Advertisement
2-9
to 4.4v
which
causes
D138
to
conduct.
When
D138
conducts,
it
shunts
the
cathode
network
of
V133,
R133
and
R136.
The
tube
is
now
supplied
current
from
ground
through
the
low
impedance
of
the
VAR
ATTEN
BAL
circuit.
The
Ri
of
the
tube
is
about
1252
at
this
time
and
the
gain
of
the
Input
CF
stage
will
fall
to about
.01.
Since
the
undriven
side
of
the
paraphase
amplifier
is
tied
to
the
VAR
ATTEN.
BAL
control,
its
base
will
go + as
the
base
of
the
driven
side
goes
+.
With
both
bases
following
the
same
rise
in
potential,
the
effects
of
an
excessive
+ Signal
are
partially
cancelled
common
mode
at
the
paraphase
amplifier.
Thus
the
same
circuitry
protects
both
the
input
CF and
the
paraphase
amplifier
from
excessive
+ signals.
If V133's
current
increased
to about
12.5
ma,
the
plate
voltage
would
go ''down'',
cutting
off
D134.
Now
further
increases
in
tube
current
are
resisted
by
R134
and
the
maximum
tube
current
is
limited
to approximately
15
ma.
The
conditions
described
above
occur
when
an
excessive
+ signal
is
first
applied.
There
will
be
a finite
time
lag
before
the
neon
will
ionize
and
before
the
effects
of
grid
current
will
be counteracted
by
R117.
The
paraphase
amplifier
is
also
protected
against
excessive
- signals
by
D137.
When
the
potential
at
the
cathode
of
D137
falls
to
-./7v,
D137
conducts
protecting
the
paraphase
amplifier
from
an
excessive
-
drive.
R136
prevents
the
Input
CF
from
presenting
a negative
input
impedance
to
the
circuit
under
test.
The
maximum
allowable
grid
current
is
5 nanoamps
which
devleops
a 5 mv
drop
across
R115.
This
is
only
1
mm
of
deflection
as
seen
on
the
CRT
and
is
not
considered
sufficiently
annoying
to warrant
a grid
current
null
adjustment.
Advertisement
Need help?
Do you have a question about the 1A2 and is the answer not in the manual?