main anode circuit
or
by
decreasing the positive voltage on the main
anode to a value insufficient
to
maintain ionization
(approximately
65
to 70 volts for an OA4G tube).
General Operation
The
flow
of current
to
the sort magnet
is
furnished
by
three 25L6 beam power tetrodes connected in paral-
lel. This
flow
of current
is
under control of an OA4G
cold cathode gas triode which serves the same purpose
as
does
the card brush relay in the Type 80 sorter.
The
OA4G, or trigger tube,
is
fired
by
the action of
the card brush sensing a punched hole and, being a gas
tube, it remains in conduction until the anode circuit
is
broken
by
the center brush of the commutator at
the
end of the card cycle (principle
9).
Oscillator and Rectifier Tube
Approximately
-40
to
-45
volts
bias
(principle
7)
for the 25L6 power tubes and the OA4G trigger tube
is
supplied
by
a diode connected 12SN7, which recti-
fies
a high audio frequency voltage (approximately
4.3KC) supplied
by
a Hartley type oscillator using a
triode connected 25L6.
Since a transformer and rectifier
bias supply
is
not feasible for
DC
machines, an oscillator-
rectifier setup
is
used
so
that machine circuits will be
applicable on both
AC
and DC. The bias rectifier and
oscillator operate continuously when the machine
is
turned on and the contact roll cover
is
down,
so
that
-40
to
-45
volts
is
always available
as
required.
The oscillator and rectifier are shown schematically
in Figure 24. The rectified machine supply voltage
is
shown
as
150 volts DC because the power supply capa-
citors tend to charge to peak line voltage under a light
load.
As
the 25L6 oscillates, there
is
a constant rising
and falling of the plate current through the tube. The
speed at which this rising and falling of plate current
occurs
is
dependent on the resonant frequency
as
deter-
mined
by
the tank circuit made up of the .05 mfd.
capacitor and sections A and B of the oscillator coil.
Because the B section of the coil
is
in
series with the
tube, variations in plate current cause variations in the
current flowing through the B section of the coil.
Rising and falling values of current
in
the B section
of the coil induce voltages in sections A and C of the
coil. These induced voltages are alternating voltages
which change polarity with each
rise
and fall of the
current through the B section. The voltage induced
in the A section of the coil
is
applied to the grid of
the tube
to
keep the tube oscillating. The voltage
induced in the C section of the
coil
is
applied across
terminal 5 and the cathodes of the
12SN7. Since this
voltage changes polarity each half cycle, the
12SN7
conducts only on those half cycles during which its
anode
is
positive in respect to its cathode (principle
5).
Because of this action, the 12SN7 rectifies the
output from section C of the
coil
to provide approx-
imately
-40
to
-45
volts DC with respect to the
negative or zero side of the power supply.
When
the oscillator first starts to operate, the 4 mfd.
capacitor tends to charge through the circuit shown
dotted in Figure 24 on each half cycle that the
12SN7
conducts. Because of its comparatively large
size,
sev-
eral oscillator cycles are required before the capacitor
becomes fully charged. As the condenser becomes
charged to the value of the voltage drop across section
C of the coil (approximately
40 to
45
volts), current
flow
in the 12SN7 diminishes greatly because of the
lack of a difference in potential between its anode and
its cathode.
Once charged, the 4 mfd. capacitor remains
charged except for a slight leakage through the 1
megohm resistor on those half cycles during which the
12SN7
does
not conduct. Any loss of charge across
the capacitor due to leakage
is
replaced
by
conduction
through the
12SN7. Except for the small amount of
current required to replace the charge that leaks from
the 4 mfd. capacitor, current
flow
through the 12SN7
is
practically zero after the capacitor
is
initially charged
and before a hole
is
sensed in the card. Once the 4
mfd. capacitor becomes charged with the polarity
as
shown in Figure 24, a constant negative bias of
-40
to
-45
volts
is
supplied, even though the induced
voltage in section C of the coil reverses polarity each
half cycle. Without this capacitor, negative bias would
be lost each half cycle that the
12SN7 did not conduct.
The 1 megohm resistor connected between the plate
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