April 1947 Radio News
[Table of Contents]
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
While not many people are likely to build this R-C bridge
circuit with vacuum tubes, the article has a good basic description of operation of any calibrated bridge
circuit used to measure an unknown value. Interestingly, a 'magic eye' or 'cat's eye' tube is used in
lieu of a meter movement to give a visual indication of an open, short, intermittence, poor power factor,
and low 'Q', as well as when the selected switch position correctly identifies the value of the resistor
or capacitor under test.
Note: μfd = μF, μμfd = pF
Build This Radioman's R-C Bridge
By Rufus P. Turner, W1AY
Consulting Eng., Radio News
Fig. 1 - Front panel view of home-built test instrument. For correct identification
of various controls refer to Fig. 3.
Complete construction details for a resistance capacitance bridge. This wide range instrument will
find many applications in the service shop.
Here is a compact, wide-range resistance-capacitance bridge of marked usefulness, that can be built
by any serviceman. Although it is sensitive, stable, and rugged, this instrument is inexpensive. Its
attractive ranges will catch the eye of the amateur experimenter as well as the repairman. This small
bridge is completely self-contained, as Fig. 1 shows, and it is fully a.c. operated. There are no tricks
either in its operation or calibration, this instrument being entirely conventional and fool-proof and
direct reading. We believe many radiomen will want to build it.
There are three capacitance ranges: 1 μμfd. to 0.1 μfd., 0.0001 μfd., to 10 μfd.,
and 0.01 μfd. to 1000 μfd. There likewise are three resistance ranges: 1/10 ohm to 10,000 ohms,
100 ohms to 10 megohms, and 10,000 ohms to 1000 megohms.
The main dial has a single set of graduations from which both capacitance and resistance are read.
The range switch, in its various capacitance positions multiplies the main dial readings by 10, 1000,
and 100,000 and in "resistance positions multiplies the readings by 1, 1000, or 100,000. This arrangement
provides the maximum simplicity and directness in operation of the bridge.
The exceptionally wide range of this bridge enables the operator to measure a variety of capacitances
ranging all the way from small circuit values (down to 1 μμfd.) to the large capacitances of motor
starter capacitors. Also, resistance measurements may be made from values considerably lower than 1
ohm to the high leakage resistances (up to 1000 megohms) encountered, say, in tubular capacitors.
Fig. 2 - Schematic diagram of RC bridge. Test instrument covers capacitance ranges
from 1 μμfd. to 1000 μfd. and resistance ranges from 0.1 ohm to 1000 megohms.
The compact RC bridge has several special features worth mentioning. These contribute to its versatility
Sensitivity. Very high sensitivity is provided by use of (1) a high-gain 6SJ7 bridge
amplifier and (2) a high signal voltage. The signal voltage is approximately 50 volts at 60 cycles,
supplied by the secondary of the signal transformer, T1 (See Fig. 2).
The sensitivity is under full control, being variable from zero to maximum by means of the amplifier
gain control, R4. Sensitivity may be made so high that an almost imperceptible adjustment
of the main dial will show up in the null detector.
Provision for External Signal Input. Transformer T1 delivers a 60-cycle
signal voltage to the bridge circuit. This will be satisfactory for all ordinary purposes. However,
some operators may prefer to use a signal of some other frequency, such as 400 or 1000 cycles, for occasional
experimental measurements. For this purpose, an external signal jack, J2, is provided in
the circuit. When a plug, connected to an external audio oscillator, is inserted into J2
the jack contacts open and automatically disconnect the secondary of the 60-cycle transformer, T1
from the bridge circuit.
Provision for External Null Indicator. The self-contained bridge null detector is
a 6E5 magic eye tube which, driven by the 6SJ7 amplifier, makes a sensitive indicator. However, some
operators may prefer to use an external null indicator (such as an oscilloscope, a.c. vacuum tube voltmeter,
or headphones) in occasional experimental bridge measurements. Jack J1 permits the quick
connection of such an external indicator. Neither the 6SJ7 amplifier nor the 6E5 indicator need be placed
out of operation when an external indicator is plugged in.
Power Supply. A regulated a.c. power supply is employed. Our experience with various
small bridges convinces us that this feature is most desirable. A line operated (a.c.-d.c.) type of
power supply would cause one of the measuring terminals to be "hot," especially on the resistance ranges,
thereby creating a shock hazard. Amplifier and null detector operation tends to be unstable with an
unregulated power supply.
Voltage regulation is obtained by means of the two gaseous regulator tubes connected in series, as
shown in Fig. 2. The OC3/VR105 and OD3/VR150 together give a regulated d.c. voltage of 255 for the amplifier
and indicator tubes. The regulator limiting resistor, R10, with filter capacitors C10
and C11, provides sufficient filtration without having to use a filter choke.
The power supply components are small in size and accordingly do not require a great deal of chassis
Ease of Calibration. The builder has to calibrate only one resistance range on the
main dial. If the range switching resistors and capacitors are selected with care as to their values,
the other ranges then automatically "fall in line."
The bridge calibration can be made easily with a number of hand-picked resistors, as will be explained
in detail later in this article. This is a decided advantage, since most radiomen find it easier to
obtain accurately measured resistors than a set of precision capacitors.
Fig. 3 - Front panel layout showing placement of the various operating controls.
Power Factor and Q Indication. Capacitor manufacturers have not given servicemen
much definite, practical information in the way of limiting power factor and Q values. For this reason,
repairmen are inclined to interpret power factor percentages in a variety of ways. The circuit of the
bridge shown here has not been complicated, therefore, by the addition of a separate power factor control.
But that does not mean that a capacitor with a bad power factor or Q cannot be spotted.
Normally, the 6E5 shadow pattern opens quickly and cleanly at null. However, when a capacitor under
test has high power factor or low Q, the opening of the eye is not clean - the width of "fuzz" on each
side of the shadow being proportional to the power factor. A very bad capacitor will produce a narrow
eye opening with a great deal of fuzz on each side. There is no blurring at all, but a sharp line between
the dark and bright portions of the eye pattern, when a good capacitor is connected to the bridge.
Indication of Shorts, Opens, and Intermittents. On any of the resistance or capacitance
ranges of the bridge, the 6E5 indicator will open wide at the extreme left-hand (zero) end of the dial
for shorts, and at the extreme right-hand (full-scale) end of the dial for opens. Intermittents are
indicated by a flickering of the eye, when the dial is set for null, especially if the capacitor or
resistor is rapped sharply.
The complete bridge schematic appears in Fig. 2, and an examination of this drawing will show the
measuring circuit itself to be of simple, conventional design. Capacitors C1, C2
and C3 are employed as standards for capacitance measurement. Resistors R1, R2
and R3 are standards for the three resistance ranges. The bridge balance potentiometer, RA,
is a 3000-ohm wirewound volume control-type component. The rather high signal voltage does not cause
bad heating in this potentiometer.
The 2-pole, 6-position range switch, S1 automatically shifts the standards and measuring
terminals from one bridge arm to the other when switching from resistance to capacitance functions.
This enables the main dial to read in the same direction for both resistance and capacitance.
The 10 ohm, 10,000 ohm, and 1 megohm standard resistors, R1, R2 and R3,
must be hand picked for exact values. An accurately calibrated ohmmeter may be used for this purpose,
if no other instrument is available. The author found that not many resistors had to be checked at the
store in order to obtain three satisfactory standards. If the reader desires, precision instrument resistors
may be employed as standards and the need for hand picking thereby eliminated.
The 0.0001, 0.01, and 1 μfd. standard capacitors, C1, C2, and C3
likewise must be selected for exact values. It is best to order directly from the supplier or manufacturer
capacitors having a tolerance of 1% or better. If an exact 1 μfd. value cannot be obtained for C3,
it is advisable to accept a somewhat lower capacitance and to connect enough mica capacitors in parallel
with it to build its capacitance up to the required 1 μfd.
The accuracy of the bridge depends a great deal upon the accuracy of these resistance and capacitance
standards, and the individual builder will do well to select them with the greatest possible care.
The signal transformer, T1, is a 2-to-1 ratio interstage audio transformer with its secondary
connected to the 115-volt line. The proper primary and secondary connections are indicated by standard
lettering in Fig. 2. This transformer provides a bridge signal of a little more than 50 volts; and on
long test runs, no large amount of heating was detected either in the signal transformer or in the bridge
potentiometer. Any other type of transformer may be used, provided it will deliver 50 to 60 volts to
the bridge circuit.
The remainder of the circuit is entirely straightforward, consisting of a conventional voltage-regulated
power supply, previously described, and a standard 6SJ7 amplifier and 6E5 indicator.
With the arrangement shown, the eye opens up to indicate null points.
Mechanical and Electrical Construction
Fig. 4 - Rear view of resistance-capacitance bridge. While the two OD3/VR150 tubes
are visible, the 6SK7 and 5W4 tubes cannot be seen in this photograph. They are mounted directly behind
the two voltage regulator tubes.
Fig. 5 - Photograph shows placement of various under-chassis components.
The bridge is built on a metal chassis, 12" long, 6" wide, and 2" high, and a metal front panel,
14" long and 7" high. It is enclosed in a metal cabinet. The external and internal appearance of the
instrument and the arrangement of its parts may be seen in Figs. 1, 3, 4, and 5.
The main dial has a 3 3/4" diameter metal plate to which has been cemented a disc of thin white cardboard
on which the special scale is hand drawn. After completing the calibration and drawing in the graduations
with black India ink, this dial plate is covered with a matching disc of transparent celluloid or other
plastic to prevent soiling.
The range switch plate (see Fig. 1) is also made of thin white cardboard which, after lettering,
has been cemented to the front panel and covered with transparent celluloid. Mark this plate Rx1, Rx1000,
Rx100,000, Cx10, Cx1000, and Cx100,000, corresponding to the switch positions shown in Fig. 2 and listed
in detail in the range data in that drawing.
All of the standards, except the 1 μfd. capacitor, C3, are connected directly between
contacts of the range switch, S1, and are supported by this switch. C3 is mounted
on the chassis where it may be seen directly behind the range switch in Fig. 4. Note the two parallel-connected
mica capacitors on top of, and shunting C3.
The two voltage regulator tubes may be seen in the rear of the chassis in Fig. 4. The 5W4 rectifier
tube is directly behind the left hand regulator tube.
The 3000 ohm-bridge potentiometer is mounted in the center of the front panel and is supported back
from the panel on a Mallory RB 249 metal bracket.
The 6E5 magic-eye indicator tube is mounted from the back of the front panel, where it may be seen
directly above the power transformer in Fig. 4. An Amphenol 58-MEA6 magic eye assembly holds this tube
in position. The 5 leads from the socket of this assembly pass through a grommet-lined hole in the chassis
for connections to points underneath. The 1 megohm resistor, R9 is enclosed in the tube socket
shell of the magic eye assembly. The magic eye escutcheon, also supplied with the assembly, may be seen
on the front panel in Figs. 1 and 3.
The voltage regulator limiting resistor, R10, is mounted between the two voltage regulator
tube sockets, under the chassis (see Fig. 5). The "On-Off" switch, S2; sensitivity control
R4, pilot light bracket, and jacks J1 and J2 also are mounted under
the chassis and may be seen directly back of the front panel in Fig. 5.
By employing two Johnson type 44 ceramic feed-through units as the "measuring terminals" (see Figs.
1 and 4), these terminals are kept out of contact with the metal front panel. Another one of these feed-through
terminals passes through the chassis to connect the bridge circuit with coupling capacitor C4,
which is under the chassis near the 6SJ7 socket. This particular feed-through terminal is seen directly
under the bridge potentiometer in Fig. 4.
Connections between the measuring terminals and the range switch, and between the range switch and
potentiometer RA are made with bare bus wire. These rigid connectors must be run as directly
and over as short a path as possible, in order to minimize circuit capacitances.
It is advisable to shield the lead running between sensitivity control R4 and the 6SJ7
control grid and to connect this shield to chassis at both ends. This is very important in order to
eliminate extraneous pickup, especially if this lead runs for more than about 1 inch. Fig. 5 shows this
procedure to have been followed in the author's bridge. Note that the lead from the sensitivity control
has been enclosed in shield braid.
A single return point (ground) must be employed in the 6SJ7 stage; and J1, R4,
R5, C5, and C6 must be connected to this point. The most logical point
is the No.1 contact of the 6SJ7 socket, since this contact also grounds the tube shell. The common return
point is connected to chassis by means of a short, direct lead.
Signal input jack J, must be insulated from the chassis and panel. It may be seen supported by a
bakelite washer in a large clearance hole in the panel, in Fig. 1.
Adjustment and Calibration
After the wiring has been checked; remove the 6SJ7 and 6E5 tubes from their sockets, open the lead
between filter capacitor C11 and the OD3/VR150 tube at the point marked "X" in Fig. 2, and
insert a 0-50 d.c. milliammeter in this line. The negative terminal of the meter must be connected to
the regulator tube. Set the slider on R10 at a trial point about 1/4 of the way from the
C11 end of this resistor, insert the line plug into a 115-volt a.c. outlet, and throw switch
S2 to its "On" position. Note the milliammeter reading. Again set the slider on R10
carefully to give a milliammeter deflection of exactly 30 ma. and tighten the slider at this point.
The voltage regulator now has been adjusted; and the milliammeter may be removed from the circuit, the
connection "X" restored, and the 6SJ7 and 6E5 tubes returned to their sockets.
The bridge calibration is made with an assortment of accurately known resistors - with range switch
S1 in its No. 5 (Rx1000) position. For the calibration, obtain resistors having as many as
possible of the following values: 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500,
3000, 3500, 4000, 4500, 5000, 5500, 6000, 6500, 7000, 7500, 8000, 8500, 9000, 9500, 10,000, 11,000,
12,000, 13,000, 14,000, 15,000, 16,000, 17,000, 18,000, 19,000, 20,000, 25,000, 30,000, 35,000, 40,000,
45,000, 50,000, 55,000, 60,000, 65,000, 70,000, 80,000, 90,000, 100,000, 150,000, 200,000, 250,000,
300,000, 400,000, 500,000, 1 megohm, and 10 megohms. The reader will see that a number of these values
may be secured by connecting several of the lower values in series. If one or more decade resistance
boxes are available, they may be used very satisfactorily to obtain the above values. Also, a series
of volume control-type resistors might be set to the above calibration values, provided some means is
available for measuring the control-settings.
The following calibration procedure must be followed: (1) Connect bridge to 115-volt a.c. supply
and throw switch S2 to "On." (2) After tubes have heated and 6E5 indicator glows brightly,
throw range switch to position 5 (Rx1000). (3) Connect 100-ohm resistor to measuring terminals. (4)
Set sensitivity control so that 6E5 eye just closes, and adjust main dial (RA) for null,
as indicated by sharp, complete opening of 6E5 shadow. This null should be obtained at left-hand end
of main dial. If it occurs at right-hand end of dial, reverse the two outside connections to potentiometer
RA. (5) Adjust sensitivity control R4 for sharpest and clearest opening of eye
pattern. (6) Mark this dial setting 0.1. (7) Repeat process with 200 ohm resistor, marking corresponding
null point 0.2 on main dial. (8) Repeat with the various resistors listed earlier, marking dial points
according to the following system: 100 ohms = 0.1, 200 ohms = 0.2, 1000 ohms = 1, 1500 ohms = 1.5, 10,000
= 10, 1 megohm = 1000, 10 megohms = 10,000, etc. The reading should be marked in hundreds up to and
including 900, then from 1000 through 9500 divisions should be marked for each 500 ohms, from 10,000
to 20,000 should be marked in steps of 1000. From 25,000 through 65,000 markings should be at intervals
of 5000: From 70,000 the readings should be marked at: 80,000; 90,000; 100,000; 150,000; 200,000; 250,000;
300,000; 400,000; 500,000; 1 megohm and 10 megohms.
It is not possible to calibrate a few points and then to divide the dial scale by hand to obtain
the rest of the graduations since response of potentiometer RA is not linear. It becomes
necessary therefore to calibrate as many individual points as practicable. If the reader has the required
resistors, he is advised to calibrate even more points between 500,000 ohms and 10 megohms than we have
After the calibration is completed, the dial scale may be drawn-in permanently with black India ink
and covered with transparent celluloid or other plastic, to prevent soiling and marring.
Use of the bridge is straightforward and rapid. (1) Switch-on bridge power. (2) Connect capacitor
or resistor to be tested to the "Measuring Terminals." (3) Set range switch S1 to trial resistance
or capacitance range. (4) Set sensitivity control R4 so that 6E5 eye just closes, and adjust
main dial for null, as indicated by wide, clear opening of 6E5 eye at some point along dial range. (5)
Adjust sensitivity control R4 for clearest and least "jumpy" operation of eye. (6) Read capacitance
or resistance value on main dial and multiply dial reading by multiplier indicated by range switch setting.
(7) If null is not found, switch to next R or C range, as case may be, and repeat adjustments. (8) If
null occurs in upper fifth of main dial, where divisions are crowded and comparatively difficult to
read, switch to next highest range to obtain null in wide open (more accurate to read) portion of dial.
A capacitor with high power factor or low Q may be discovered by a fuzzy appearance of the edges
of the 6E5 shadow pattern at null. If no null point is found (either with a resistor or capacitor under
test) and the eye opens sharply at the extreme right-hand (full-scale) setting of the dial, an open
circuit is indicated. If no null point is found and the eye opens sharply at the extreme left-hand
(zero) setting of the dial, a short circuit is indicated. An intermittent is shown by a flickering
of the eye at null, especially if the capacitor or resistor under test is rapped sharply.
The operator will observe that by advancing sensitivity control R, the bridge may be made so sensitive
that the eye indicator responds to individual wire turns of potentiometer RA as the main dial is adjusted!
If it is desired to employ some other bridge signal frequency than 60 cycles, obtain the desired
frequency from an audio oscillator or combination of oscillator and audio amplifier, plugging the output
of the external signal source into jack J2. When the plug is inserted into this jack, the internal 60-cycle
signal source will be removed from the bridge automatically.
If it is desired to use some other null detector than the magic eye tube, plug the external detector
into jack J. This connection will not disturb the bridge operation and no changes need be made. The
bridge sensitivity control will have no effect upon the external detector; however, this will be no
disadvantage, since, in most cases, external null detectors have gain controls or input voltage adjustments
of their own.
Posted August 23, 2016