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INSTRUCTIONS
FOR MODEL 686- TYPE lOA
TRUE
MUTUAL CONDUCTANCE VACUUM TUBE ANALYZER
GENERAL
DESCRIPTION
The Model 686 is a complete direct reading True
Mutual Conductance
Vacuum
Tube Analyzer
de-
signed to operate from any 105 to 125 or 230 volt
50-60 cycle outlet. It has eight mutual conductance
ranges with full scale readings of 300, 600, 1500 and
3,000 micromhos and also 3,000, 6,000, 15,000 and
30,000 micromhos.
Instruments are provided for ac-
curately
measuring
all electrode voltages
and for
reading
electrode
currents
including
minute grid
currents.
Internal power supplies and a signal source provide
all necessary
potentials
to panel controls, wherein
adjustments
can be made in accordance
with meter
readings.
Tube sockets for all commercial type re-
ceiving tubes are mounted on a removable
socket
panel across the top front section of the equipment.
These in turn connect through short-test switches to
patch cord jacks which are marked with R.M.A. pin
numbers and are used with patch cords for any or
all electrode connections.
Thus with complete con-
nector flexibility and complete voltage control, all
kinds of static characteristics
can be plotted, in addi-
tion to the measurement
of Gill under any or all ap-
plied potential conditions.
DETAILEDDESCRIPTION
OF EQUIPMENT
THEG./II M ETER: The fan shaped instrument in the top
center of the main panel is the Gill meter. The scale
is calibrated in two arcs reading zero to fifteen and
zero to thirty and sixty. This meter operates
with
two multiplier switches marked "GM RANGE" and
"GM Factor".
The Gill Range switch is used for selecting the
300,600, 1500, or 3,000 micromhos range as required,
with corresponding
grid signal voltages of 1.0, 0.5,
0.2, and 0.1 volts respectively.
The GIllFactor switch
is used to extend the ranges by a factor of ten times
so that ranges of 3,000, 6,000, 15,000 and 30,000 are
also available.
On low mu tubes with low plate resistance,
the
meter resistance is a reasonable
part of the total tube
and tube tester plate circuit impedance;
measure-
ments on these tubes would be in error unless .this
was taken into consideration.
The compensation
is
made automatically
for all tubes having
a plate
resistance above 250 ohms.
On high mu tubes the peak value of the grid signal
.
voltage should be kept smaller than the dc grid bias
voltage to prevent direct grid conduction.
On tubes
having sharply curved characteristics
the grid signal
should
be kept small
to avoid
second-order
effects.
The low signal
voltages
available
make
it possible
to measure
all low bias
tubes
operated
and tested
as Class A amplifiers.
It is advisable
for the operator
to select
the low-
est grid signal
voltage
that will render
reasonable
pointer
deflection
when testing
all low bias high mu
tubes.
Note that a 3,000 micromho
range is available
with
either
a 1 volt signal
or a 0.1 volt signal.
Use the
0.1 volt signal for low bias high mu tubes.
The 1 volt
signal
is useful
for tubes having
a-c operated
filia-
mentary
cathodes,
where
it is desirable
to increase
the signal
voltage
to minimize
hum effects.
The a-c grid signal
voltage
is calibrated
on the
Gill meter to eliminate
possible
errors due to temper-
ature and amplifier
gain.
If the instrument
is reading
the a-c plate current
component
2% low because
of
these effects, it will likewise
require
a signal voltage
2 % higher
in value
to bring the meter
to top mark
on Signal
Calibrate.
Since
the
signal
voltage
is
higher,
the GIll indication
will also
be higher
and
hence
compensation
is obtained.
GRID CURRENT METER:
A two range microammeter
for grid current readings is mounted to the left of the
Gill meter.
This instrument
has a range of 15-0-15
microamperes.
Readings down to and including one-
half microampere
are easily read. The meter is nor-
mally shunted to 1500-0-1500 microamperes
and is
switched to the low range by manipulation
of a mo-
mentary
toggle switch located to the right of the
meter.
The meter is a zero center instrument to indicate
any or all components of grid current resulting from
gas, leakage resistance, or secondary emission. Grid
current readings
are especially
important in segre-
gating defective power tubes such as the type 6L6
where a limit of 3 microamperes is specified.
In taking grid current readings, the operator will
note that there is a red line on each side of the cen-
ter scale zero mark. This red line indicates 15 micro-
.
,
amperes
on the 1500 microampere
range.
If the
pointer does not deflect beyond the red line after the
tube is heated, then the Microamperes switch can be
shifted to the 15 microampere
position, and readings
taken on this low range. The instrument is in series
with the control grid patch cord jack at all times and,
therefore, will indicate grid current under all operat-
ing conditions.
-3-
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