Tektronix 570, Manual

Page: 22 / 134

P LATE - S C REEN­
GRID
HO RIZONTA L/
VOLTS
PLATE-GRID
POSITIONING
( V E RTICAL and
HO RIZONT AL)
PLATE
2
0
P L ATE
Midrange
Now insert the nine-pin-miniature socket
adapter into the test panel and connect the
patch cords as follows :
Pin No. Connect to
Tube element
1
P Triode plate
2
GRID B Pentode grid
3 + DC
Pentode screen
4
HTR Heater
5
HTR
Heater
6 P
Pentode plate
7 K Pentode cathode
and suppressor
8
K
Triode cathode
9 GRID A Triode grid
Install a type 6U8 tube in the socket adapter
and turn the P OWER-TEST switch ON. The
H EA TER VARIABLE should now be adjusted
for a reading of 100%. The 100% marking will
b e found on the 0- 140 meter scale. Switch the
INDICATION Control to + DC and adjust the+ DC
V ARIABLE Control for a meter reading of 100
volts.
Position the TEST POSITION switch to the
GRID A position. Adjust the POSITIONING
Controls so that the displayed curves start
in the lower left hand corner of the graticule.
Using the display of curves adjust the FOCUS,
ASTIGMA TISM and INTENSITY Controls for
a sharp trace of comfortable brightness.
The Z E RO B lAS button should now be
depressed and the position of the zero bias
curve noted. With the Z E RO B IAS button
released adjust the START ADJUST until the
uppermost curve is superimposed upon the
position that the zero bias curve occupied with
t h e button depressed.
You are now displaying the 6U8 triode plate­
c h aracteristic curve. Move the STEPS/SEC
control to each of the 1 20 positions and note
that the Grid Step Generator switches at only
one end of the sweep in each of these positions.
Now move the TEST POSITION switch to
G RID B and display the plate-characteristic
c u rves of the 6U8 pentode section. Move the
P L ATE-SCREEN-GRID Control to SCREEN.
This will result in a display which shows the
s creen current plotted against plate voltage.
Now return the control to the PLATE position.
To expand the display of the first curve reset
the following controls:
PEAK VOLTS 20
V E RTICAL MA/DIV
. 1
HORIZONT AL 2
VOLTS/ DIV
You will notice that the curves are not plotted
all the way to the origin and that the curves
intersect the current axis above the origin.
These are normal effects and do not indicate
m i s calibration of the instrument. The correct
ho rizontal positioning can be determined by
momentarily grounding the plate connector, P .
T h e line which results should lie along the
c urrent axis. The P connector should not be
left connected to ground any longer than is
necessary.
The initial velocity of the emitted electrons
c auses plate current to flow at zero plate
voltage as indicated by the curves intersecting
the current axis above the origin. This effect
is more pronounced with increased series load
r e s istance. A similar effect in the plate­
s weep-generator rectifiers prevents the plate
s weep voltage from dropping completely to zero.
THERMIONIC D I O D E CURVES
To plot diode curves , connect the diode in the
normal manner with the cathode to ground ,
K , and the plate to the Plate Sweep Generato r ,
P . Set the PLATE-SC REEN-GRID Control
to PLATE and the PLATE-GRID Control to
P L ATE.
Before turning the POWER-TEST
s witch on , set the PEAK VOLTS Control to a
low voltage. After the POWER-TEST switch
is turned ON , wait until the cathode of the tube
under test has reached operating temperature
before raising the P E A K VOLTS. The voltage
can then be raised until maximum operating
conditions are reached.
It is sometimes of interest to sweep the plate
voltage negative with respect to the cathode,
b eyond the point of plate current cutoff. This
c an be done by connecting a battery between
the cathode and ground to raise the cathode
positi
ve. Three volts is normally sufficient.
The + DC Control and test panel connector
can be used instead of a battery if the peak
plate current is to be less than 50 rna. To
use the
+
DC
test panel connector, connect the
c athode of the tube under test to
+
DC. Set the
+
DC Controls to minimum , about ten volts.
2-4
O perating Instructions - Type 570
AA