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FIG. 3
D-C POWER SUPPLY CONTROLS:
Individual suppressor
grid, screen grid, and plate adjusters are mounted
in line across the lower section of the Model 686
panel. These are 150 watt or 100 watt vitreous type
potentiometers
connected in the d-c supply circuit.
These high wattage ratings are used to provide a
long peripheral length of contact travel, thus giving
accurate potential settings on the tube electrodes. The
element voltages should be adjusted in the following
order to prevent damage to the screen and cathode
due to excessive currents:
(1) control grid (2) plate
(3) screen.
All electrode potentials can then be re-
adjusted
and any changes
due to tube loading of
the power supply can be corrected.
The accuracy
of the mutual conductance
readings
depend to a
great extent on the accuracy of these electrode poten-
tial adjustments.
SIGNALAMPLITUDE ADJUSTER:
By effectively chang-
ing the signal voltage this potentiometer
is used to
compensate
for different line voltages.
The signal
voltage is checked at full scale on the Gillmeter and
any necessary
correction is then made with this
control.
To correct for different line voltages
the Signal
switch should be placed on the "Ca1." position and
the Signal Amplitude Adjuster rotated to obtain full
scale deflection on the Gm meter.
HUM CONTROL:
Filamentary tube types require an
accurate
electrical
center tap on the filament for
correct Gm readings,
and it is the purpose of this
control to provide such an adjustment.
On heater
voltage switch positions 12.5 volts and above, the
Hum Control is disconnected
to prevent excessive
heat dissipation.
After the tube is in position and all electrode
potentials have been adjusted to the specified val-
ues, the Gill Factor toggle switch to the right of the
Gill meter should be set to the "x 1" position and the
Hum Control set for a minimum or zero reading on
the Gill meter.
(Serious errors in mutual
conductance
readings
can be expected
if this operation
is not per-
formed carefullyJ
SELFBIAS RESISTANCE C ONTROL:
The Model 686 is
equipped with a Self Bias Resistance control mounted
on the left side of the top panel section. This control
provides for checking certain tube types such as the
6I4, 6I6 and 1231 where self bias is definitely speci-
fied by the manufacturer.
These tubes tend to draw
grid current or are unstable under equivalent
fixed
bias conditions, thus causing errors in Gm readings.
The control may be varied from 0 to 700 ohms,
and to prevent degeneration
the unit is by-passed
by a 1,000 m.f., 50 volt condenser.
When checking
the normal fixed bias types this control must be set
to zero (0) but on those types requiring self bias the
Control Grid Voltage Adjuster must be set in the
most counter-clockwise
position.
This procedure is
necessary
to prevent fixed bias from being applied
in addition to the self bias.
To prevent damage
to the 1,000 microfarad
con-
denser, a simple calculation should be made to see
that the product of the expected
cathode
current,
and the resistance in the self bias circuit does not
exceed 50 volts: V
==
1 x R x .001
Where V
==
Voltage appearing across 1,000
microfarad condenser.
I
==
Expected cathode current in mill-
amperes.
R ==Resistance
selected
by Self Bias
switch position.
SHORTTEST - METERREADINGAND ELEMENT SHORT
TEST SWITCHES:
The Short Test-Meter Reading and
Element Short Test switches mounted on the right
side of the top panel section provide the necessary
means for short checking tubes with a d-c potential.
Tubes, having a filament or heater, may be short
checked, either hot or cold. The Short Test-Meter
Reading
switch disconnects
the d-c potential
and
meters from the tube elements and connects a small
d-c power supply and a neon lamp into a group of
circuits controlled by the Element Short Test switch.
This switch segregates
the element
to be short
checked, leaving all the other elements tied together.
The filament or heaters should be at normal op-
erating temperature
when hot short checking, and
the Short Test-Meter Reading switch should be in-
dexed to "Short Test" position.
The Element Short
Test switch is then rotated through its six positions,
stopping at each position to tap the tube.
If the patch cords have been connected so as to
use both PI = Plate 1 and Pz = Plate 2, index the
Plate 1
-
Plate 2 toggle switch located on the upper
right top panel section to its other position and rotate
the Element Short Test switch through its six positions
again.
A slight flicker of the neon lamp between
positions on the Element Short Test switch does not
indicate a short in the tube.
-5-
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