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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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