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MODEL 6008 ELECTRCME!I!EB
SECTION IV - CIRCFIT DISCUSSION
CIRXJlT DISCUSSION
The basic element of the Model 6OOA is a highly accurate,
stable dc
voltmeter with a full
scale se
1C
sitivity
of 10 millivolts
and an in-
put impedance greater than 10
ohms shunted by 30 micro-microfarads.
Amperes and ohms are measured by the use of resistance
standards.
The various connections necessary for ampere and ohms measurements
will
be discussed following
the detailed
description
of the amplifier.
A.
VOLTMETER
Refer to DR1201hC at the rear of the manual.
FIOURE
BOO-
2
The amplifier
proper con-
sists of Vl, V2, and V3.
VL is a cathode follower
which drives the amplifier
at the same instantaneous
potential
as the input
signal.
In other words the
neutral
or ground terminal
of the amplifier
is not
grounded to the chassis out
is attached to the output
cathode follower.
Fig.
600-2 shows this diagram-
atically.
The amplifier
A
is driven by the cathode
follower.
Input dividers
are neither
stable nor accurate at high impedance. Con-
sequently the Model 600A will
accept 10 volts without
the use of input
dividers,
preserving the high input impedance and accuracy of the ampli-
fier.
Accessory probes are available
for extending the voltage range at
reduced input resistance
and accuracy.
Since the amplifier
proper is driven by the cathode follower,
the plus
and minus 30 volt batteries
for the cathode follower
are referred
to
input ground while the battery
supplies for the amplifier
are referred
to amplifier
ground which is "floating".
In subsequent discussion,
reference will
be made to the amplifier
ground as "floating
ground" and
to cathode follower
ground as "output ground".
The amplifier
input consists of two balanced 5886 electrometer
tubes.
The filaments
are operated in parallel
from battery
Bl.
The control
grid of Vl, the active electrometer
tube, is protected by R2, a 109 ohm
resistor,
bypassed for high frequencies by C2. The control
grid cf 712,
the 'tdummy" tube, is returned through a variable
voltage to floating
ground.
This variable
supply is made up from B2a, B2b, R15 (ZERO on the
front panel) and Rll, ~28, R29;R30, R31, R32 on SW-& (COARSE ZERO on
rear panel).
og62R
4-1
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