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nent of plate current, the Gill meter is switched to
the "Gm Reading" position. Inasmuch as the value
of the grid signal and the a-c component of plate
current are known, the mutual conductance
is the
ratio of the two. Since the ratio
2~:
==
G1l1
consists
of one known value and one measured quantity, the
scale can be calibrated directly in micromhos.
GENERAL INFORMATION
INSTRUCTIONBOOK: This edition of the instruction
book applies to Model 686, Type lOA.
Any questions
concerning
a special application,
the use, maintenance
or repair
of these models
should be addressed to Weston Electrical Instrument
Corporation, giving all the information listed on page
11 under paragraph
ORDERING INFORMATION.
TUBE
MANUALS: It is advisable for the operator of
this device to have at his disposal a tube manual
which can be obtained from any of the tube manu-
facturers.
A manual facilitates the measurements
of
tubes at potentials not normally listed in the manu-
facturer's specifications.
For example, if the mutual conductance of the 6C8
were to be measured
at 150 volts and -I ¥2 volts
bias a glance at the Ep-Ip curves would show that
on the vertical line corresponding
to 150 volts a
change in plate current of 1.6 milliamperes
would
result with a grid bias change of -1 to -2 volts.
The expected mutual conductance,
therefore would
be approximately
1600 micromhos, this value being
obtained by dividing plate current change in milliam-
peres by the grid bias change in volts and multiply-
ing by 1,000. Computing the approximate
Gm to be
expected as explained above sometimes eliminates
an incorrect reading due to improper testing of the
tube.
TUBE
DAT A CHARTS: T ube Data Charts, supplied sep-
arately, list the manufacturer's
nominal ratings for
the element potentials, mutual conductance,
ampli-
fication factor, and the tube base diagram number.
These charts will in most cases handle all of the tubes
that will be checked, and should be used as a quick
reference supplement to the tube manual.
Tubes not
listed in the charts can be checked if the basing and
element potentials of the tube are known.
REJECT LIMITS:
The reject limits listed should be con-
sidered as nominal limits only. It is possible that in
certain special applications
it will be necessary
to
reject a tube when its G1l1 has fallen to possibly only
75 or 80% of its nominal value.
In other cases a
reduction of 60% in the G1l1 would have no adverse
effect on the operation of the circuit. If an end point
figure or an end point range is specified by the
tube manufacturer,
the operator should be guided
accordingly.
If it is not known what reduction in Gm'is allow-
able, the limits in percent of normal listed below are
suggested
as the proper end points for use in gen-
eral electronic equipment:
RF, IF, and Pentode Voltage Amplifiers
65%
General Purpose and high mu Triodesm___50%
Power Output
Types
m
m
50%
Converters and Mixers
(Refer to Tube Data Chart)
Rectifiers (Refer to Tube Data Chart)
Diode Detectors (Refer to page 8, Step
46.)
D-C FILAMENT SUPPLY:
I n certain applications it may
be desirable to measure the mutual conductance
of
a filamentary
type using d-c voltage instead of an
a-c source to energize the filmaent.
In such cases
index the Heater Voltage switch to zero position, run
jumper leads from the d-c filament supply source to
the proper numbered pin jacks in the top two rows.
Connect an additional
lead from the negative fila-
ment supply to the second or third Cathode pin jack.
A separate d-c instrument will be required to meas-
ure the filament voltage and should be connected
into the two pin jacks in the top two rows corre-
sponding to the filament connections.
HELPFUL
SUGGESTIONS
IN TUBE
TESTING
GRIDCURRENT:
In using the Model 686 care should
be exercised to see that the grid current in micro-
amperes indicated on the Grid Current meter does
not exceed three to four microamperes.
This value
changes
somewhat
between
tube types, but the
above
value
can be assumed
in general,
to be
satisfactory.
Excessive grid current will cause an
error in the Gill readings and it is advisable
that a
limit of 4 microampere3 be strictly adhered to and
that some means of eliminating this condition should
be tried as outlined below.
If the Grid Current meter deflects to the left of
zero the tube is either gassy or the element potentials
applied are not correct.
Check the manufacturer's
specification and note whether the proper potentials
have been applied.
If the potentials are correct and
the meter indicates 4 microamperes
or more to the
left of zero the tube should be rejected.
If the Grid Current meter indicates to the right of
the zero with the correct potentials applied, the tube
is oscillating.
This condition must be eliminated be-
fore accurate Gillindications can be obtained. Usually
a 15 to 20 ohm resistor placed in some one of the
electrode
leads,
except filament
or cathode,
will
eliminate the tendency to oscillate.
There are some tubes requiring low bias and if the
signal voltage applied is too high, grid rectification
will cause the Grid Current meter to indicate to the
right of zero. This condition can be readily detected
by noting the increase in Grid Current meter deflec-
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