Any
error
in this calibration
is
called "offset" since
it
offsets
the
actual readings by a fixed
amount
from
zero.
An
error
in
this adjustment shows up
as
a constant number added
to
the
measured transconductance.
R8
is
a balance control
that
adjusts
the
relative amplitude
of
the
plate supply rectified 120Hz (100Hz) pulsating
DC
peaks.
Adjustingthe
balance
ofthe
peaks directly affects
the
balance
ofthe
plate current measurement bridge
and
thusthe
zero point
of
the
readings displayed, sort
of
the
electrical equivalent
of
zeroing
the
meter
so
that
all
of
the
readings
won't
have a consta nt value added. The actual sensitivity
or
slope
of
the
ca
libration line
is
fixed by
the
hardware and
can
only
be
changed by replacing
the
scaling resistors
in
the
meter
circuit.
R15
adjusts
the
balance
of
the
amplitude
ofthe
peaks
of
the
DC
grid
bias
and screen voltages. Any difference in
amplitude
of
the
peaks
in
alternate pulses
of
unfiltered
DC
bias
is
exactly
the
same
as
addingan
AC
voltage
to
the
bias. This directly affects
the
apparent
AC
signal on
the
grid
so
it must
be
balanced
or
additional signal
will
appear
onthe
grid
alongwith
the
intended signal. The
amount
of
any additional
AC
signal added
to
the
grid
by
an
imbalance
in
peak voltages
of
the
DC
bias
is
directly proportional
to
the
DC
bias setting used. A setting
of
-3 volts
will
add
only
half
of
what
a setting
of
-6 volts will. Forthis reason it
is
very important
that
the
grid
bias
supply be
close
Iy
ba
la
nced
in
order
not
to
add any
AC
signa I
to
the
grid.
The
common Hickok calibration procedure
forthe
539C
used
the
calibration
tube
test
to
adjust
R15
in
orderto
trim
out
any residual unbalance and bring
the
readings
to
the
nominal value
of
the
calibration tube. The
assumption being
that
everything else
is
good and adjusting
R15
will
set
the
true
balance by default. This
is
a
poor
method because it opens the possibility
that
the
DC
bias
is
not
actually balanced but only compensating
for
errors
in grid signal and metering accuracy. It would have been better
to
split
the
function
of
bias balancing and
ca
libration adjustment
by
adding another
potentiometer
to
the
grid signa I divider resistors. Doing
so
would
have
added add itiona
I cost a
nd
complexity
to
the
tester. Proba bly
the
problem was dea It
with
at
the
factory by
selection
of
resistors
in
certain areas
of
the
circuit
to
get close enough
or
it
was
not
considered a particular
problem
since service type test equipment does
not
need laboratory grade accuracy.
Testing
with
a calibrated
tube
to
match
the
readings
to
an
actual known value
as
in step 17 (MUTUAL
CONDUCTANCE CALIBRATION USING A CALIBRATION TUBE) acts
as
a fina I
test
of
overa
II
ca
libration.
If
the
readings do
not
measure correctly
after
the
tester
has passed all
of
the
other
tests
there
is
little else but
to
try
and
adjustthe
grid signal by replacement
of
the
fixed grid
bias
divider resistors
to
make
it
comply. If a
calibration
tube
is
not available, following
the
balance adjustment step
14
then adjusting
the
AC
grid signal voltage
to
the
design values
as
in
step 11.1
can
do a reasonable calibration.
30
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