Quote: Hickok testers, like the WWII vintage 540 model I currently have do not appear to have any calibration adjustments possible. The basic accuracy is not mentioned in the manual, just that it will give superior results to any other tester then available. I would say that the accuracy of the instrument was provided by the special circuit Hickok designed, the quality of the parts they used, and the tube test data which they derived for tests.
I think the quality referred to, is in the Fact
that most other tube testers were EMISSION TYPECalibration AND REPAIR VIDEOS FOR TV-7
Calibration OF A tv-7 1 HOUR 28 MINUTES
[You must be registered and logged in to see this link.] TV-7
LINE SET ISSUES 17 MINUTES
[You must be registered and logged in to see this link.] Installing a Digital Meter in a TV-7 Tube Tester 20 MINUTES
[You must be registered and logged in to see this link.] Hickok 6000A
restoration and Calibration 28 MINUTES
[You must be registered and logged in to see this link.]--------------------------------------
Another good source is printed out below.
Its kinda lengthy but you can also download it from the Link . click here to download
Note: Since its a copy, and a quote in a small part, it comes under the Fair use Doctrine.
Published by:
" THE HICKOK ELECTRICAL INSTRUMENT COMPANY 10514 DUPONT AVENUE • CLEVELAND 8, OHIO
INSTRUCTION MANUAL FOR TRANSCONDUCTANCE TUBE TESTER MODEL 539C
.........20
Transconductance Calibration Notes Hickok calibration is always an inexact science. Hickok tended to change production details on an individual basis
and tweak individual testers as they were assembled to cover irregularities. The test procedure outlined above is
to be used as a guideline for the calibration of the typical tester. Individual testers may vary from the design norm.
Keep this in mind if certain details in your tester do not match the information given here. It would take a custom
test procedure written for each tester in order to be absolutely accurate in every detail.
The sensitivity of the transconductance readout circuit is essentially fixed by the hardware of the 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. If the alternate 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 higher than or lower than 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 higher the reading.
Hickok determined a nominal grid signal value and displayed the AC plate current as a transconductance reading
assuming the grid signal value is the design value. The 539C has no means of measuring the actual AC grid signal
applied to the tube nor does it regulate that voltage in anyway so that it is always correct. The tester simply
calculates the transconductance of the tube under test 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 adjusting the AC
signal to the control grid in order to bring the reading to normal.
The zero transconductance reading, that is the meter reading at which the tube plate current has no AC
component whatsoever due to transconductance, is trimmed by balancing the plate power supply in step 14A.
29
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. Adjusting the balance of the peaks directly affects the balance of the plate current measurement bridge
and thus the 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 constant 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 of the 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 adding an 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
on the grid along with 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. For this reason it is very important that the grid bias supply be
closely balanced in order not to add any AC signaI to the grid.
The common Hickok calibration procedure for the 539C used the calibration tube test to adjust R15 in order to
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
calibration adjustment by adding another potentiometer to the grid signal divider resistors. Doing so would have
added additional cost and complexity to the tester. Probably the problem was dealt 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 final test of overall calibration.
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 adjust the 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
21 Notes on using the Calibrated 616
Hickok part number 20877-1
The calibration tube is intended to be used as a tool to compare and adjust the final overall accuracy of any 539C
tester against a factory standard 539C tester in order that your tester will give comparable results to the factory
standard and comply with the roll chart results. It should be noted that this value is not necessarily the actual
"book value" transconductance of the tube measured on precision test equipment according to tube
manufacturer's standard procedures but a representation of the transconductance that a factory calibrated
Hickok 539C should measure.
Even though the mutual conductance calibration tests performed with the simulated test circuit show the correct
readings, other factors and irregularities will noticeably affect the final readings. Power supply loading, small
imbalances in screen voltage, plate voltage, mismatches between the two halves of the 83 and 5Y3 rectifiers,
resistor values and other things all add up to affect the readings. Even using the wrong type of fuse lamp can
substantially change the calibration of the tester.
The tube is not intended to be used to adjust out significant problems or faults in the operation of the 539C. It is
only to be used to match the reading of a properly functioning tester to that of the factory standard. If significant
errors in operating voltages are found at the time of calibration, first determine that the rectifier tubes are
working properly and have substantially equal output from each half of the tube. A good test is to substitute
known good re placement rectifier tubes and see if the problem resolves. Replacement of a rectifier tube will
always require a recalibration of the tester. Verify that all of the test voltages are correct and that the fixed
resistor values have not shifted with age or by electrical damage.
It should be noted that even ca lib ratio n by this method will never guarantee perfect accuracy. A different
calibration tube will seldom read the exact calibration value on any given 539 calibrated with another tube. They
will be very close but the tester is simply not capable of that kind of repeatable accuracy from sample to sample.
Slight differences in component tolerances, calibration tubes and mostly the lack of regulation of operating
voltages due to loading. meter reading inaccuracy and the coarse adjustment capability of the line adjust control
will cause a discrepancy in test results.
While Hickok designed the 539C to be a better than average tube tester it is still not a laboratory grade
instrument. Some deviation from perfect accuracy must be expected. The calibration tube is carefully tested and
specified for use with the Hickok 539C only. Because other Hickok model testers and other manufacturers testers
use different voltages for testing than the 539C they may, but more likely will not, read the same value of mutual
conductance for the calibration tube. Only a perfect tube would test out to the same mutual conductance value
for every tester and every set of ope rating voltages applied to it. Since there is no perfect tube this should not be
expected.
Never place the tube in emission type testers or testers of other brands because of the possibility of permanent
changes in calibration value. Because it was specifically calibrated for the 539C circuit using 539C operating
conditions its value for checking other testers using different test conditions is marginal at best..."
Subject: Military Model TV-7 Tube Tester Corner 
April 12th 2019, 10:42 am
Though, if the test button is within reach, I may see if I can clean that up if nothing else.
