to the advent of FET-input multimeters, obtaining a very high input
impedance meter required the use of a vacuum tube circuit that used
a buffer stage to isolate the measured signal from the loading effects
of the meter movement. As most people reading this article already know,
the voltage value indicated by a non-buffered meter can be greatly affected
by the meter's loading of the device under test (DUT) if the meter's
impedance is not many times greater than the DUT's impedance. The voltmeter
is used in parallel with the circuit under test, so for example if the
impedance of the DUT is 100 kΩ and the meter's impedance
is also 100 kΩ, the meter will display a value as if the DUT itself
had only a 50 kΩ impedance, which represents a huge error. The
problem was that VTVMs were relatively expensive and beyond the budget
of most amateurs. This article from the June 1994 edition of QST presents
a simple vacuum tube voltmeter VTVM project that allows the user to
measure both resistance and capacitance. Nowadays you can buy a low-end
equivalent with a digital readout for $20 at Sears.
June 1944 QST
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. All copyrights (if any) are hereby acknowledged.
See all available
vintage QST articles.
Resistance and Capacitance Measurements with the V.T.V.M.
Extending the Usefulness of a Versatile Instrument
BY A. D. Mayo Jr., W4CBD
Few garden-variety hams have either the equipment or the inclination
to construct elaborate measuring apparatus for checking either condenser
capacity or high values of leakage resistance. This article describes
a simple and effective way of making such checks by the reactance method,
requiring only an all-purpose v.t.v.m. as a non-loading voltmeter.
Almost any a.c, vacuum-tube voltmeter can be used to indicate
the approximate capacities of small condensers without requiring the
addition of extra parts. All that is necessary is to apply the filament-supply
voltage to the input terminals of the meter, in series with the condenser,
and note the resulting voltmeter reading. The voltmeter can be calibrated
in micromicrofarads and a separate graph prepared to make it direct-reading
for this use.
This method of checking capacity is similar to
that described on page 407 of the 1944 edition of the ARRL Handbook,
which shows how an ordinary 1000-ohms-per-volt a.c. meter can be used
to check capacities down to 0.001 μfd. The principle is similar to
that of the d.c. ohmmeter, except that impedance is measured instead
of resistance. The limitation in the use of this method with an ordinary
voltmeter lies in the fact that an external source of a.c. is required,
as well as a resistor or two and some terminals. Nor does the capacity
range extend quite low enough to check small mica condensers. A vacuum-tube
voltmeter using a voltmeter tube on extended leads overcomes these objections,
since a.c, voltage is available at the tube from the filament supply.
With the very high input resistance of the v.t.v.m., the capacity range
covered can be extended down to 50 μμfd. or less. The only leads
necessary are one to the probe tip and one to the ungrounded side of
When using a 3-megohm input resistor on the probe
tube and a filament voltage in the neighborhood of 6 volts r.m.s., capacities
of from 50 μμfd. to 0.002 μd. will give an indication on the
10-volt scale. The filament voltage will divide between the input resistance
of the meter and the reactance of the unknown condenser. The internal
resistance of the small condenser does not affect the reading unless
the condenser has high leakage or is otherwise defective.
reactance of a 50-μμfd. condenser at 60 cycles is about 32 megohms.
When this reactance is placed in series with the filament supply and
the voltmeter input terminals, about one-tenth of the supply voltage
will appear across the meter.
Fig. 1 - Changes in wiring required
to convert the author's v.t.v.m, (originally described in November,
1943. QST) into a wide-range instrument for measuring capacity and resistance.
RA - 90 megohms.
RB -1 megohm.
- 700,000 ohms.
RD - 10,000
RE - 200-ohm variable.
RF - 3 megohms.
The homemade v.t.v.m. described in the November, 1943,
issue of QST1 has been used in this manner for a
rough check of small capacities, and it has turned out to be a very
The changes required in the original circuit may
be noted by comparing the diagram of Fig. 1 with that shown in the November
article. To use the meter for this purpose it was necessary to change
the ground connection from the center-tap of the filament to one side,
as shown in the circuit diagram. The voltage at the end of the tube
prod was 5.7 volts r.m.s., which gave a reading of 8 volts peak on the
meter scale. A separate calibration curve was made for capacity against
voltage by taking readings on several condensers which were known to
be close to marked capacity.
In testing a handful of new and
junk-box condensers we noted some surprising readings. Out of about
a dozen new mica postage-stamp condensers tested there was one which
showed no capacity at all and another which read so high it was tested
for d.c. resistance and found to have 10 megohms leakage resistance.
Any attempt to use either of these condensers at very high frequencies
would probably have led to a long headache before the trouble could
have been found. On the other hand, one very old condenser of about
1925 vintage, of the type having mica and brass strips clamped together
without any molded Bakelite covering, tested 0.001 μfd. as marked
and did not show any abnormal leakage.
Fig. 2 - Typical resistance calibration curves
for the v.t.v.m.
It is apparent that, in order to check a condenser thoroughly, it should
be tested for leakage resistance as well as capacity. The v.t.v.m. also
lends itself very well to conversion into an ohmmeter for reading extremely
high values of resistance. For this purpose the d.c. plate supply is
applied to the d.c. voltmeter section through the unknown resistor in
much the same manner as that previously described for measuring capacity
with the a.c. section.
Front view of the v.t.v.m. described in the November issue of QST,
as modified with pin jacks added on the panel for making connections
for resistance and capacitance measurements.
Fig. 3 - Calibrating resistors for extending the range of the v.t.v.m.
are constructed by making pencil marks on an insulating strip, as
described in the text.
The internal plate-supply voltage of
the instrument runs 170 volts above ground. Of this, 100 volts is tapped
off on a voltage divider and applied to the 100-volt input terminals
in series with the resistance to be measured. This scale reads from
1 megohm to 100 megohms and is called the LO-OHM scale. To read higher
values of resistance the 100-volt supply is applied to the 100-volt
scale through the unknown resistor, with an additional resistor of 90
megohms added in series to limit the maximum voltage applied across
the meter input to 10 volts. This scale is labeled HI-OHMS and it reads
from 1 megohm to 1000 megohms.
The HI- and LO-OHM scales
worked so well that two additional ones were added (XLO and XXLO in
Fig. 2). The XLO scale is obtained in a manner similar to that used
in the higher resistance ranges, but the input resistance of the meter
had to be reduced by connecting in an additional switch point, shunted
with a 12,000-ohm resistor, as shown in Fig. 1. Since some current was
required to operate this section, a battery was added as the easiest
way out. The XXLO scale is made up by using the milliammeter in a regular
ohmmeter circuit with another 1 1/2-volt battery. It is important that
the power be turned off in the meter before using the latter range,
since the meter is in the "B"+ side and is above ground by about 170
volts. Finally, a terminal was added to the panel to supply one side
of the filament voltage.
In using the 1000-megohm range it is
important to keep down leakage in the test prod leads if they are used.
The leakage through many insulators will be less than 1000 megohms.
Newsprint paper on a damp day will show a reading if the prods are pressed
on it a couple of inches apart. It is best to use a couple of bare wires
pushed in the HI-OHM terminals with the condenser connected to them
as close to the terminals as possible.
It turned out to be fairly easy to calibrate the meter at the
high ranges. Perhaps the accuracy of the method used is less than that
obtained on the best commercial bridges, but it is sufficient for our
In constructing the calibrating resistors, a piece
of fiber was drilled with three holes in a row and machine screws and
washers put in the holes, as shown in Fig. 3. Pencil lines were drawn
from under the washers on to the next screw, making a pair of 1920-model
pencil grid leaks in series. A 10-megohm resistor was obtained and one
of the grid-leak sections adjusted to the same resistance as measured
by the meter scale. Then the other grid-leak section was adjusted to
the same resistance. Since the total resistance of both grid-leak sections
is 20 megohms, the meter deflection for 20 megohms was recorded. Then
each section was made 20 megohms, making a total of 40 megohms. Thus,
by doubling resistance each time, the calibration was carried on up
to 1200 megohms.
The resistances were adjusted by marking on
a little pencil lead or erasing a little of it until the resistance
was correct. The meter was calibrated at the lower ranges by plotting
points from resistors which were measured by another ohmmeter of good
The original meter as shown in November QST did not
have a case, but after its conversion to read ohms and whatnot it was
mounted in a wooden case and some additional terminals put on the panel,
as shown in the photograph. It was a case of something that started
out to be a voltmeter and ended up being a meter to read nearly everything
else as well.
For something that was born on the kitchen table
from parts out of the junk box, this thing turned out to be a good little
1: Mayo. "A
V.T. Voltmeter for A.C. and D.C .," QST. November. 1943. p. 36.