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9. LIMITS: If the tube fires between
-
3 and
-
1
volts on the grid, the tube is within manufac-
turing limits for both end point and variation
in new tubes.
Note: Once the tube fires, the grid loses control.
If it should be necessary
to repeat the test, in-
crease the grid bias to
-
10 volts and decrease
the plate voltage to zero, and then reset it to
212 volts and repeat the above procedure.
The plate current for tubes 2050 and 2051
should not exceed
100 milliamperes
and 75
milliamperes
respectively.
If it does, reduce
the plate voltage slightly to bring it within these
values.
Note also that when the thyratron fires the
plate voltage
will drop to about 7-1/2 volts.
This is normal.
For low current thyratrons,
a lower initial
plate voltage should be used, or a current lim-
iting resistor inserted in the plate patch cord
lead.
VOLTAGE REGULA TORS:
The Model 686 can check voltage regulator tubes
similar to the 874 and VR-150-30 types.
The pro-
cedure outlined below is for checking the 874 type.
1. Make connections
by use of patch cords in
accordance
with the base diagram.
In the case
of the 874, Pin 1 is the cathode and Pin 3 is
the anode or plate.
2. Set Gm Factor switch to "x 10".
3. Turn "Signal Amplitude"
control down to zero.
4. Set Element Current Range switch to "120" milli-
amperes.
5. Set Element
Current switch to "Plate" posi-
tion.
6. Increase the plate voltage until conduction be-
gins. This should be at approximately
115 volts
for the type 874. (For other voltage regulators
refer to manufacturer's
ratings).
7. To check the regulating
characteristics
vary
the plate voltage to produce anode currents be-
tween 10 and 50 milliamperes
maximum.
(For
other
voltage
regulators
see manufacturer's
ratings.)
Note: The voltage applied to produce currents
between
10 and 50 milliamperes
should be
within the limits indicated
by the man ufac-
turer's ratings. In the case of the 874 the voltage
after conduction should not vary more than 7
when the current is varied from 10 to 50 milli-
amperes.
THEORY OF OPERATION
Essentially
the Model 686 is a low impedance
power supply metered for potentials
and currents
and provided
with a means of introducing
an a-c
signal into the grid bias line and measuring the a-c
component in the plate circuit.
The incoming a-c line energizes
the d-c power
supply, the heater voltage supply and the a-c grid
signal transformer.
The filament transformer control
is located in the primary leads thus providing proper
adjustment to take care of varying line voltage con-
ditions. A rotary switch selects secondary
taps to
give the necessary
heater voltages for all tubes.
The d-c power supply delivers potentials to the
plate, screen and suppressor
controls.
Following
these controls the potentials are metered and the cir-
cuit is so arranged
that the element milliammeter
can be placed in each of the lines. A separate reg-
ulated power supply develops the voltage for grid
bias. This potential is likewise controlled by a poten-
tiometer. A separate
voltmeter is used to measure
this potential and a microammeter
is placed in the
circuit to detect the presence of grid current.
The a-c grid signal winding together with a signal
voltage divider is placed in series with the control
grid circuit to the tube. The proper signal voltages
are selected by the Gm Range switch which is con-
nected to the signal voltage divider.
The injection
of the a-c grid signal into the grid bias circuit is a
function performed by the Gm Reading toggle switch.
The Gm meter is an amplifier-rectifier
type a-c
instrument which is connected to the plate circuit for
measuring
the a-c component of plate current.
The element potentials are fed to a multi-circuit
Short Test-Meter Reading switch.
This switch pro-
vides the necessary
circuit connections so that tube
elements may be either short checked by means of
the Element Short Test switch or energized by the
potentials from the power supply.
The a-c grid signal transformer
has a separate
winding feeding a type 3A4 tube which supplies the
necessary d-c voltage for high sensitivity short check.
The Hum Control is simply a potentiometer placed
across the filament winding to provide the necessary
balance on the filament return when checking these
types. This is to prevent an additional signal (which
may either add to or subtract from the true grid sig-
nal) from appearing
in the grid circuit causing a
modification in the a-c plate current component.
The
Hurn Control is switched out of the circuit on heater
voltages above 10.
In checking a tube, the d-c potentials are applied
to the tube through the various controls. The a-c grid
signal is applied in series with the grid bias voltage,
and is measured by switching the Gm meter to "Sig-
nal Cal." and held to a fixed value by rotating the
Signal Amplitude
Adjuster. To measure a-c compo-
-9-
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