19 GAS
TEST
Obtain a
6L6tube
that
is
known
to
be
free
of
gas.
Put
the
tube
in
the
tester
and set
it
up
for
the
standard 6L6
test.
Set
the
BIAS
VOLTMETER switch
to
the
50
volt position. Set
the
FUNCTION switch
to
the
D position.
Press
PS,
the
gas
1
test
button. Adjust
the
BIAS
control
for
a reading
of
500
on the 3000 scale. Hold
PS
and press
P6,
the
gas
2
test
button and verify
that
the
reading moves up by
less
than
one small division. Release
P6
and
PS.
Connect a
10M
resistor between pin 5 and pin 8
of
the
nine pin miniature socket.
Press
PS,
the
gas
1
test
button. Adjust
the
BIAS
control
for
a reading
of
500
on
the
3000 scale. Hold
PS
and press
P6.
This
time
verify
that
the
reading goes up by 3
to
4 small divisions on
the
meter. Release
P6
and
PS.
Switch
off
the
AC
power
to
the tester and disconnect the power cord. Reinstall the
tester
in
the
cabinet.
19.1
Adjustment
The
gas
testing
circuits depend on the
meter
bridge circuit and several resistor values.
If
the
mutual
conductance tests are functioning properly, check
for
bad switches
or
dirty
contacts
on
P5
and
P6.
Also check
R2/470K
and
R53/220K
resistors.
R2
is
placed
in
series
with
the
grid
forthe
gas
test
when
P6
is
pressed. Any
grid
current
due
to
gas
will
cause a voltage
drop
across
R2.
The plate
current
goes up
as
the
grid bias drops
indicating
gas
in
the
tube.
20 Transconductance Calibration Notes
Hickok calibration
is
always
an
inexact science. Hickok tended
to
change production details on
an
individual
basis
and twea k individ ual testers
as
they were assembled
to
cover irregula rities.
The
test proced ure outlined a bove
is
to
be
used
as
a guideline
for
the calibration
of
the typical tester. Individual testers may vary
from
the design norm.
Kee
p this
in
mind
if
ce
rtain deta
ils
in
your
tester do
not
match the information given here.
It
would ta
ke
a custom
test procedure written
for
each
tester
in
orderto
be
absolutely accurate
in
every detail.
The
sensitivity
ofthe
transconductance readout circuit
is
essentially fixed
by
the
hardware
ofthe
tester,
the
meter, power transformer and the various fixed resistors
in
the metering circuit.
The
transconductance circuit
is
in
the
form
of
a bridge
that
is
sensitive
to
the relative amplitude
of
the alternate peaks
of
the
full
wave rectified
120Hz (100Hz) pulsating
DC
plate current. Any difference
in
the
amplitude
of
the
peaks
is
read
as
an
indication
of
transconductance. This
is
why
it
is
important
to
adjust
out
any difference
in
the
voltage
of
the alternate peaks
of
the unfiltered plate voltage. 1ft
he
a Ite rn ate peak voltages differ constant errors
will
be
added
to
the
transconductance readings.
An
unfiltered pulsating 120Hz (100Hz)
DC
bias
with
a n added
AC
signal
is
applied
to
the
control grid
of
the tube.
For each peak
of
plate voltage
the
plate current
is
alternately higherthan
or
lowerthan
the
average by
the
amount
of
the
AC
grid signal times the transconductance
of
the tube. The measurement circuit
senses
this
difference and reads
it
out
as
a value
of
micromhos
of
transconductance.
The
greater the difference there
is
in
the
amplitude
of
the
peaks because
of
the transconductance
of
the tube, the higherthe reading.
Hickok determined a nominal grid signal value and displayed the
AC
plate current
as
a transconductance reading
assumingthe grid signalvalue
isthe
design value.
The
539C
has
no
means
of
measuringthe actual
AC
grid signal
applied
tothe
tube
nor
does it regulate
that
voltage
in
anyway
so
that
it
is
always correct.
The
tester
simply
calculates the transconductance
of
the
tube
undertest
assuming a correct grid signal voltage. Therefore if
the
AC
grid signal
is
something different than
the
intended value,
say
for
example because
the
AC
line
meter
is
inaccurate,
the
readings
will
be
off
by the amount
that
the grid signal differs
from
what
it
is
intended
to
be.
Calibration using the standard 6L6 calibration tube attempts
to
adjust
out
any residual
error
by adjustingthe
AC
signal
to
the control grid
in
order
to
bring
the
reading
to
norma
I.
The
zero transconductance reading,
that
is
the
meter
read ing at which
the
tube
plate current
has
no
AC
component whatsoever due
to
transconducta
nce,
is
trimmed
by
bala
ncing the plate power supply
in
step 14A.
29
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