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58
T Y P E
6 0 3
E L E C T R O N I C
M U L T I P L I E R
tiometer
P.
The voltage at rhe top of each com-
ponent is shown in Figure 57. As previously shown,
these values are easily computed by the ratio of
resistances. The voltage at the top of Rz is +12.5
volts and at the top of
P
is
+
23 volts; hence by
adjusting the potentiometer tap, the grid can be
set anywhere from $12.5 volrs (-6.5 volts with
respect to cathode which is at
+
19 volts) to +23
volts
($4
volts with respect to cathode). This ad-
justment is made only once when the 65 volt sup-
ply is being adjusted to the line voltage. Thereafter
adjustment is automatic. Assume, for example,
that the +I 50 volt line .tends to drop in voltage.
This will in turn reduce the voltage across the 65
volt supply, meaning a lower voltage drop between
ground and the tap on potentiometer
P.
Less volt-
age at this tap means a more negative grid on the
6SJ7, since the cathode remains at essentially +19
volts at all times. The more negative grid means
a decreased anode current with a consequent re-
duction in IR drop across R3, the anode resistor of
the 6SJ7. A lower IR drop across Ra means a less
negative voltage at the anode of the 6SJ7.
This
is the voltage of the 25L6 grids; hence the 25L6
grids will be less negative than before and more
anode current will flow with athe consequent re-
duction in drop across the 25L6's and increase the
65 volt line. In this manner the 65 volt supply
is restored to its proper value.
Any change in the opposite direction will make
the 6SJ7 grid more positive which, in turn, will
make the 25L6 grid more negative. This will in-
crease the drop across the 25L6's and again 65 volts
will be maintained across this supply.
The 100 ohm screen grid resistors in rhe 25L6
circuits are current limiting resistors. Low values
were selected to allow maximum current without
damage to the screen grids. The 10,000 ohm con-
trol grid resistors are also current limiting resistors.
These resistors also provide more stable tube opera-
tion. The
8
mfd capacitor across the 65 volt line
serves as a filter with the 10,000 ohm resistor
R e
as its bleeder. Rc, serves the purpose of a dummy
load in case the operating load is removed.
An
extremely high resistance is used at R3 in the
anode circuit of the 6SJ7 to provide very close
control.
A very slight change in anode current
produces a large change in voltage drop across R3.
For example, a I micro-nrrzpere increase in anode
curren,t increases the IR drop across R3 by 5 volts.
NOTE:
The 6SJ7 may be replaced by a 6SK7 in an
emergency.
A slight readjustment of the
potentiometer may be necessary.
O n some early models the 12,500 ohm
resistor, which is part of the voltage divider
which establishes the cathode potential of
the 6SJ7 tube B-26 (Section 30B on wiring
diagram) is a 0.1 watt carbon resistor. The
load across this resistor is considerably more
than 0.1 watt, consequently this resistor
overheats and changes in value, thereby
causing fluctuations in the $6 5 volt supply.
This carbon resistor should be replaced by
a 12,500 ohm, 5 watt, wire-wound resistor
as indicated on the wiring diagram.
Voltage Adjustments
When it is desired to adjust the voltage supplies,
a definite procedure should be used to insure thar
one change does not affkct the other.
First, the
fiIament voltage should be checked for 110-11 5
volts. If a change is necessary, the voltage tap on
,the main transformer primary can be moved to
either post 18, 19, or 20.
The series filament
strings are designed for approximately 11 3 volts,
but a slightly lower voltage will considerably in-
crease the life of the tubes. However, a slightly
longer time must be allowed for the tubes to heat.
Changing the primary tap on the main transformer
will change the voltage output from the selenium
rectifiers slightly, but these voltages are not criti-
cal and are usually higher than necessary.
The first adjustment in the tube power supplies
is the $110 volt supply. This output is adjusted
by varying the position of the tap switch in the
primary of the power supply transformer.
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