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ELEMENTCURRENTMETER:
A four range d-c milliam-
meter used to measure
cathode,
suppressor,
screen,
or plate current.
The Element
Current Range
switch
below the instrument
serves to select either the 3, 12,
60 or 120 milliampere
range.
The Element
Current
switch directly
to the left of the range
switch selects
the electrode
in which the current
is to be measured.
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The Element Current meter has less than 75 milli-
volts drop in the electrode circuit to which it is con-
nected. The switch is designed so that all other posi-
tions are short circuited. This switch may be indexed
to any position at any time without effecting read-
ings.
CONTROLGRID VOLTMETER:
M easurements of grid
bias potential are made on the Control Grid Voltage
meter. The switch to the right of the meter selects
either the 10, 50 or 100 volt range as required.
Be
sure to note the position of this switch when unex-
pected plate current exists in the tube under check.
This switch also changes
the bias network, de-
creasing the drop across the control grid potentio-
meter on the 10 volt range, providing better voltage
control when adjusting low bias potentials.
Note that the grid bias supply is regulated by an
OB2 regulator tube.
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FIG. 1
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HEA TERVOLTMETER:
Filament potentials from 0.6 to
120 volts may be selected by means of a rotary
switch and potentiometer. These potentials are meas-
ured on a multirange a-c voltmeter (Refer to Fig. 2).
Note that the Heater Voltage Selector switch is
used to select the nominal voltage, and exact adjust-
ment is made by rotating the Heater Voltage Adjuster
which controls the primary potential on the filament
transformer. An interlock circuit is used on the rotary
switch to automatically
shift ranges on the filament
voltmeter as the switch is rotated. Thus the operator
always has this meter on the correct range, and the
possibility of instrument overload is eliminated.
The
particular range connected into the circuit is marked
for each group of potentials on the Heater Voltage
Selector switch plate. The voltmeter connections are
brought back through separate leads directly from the
tube socket thus providing a more accurate reading of
filament potential at the tube pins. Thus any voltage
drop in the leads from the filament transformer to
the tube socket will not be in the meter circuit.
FIG. 2
ELEMENT VOLTMETER:
Electrode potentials are meas-
ured on the two range Element Voltage instrument
mounted below the Element Current meter.
Plate,
screen, positive suppressor
grid, or negative
sup-
pressor grid potentials can be selected by using the
rotary switch below the meter (Refer to Fig. 3).
When selecting either Pos Supp or Neg Supp volts
position on the rotary Element Voltage switch, choose
the position indicated
by the Supp-Pos-N eg toggle
switch located just to the left of the Model 686 name
plate.
If the rotary switch is indexed to the wrong
position with respect to the positioning of the toggle
switch mentioned above, the Element Voltage meter
will read down scale.
No damage will be done to
the voltmeter if a down scale reading should occur.
It must be remembered
that when checking any
one tube, either a negative
suppressor
volts or a
positive suppressor
volts (used for second anode
voltage purposes) can be selected, but not both for
the same tube test.
The Element Voltage Range switch directly to the
left of the meter selects the Element Voltage meter
range of 60, 150 or 300 volts. This switch is compen-
sated for meter loading so that the effect on validity
of readings is within 0.2%.
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