22 Notes on Making an Accurate 539C Calibration Tube
Calibration tubes
for
the
539C
must
be
measured at the specific conditions and voltages
that
the
539C
testers
subject
the
tubes to. According
to
Hickok documents, a calibration tube must
be
produced by measuring a
sample candidate tube on a Hickok factory prototype instrument known
to
be
in
proper calibration.
This
sets
up a kind
of
chicken-egg situation in which a calibration
tube
is
required
to
calibrate a tester and a calibrated
tester
is
required
to
create a calibration tube. Hickok never revealed
how
the
factory prototype tester was
calibrated in the first place before calibration tubes were available.
Hickok factory calibration tubes were probably not available
to
anyone
other
than internal factory personnel.
None have ever been
seen
even by Hickok authorized repair stations outside
the
factory.
To
obtain
an
accurate calibration tube one must
be
either
made by testing on a known accurate
539C
tester
or
bought
from
someone
who
has
such a tester and offers
the
tubes
for
sale.
An
individual cannot
test
one
on
his own using
lab equipment and certify it.
As
stated above
in
the
notes
on
using the calibrated
6L6,
539C
testers do not
represent
the
actual book value transconductance
of
a particular tube. They only determine the Hickok
derived representation
of
it
for
comparison
to
the
539C
roll chart
to
indicate
the
tube's
worth.
Certifying a
tube using any
other
means than a calibrated
539C
tester
will
not
provide
the
same values.
A used tube
with
good transconductance
is
likely
to
be
more stable than a new tube due
to
the need
for
a
certain amount
of
initial break-in. Test candidate tubes
in
the
usual way
for
transconductance, leakage and
gas.
Candidates
for
calibration tubes must
be
stable
and
have no heater hum.
It
may take a dozen samples
to
find a suitable
tube
for
a calibration standard. Metal shell tubes tend
to
be
less
prone
to
heater hum but this
is
not always
the
case.
Heater hum affects
the
transconductance reading
of
the tube by artificially adding a 60Hz signal
to
the plate
current causing the reading
to
be
either abnormally high
or
low
depending on the phase
of
the
added hum
with
respect
to
the grid signal.
To
determine
if
a
tube
has
hum, set up a candidate
6L6
on
a
539C
and test
it
in
the usual way. Note
the
transconductance reading. Change
the
filament switches
from
the
H S setting
to
C X
and repeat the test.
If
the reading
is
different by even one small division reject
the
tube and
try
another one.
If the tube
passes
the
hum test the stability should
be
checked. The quick easy way
is
to
set up a normal
tube
test
on
a
539C
tester
for
the
candidate tube.
Take
the
reading and
write
it
down. Shut
off
the
tester and
repeat
the
test
an
hour later. Several repeated cold/warm cycles should
be
tried
to
verify
that
the
candidate
tube would reliably read
the
same every time.
If
not then you cannot depend on
it
to
be
used
as
a calibration
standard
that
is
repeatable.
A
better
verification
is
to
set up
the
tube on stable laboratory equipment
for
a
DC
grid shift test.
Use
the
manufacturers design center book values
of
250 volts
forthe
plate, 250 volts
forthe
screen and a
nominal-16
volts
for
the control grid.
Do
a grid shift
of
2.0 volts around nominal (-17
to
-15 volts) and verify
that
the
DC
plate current does
not
take
an
inordinate amount
of
time
to
stabilize and remains stationary
at
the
new value
after
either
the
shift up
or
down. The plate curre
nt
sho
uld a
Iso
return
to
close
to
the
same measured lowe r
or
h ighe r
va
I
ue
with
every up
or
down shift. Plate current stability over
time
and repeatability are essential.
Tubes
that
tend
to
wander
or
take a
longtime
to
settle are not acceptable. Perfection
is
not necessary but
the
best sample you
can
find
will
help assure
the
best results in
the
long run.
32
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