Tektronix 502 series, Instruction Manual

Page: 27 / 114

V414 is connected to the input circuit and the
grid of V444 is returned to ground either through
the switch or through one of the series attenuator
networks. When in position B (AC or DC), V444
is the input tube and V414 is the grounded-grid
tube. With either of these configurations, the In­
put Amplifier is a cathode coupled, paraphase
amplifier; it converts a single-ended input signal
to a push-pull output signal.
The cathode resistor R408 plays an important
role in determining the amount of negative feed­
back applied to the Input Amplifier stage. As
mentioned previously, this feedback voltage
comes from the cathodes of the Driver C.F. stage
(actually, from the output of the Second Ampli­
fier stage). The feedback is applied through a
divider consisting of R470 on one side and R408,
and R480 on the other side and R408. The smal­
ler the value of R408 the greater the drop across
the series resistors (R470 and R480) and the less
the negative feedback applied to the Input Am­
plifier stage. Conversely, the greater the value
of R408 the greater the drop across it and the
greater the negative feedback. Thus, for very
small input voltages, when the SENSITIVITY
switch is set so that the resistance of R408 is
quite small, there is very little negative feedback
and the Input Amplifier operates with high
gain. When the SENSITIVITY switch is set to ac­
comodate larger input voltages the resistance of
R408 is increased. This means that there is a
greater amount of negative feedback and the
gain of the Input Amplifier is decreased.
The switch diagram appearing at the bottom
of the circuit diagram shows the makeup of R408.
R408M is in the circuit for all positions of the
SENSITIVITY switch. In the .2, 2 and 20 VOLTS/
CM positions of the switch, R408M alone makes
up the resistance of R408. In all other positions
of the switch, R408M is shunted by at least one
other resistor. In the 200
/x VOLTS/CM position
R408M is shunted by both R408A and C408A; in
the 50 and 100 MILLIVOLTS/CM positions, and
in the 5 and 10 VOLTS/CM positions, it is
shunted by both a resistor and an R-C network.
The total value of R408 is determined by the de­
gree to which R408M is shunted. The greater the
shunting of R408M the smaller the value of R408
and the greater the gain of the Input Amplifier
stage. Conversely, the less the shunting of
R408M the greater the value of R408 and the
smaller the gain of the stage.
The cathodes of the Input Amplifier stage are
“ long-tailed” to the —150 volt supply through
90-K resistors. With the grids of the Input Ampli­
fier at ground potential, the cathodes will op­
erate very close to ground (actually, a couple of
volts positive to bias the stage). The approxi­
mately 150-volt drop across the 90-K cathode
resistors (R427 and R428) provides a constant
supply of cathode current to stabilize the per­
formance of the 6AU6 Input Amplifier tubes.
In order for the Input Amplifier stage to re­
main in a state of DC balance, there must be no
DC voltage drop across R408 when there is no
input signal. This means that the difference in
potential between the two cathodes must always
be zero, regardless of the value of R408. In
order to provide for equal cathode voltages
under this condition, the screen voltage of the
two tubes can be varied with respect to each
other with the COARSE DC BAL. control R431
and the DC BAL. control R433. Any change in
voltage at the screens will be reflected to the
cathodes by a factor of 1 //x (the screen grid fi)
and thus the cathode voltages can be equalized.
Vertical positioning of the crt beam is accom­
plished through the action of the POSITION con­
trol R421. This is a dual control, connected be­
tween —150 volts and ground. It is wired so
that as the voltage between ground and the
movable arm in one side increases, the voltage
between ground and the movable arm in the
other decreases. Any change in the setting of
this control tends to produce a change in the
voltage at the cathodes of the tubes. However,
the negative feedback circuit reacts quickly to
prevent any change in the cathode voltage, and
as a result a change in the voltage at the cath­
odes of the Driver C.F. is produced. This change
in voltage, at the cathodes of the Driver C.F., is
amplified by the Output Ampilfier and appears
as a change in the positioning voltage at the
vertical-deflection plates.
When the Input Selector switch is set to the
A-B position both grids of the Input Amplifier
are connected to the Input circuit. With this con­
figuration the Input Amplifier is connected for
differential operation. Two input voltages are
required, and the push-pull output voltage is pro­
portional to the difference between the two in­
put voltages.
The .2 MV/CM DIFF. BAL. control R435 adjusts
the plates of the Input Amplifier for equal volt­
ages when common mode signals are applied to
the grids. This control is equally effective in all
positions of the SENSITIVTY switch, but is pri­
marily adjusted in the high-sensitivity position
(200 ^ VOLTS/CM). The .2 V/CM DIFF. BAL. con­
trol R424 provides additional differential balance
for the low-sensitivity ranges, and is most effec­
tive when the SENSITIVITY switch is in the .2
VOLTS/CM position.
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CIRCUIT DESCRIPTION — TYPE 502
4-3