NATIONAL BUREAU OF STANDARDS REPORT

4436

21 P-1 9/55

Sensitivity and Current Leakage

of

l\venty-one Cold Cathode Electron Tubes, Type WL-759

and

Thirty-one Photoelectric Tubes, Type WL-919

By

Photometry and Colorimetry Section

Optics and Metrology Division

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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specialized research, development, and engineering in the field indicated by its title. A brief

description of the activities, and of the resultant reports and publications, appears on the inside

of the back cover of this report.

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Radio Standards. High Frequency Standards. Microwave Standards. • Office of Basic Instrumentation • Office of Weights and Measures
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

0201-20 -2301 4436

December 7, 1955 21P-19/55

Sensitivity and Current Leakage

of

Twenty-one Cold Cathode Electron Tubes, Type WL-759

and

Thirty-one Photoelectric Tubes, Type WL-919

By

Photometry and Colorimetry Section

Optics and Metrology Division

For

Instrument Division

Weather Bureau

Department of Commerce

Washington 25, D. C.

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, repr Approved for public release by the

unless permission iso director of the National Institute of

25,d.c. Suchperm Standards and Technology (NIST)

cally prepared if tht On October 9, 2015

i part, is prohibited

idards, Washington

)rt has been specifiort for its own use,

4436

21 P-1 9/55

Sensitivity and Current Leakage

of

Ttoenty-one Cold-Cathode Electron Tubes, Type WL-759

and

Thirty-one Photoelectric Tubes, Type WL-919

Manufactured by

Westinghouse Electric Corporation

Elmira, New York

Tested for

Instrument Division

Weather Bureau

Department of Commerce

Washington 25, D. C.

1, SCOPE

The National Bureau of Standards was requested by the Instrument Division, Weather Bureau, by letter of June 15, 1954, to test

17 cold-cathode electron tubes, type WL-759 (also called trigger

tubes1*), and 16 photoelectric tubes, type WL-919, to determine their

suitability for use in transmissometer pulse-amplifier units.

Verbal requests were made later to include 4 additional trigger

tubes and 15 additional photoelectric tubes. This report gives the

results of tests of sensitivity, of plate-voltage plateau, and of

current leakage for the 21 trigger tubes, and the results of tests

of sensitivity to incident radiant flux, of dark current, and of

current leakage for the 31 photoelectric tubes. All tubes were

manufactured by the Westinghouse Electric Corporation.

2. INTRODUCTION

2.1 Analysis of Trigger-Tube Operation.

The WL-759 tube is a cold-cathode, starter-anode, gas triode

tube and is used in the pulse amplifier to generate pulses at a rate proportional to the current of the phototube. The minimum

starter-anode voltage at which the discharge through the tube will

be triggered with a plate voltage of 255 volts is defined as the

firing voltage. To maintain a constant relation between photoelectric current and pulse rate, it is essential that the firing

voltage be independent of the voltage applied to the plate of the

tube over the range of plate voltages encountered in service.
r v

.. I .
The range of plate voltages over which the firing voltage is constant is defined as the plate-voltage plateau. The sensitivity

of a trigger tube is a function of the difference between the

firing voltage and the voltage of the starter anode at the moment

the arc discharge within the tube is extinguished. If the

relation between the phototube current and firing voltage varies

slightly from pulse to pulse, the tube will exhibit varying

degrees of stability in operation. If there is leakage over the

internal or external surfaces of the envelope of the trigger tube,

or if there is a continuous gaseous conduction between the

starter anode and the other electrodes, the pulse rate will not

be proportional to the incident flux at low flux levels since

(assuming a perfect phototube)

x c = Ip - I L = C (Ef - Ed )/T (la)

or Ic = Ip - I L = C(Ef

- Ed )r (lb)

where

I c is the current charging the starter-anode circuit,

Ip is the photoelectric current,

1^ is the leakage current of the trigger tube generated by

the phototube (leakage current from grid to ground or to cathode

is considered positive and leakage current from anode to grid is

negative),

C is the capacitance of the starter-anode circuit (the

capacitance of the charging capacitor plus the internal capacitance of the tubes and the wiring T,

Ef is the voltage at which the starter anode triggers the tube,

is the voltage of the starter anode at the time the discharge is extinguished,

T is the time between pulses in seconds, and

r is the pulse rate in pulses per second.

Equations la and lb may be rewritten as

I c = C/(St

T) (2)

and I c = Cr/Sj

where Sj is defined as the sensitivity of the trigger tube.
.

.
-3-

2.2 Analysis of Phototube Operation.

The type WL-919 phototube is a high-vacuum photoelectric

tube with a type S-l spectral response.

If F is the light flux in lumens incident on the phototube,

then I p = S

p

F

where S

p

is the sensitivity of the phototube in microamperes per

lumen.

Because of leakage over the surfaces of the phototube and

thermionic emission, some current may flow across the tube even

when no radiant flux is falling on the photosensitive surface of

the tube and pulses will be generated by the pulse-amplifier unit.

This current is known as the "dark current" of the tube. If

there is leakage of the photoelectric current to ground, then the

pulse rate of the pulse-amplifier unit will be reduced. If the

phototube is not perfect, equation lb must be written as h

=

!p + (I D

- IP - C(Ef - E

d )r (3)

where Iq is the true dark current and If, is the leakage current to

ground of the phototube. These two currents cannot be measured

independently; only their difference, net dark current, can be

determined. The net dark current Id is defined as

iD = Id - if.

then I c = I p

+ I D

- I L

(4)

Note that Ig may be either positive or negative, depending upon

the relative values of Iq and 1^. In general, Iq is positive.

3. TEST PROCEDURE

3.1 Choice of Test Methods.

The measurement of some of the tube currents characteristic

of the tubes under test requires the use of sensitive galvanometers or electrometers which are not generally available.

Therefore, the procedures outlined below were developed to utilize

available transmissometer equipment to determine the performance

I

of the tubes under service conditions rather than to obtain absolute measurements of the tube characteristics. In general,

both the dark current of the phototube and the leakage current of

the trigger tube are not constant throughout the charging cycle

but will vary with the potential of the starter anode. Therefore,

a check of the effect of these currents under operating conditions

is often more useful than direct measurements under other

conditions,

3.2 Tests of Type WL-759 Trigger Tubes.

3.2.1 Sensitivity. In determining trigger-tube sensitivity use was made of a pulse-amplifier test unit and a portable transmissometer indicator. The test unit nas a pulse-amplifier unit

(with a 22-microfarad sensitivity capacitor) in one end and a lamp whose intensity can be continuously varied by a variable autotransformer at the opposite end. Power for the pulse-amplifier

unit is supplied by the portable indicator. The portable indicator

also measures the rate of the pulses generated by the pulse amplifier. The lamp intensity was adjusted to obtain a pulse rate which

produced a convenient reading on the portable indicator when the

standard trigger tube was installed in the pulse amplifier. Each

test trigger tube was then substituted in turn for the standard

tube; the ratio of the reading obtained to the reading obtained

with the standard trigger tube is defined as the relative sensitivity. The standard trigger tube was frequently reinstalled as a check on the stability of the system.

Note” Instead of using the lamp and phototube as a current

source, a 600-megohm resistor can be connected between the B+

voltage of the portable indicator and the trigger-tube grid cap.

The sensitivity can then be obtained in the same manner as outlined

in the preceding paragraph.

3.2.2 Firing Voltage. To perform this test and the tests described

in the following paragraph, use was made of the special tube tester

reported in NBS Report No. 2567, "Development of a Special Test Set

for Transraissometer Use," dated June 22, 1953. The voltage applied

to the plate of the tube was adjusted to 255 volts. Then the

minimum starter-anode voltage necessary to trigger the tube

(initiate intermittent flashes of the neon indicating lamp of the

test set) was determined.

3.2.3, Plate-Voltage Plateau. The plate-voltage plateau was found by

determining the range of plate voltages for which the firing voltage

(as determined above) was just sufficient to trigger the tube

regularly.
*

t

, fi . ; Of #
3.2.4 Leakage. A measure of the effects of leakage in the tube

was obtained by using a pulse-amplifier test unit and a pulsecounting circuit. The tube under test and the "standard" phototube were installed in the test unit and the unit was covered

so that no light was admitted. The pulse rate was then determined.

If a tube did not generate pulses when used with the standard

phototube,, the photoelectric current was increased by allowing a small amount of light to fall on the phototube or by replacing

this phototube with a phototube having a higher dark current

until a current sufficient to make the trigger tube generate

regular pulses was obtained. The leakage currents of trigger tubes

which generate regular pulses at pulse rates in the range of one pulse in 30 seconds to one pulse in 60 seconds from the dark current

of this phototube (0.00010 microampere) are rated as very satisfactory (V). The leakage current of the tubes which generate

regular pulses from the dark current of the standard phototube at

a rate slower than that of the very satisfactory tubes is rated as satisfactory (S). The leakage currents of the tubes which require

a current greater than the dark current of the phototube in order to

generate regular pulses are rated as fair (F) or unsatisfactory (U)

depending upon the current required.

3.2.5 Stability. Tube stability is a subjective estimate of the

constancy of the time interval between pulses (and hence the constancy of Ef) from moment to moment during the sensitivity check.

Each tube was rated as being very stable (V), stable (S) f or unstable (U) on the basis of the stability of the portableindicator meter indication.

3.2.6 Anomalous Behavior. Trigger tubes were checked for anomalous

behavior by varying light incident on the phototube, thereby varying the phototube current, so that, the pulse rate of the trigger tube

was varied continuously from about 60 pulses per second to about one

pulse in 10 seconds. The presence of any regions in which the tube

failed to operate was noted.

3.3 Tests of Type WL-919 Phototubes.

3.3.1 Sensitivity. The relative sensitivity of the phototubes

was determined in a manner similar to that used in determining

the relative sensitivity of the trigger tubes except that the

standard phototube was used in combination with the phototubes

under test. Because the conditions of illumination on the photosensitive surface and the color temperature of the lamp in the

pulse-amplifier test box did not duplicate service conditions, the

relative sensitivities determined in this manner will differ somewhat from the relative sensitivites in service. (A new test unit

which duplicates service conditions of illumination has been

designed and constructed.)

3.3.2 Dark Current, The dark-current pulse interval was measured

using the arrangement used for the determination of trigger-tube

leakage except that the standard trigger tube was used in the unit

in combination with the test phototubes. The average time in

seconds between pulses was taken as the dark-current pulse interval.

3.3.3 Leakage. If more than 500 seconds elapsed without a pulse

being generated by the dark current, sufficient light was applied

to the tube to produce regularly spaced pulses at a minimum rate.

The standard phototube was then substituted for the test phototube

and, without changing the intensity of the light, the pulse rate

with this tube was then determined. This pulse rate, when corrected

for the dark current of the standard phototube, is proportional to

the leakage current of the test phototube.

4. REDUCTION OF TEST DATA

The measurements obtained by the test procedure outlined above

are all dependent upon the characteristics of the standard phototube and the standard trigger tube. Since, if the minimum acceptable relative sensitivities are known, the relative sensitivity is as useful an index of tube quality as absolute sensitivity, relative

sensitivities were not reduced. Measurements of leakage and dark

currents are, however, more useful if the effect of the leakage

of the standard trigger tube is removed from the dark-current

measurements and the effect of the dark current of the standard

phototube is removed from the leakage-current measurements. This

was done in the following manner.

If the phototube is dark,

I D - I L = CST

r (5)

Note that the product CSTis the charge, in microcoulombs, removed

from the charging capacitor during each discharge of the trigger

tube.

The charge per pulse of the standard trigger tube, installed in the receiver test unit, was determined from the relation

x c = J

p

+ X D

" t L = csTr

by direct measurement of Ip and by computing the corresponding

pulse rate, r, from the reading of the portable indicator. The
* \
current I p

was made large enough so that Iq and I. are negligible

in comparison to it. This measurement was made by connecting a photoelectric galvanometer with an Ayrton shunt into the anode

circuit of the phototube. Careful attention to electrostatic

shielding and the use of a reversing key was required. (An alternative method to generate the current is the use of a suitably

high resistor, about 600 megohms, in place of the phototube.)

A charge of 0.0035 microcoulomb per pulse was passed by the

standard trigger tube when a 22 micromicrofarad capacitor was used

for the charging capacitor.

Neither the phototube dark current or the trigger -tube leakage current could be measured with the galvanometer connected

into the B+ lead of the pulse amplifier because at these slow

pulse rates the discharges of the trigger tube and the high

galvanometer sensitivity required produced large disturbances in

the galvanometer system. Instead, the dark current of the standard phototube was measured by connecting the photoelectric

galvanometer and the phototube in series with a 90-volt battery.

The minimum voltage applied to the phototube during the charging

cycle is about 90 volts. The photoelectric galvanometer with

with the Ayrton shunt in the XI position has a sensitivity of

1.4 x 10”5 microampere per millimeter. This is adequate for

dark-current measurement of the standard phototube. A dark

current of I.O3 x 10°^ microampere was measured.

The dark-current pulse rate when the standard phototube was

used with the standard trigger tube was one pulse in 500 seconds.

Therefore the leakage current of the standard trigger tube may

then be computed from equation 5 as follows:

1.03 x 10

-4

- II = 0.0035 x 1/500 (7)

-4

and = 0.9^ x 10

As a check, the trigger tube was installed in the pulse

amplifier without the phototube. The charging capacitor was connected to the starter anode. The starter anode was then connected

to the 255-volt B+ supply through a chain of high-value resistors.

The resistance of the chain was adjusted until the current through

it was so nearly equal to the leakage current of the tube that

the pulse rate was very slow (1 pulse in about 5 minutes, and

erratic). The current flowing was then only slightly greater than

the leakage current of the tube. A resistance of 4 x 10^ ohms
*

'
-8 - t

I

was required for the standard trigger tube. As the average voltage drop across the resistor chain was about 120 volts, the current

flowing was approximately 0 o 3 x 10“4 microampere.

That these two determinations of leakage current differ by a factor of 3 is not unexpected since the operating conditions differ

considerably and the trigger tubes are very sensitive to variations in the star ter -an ode circuit. In reducing the data, thg

leakage current obtained under service conditions, 0.9^ x 10“

microampere, was used as the leakage current of the standard

trigger tube.

Knowing the leakage current and the sensitivity of the standard trigger tube, the dark currents of all test phototubes having

dark currents greater than the leakage current of this trigger

tube were determined by using equation 5 to compute the dark currents from the dark-current pulse rates of these tubes.

Similarly, the leakage currents of the trigger tubes having

leakage currents less than the dark current of the standard

phototube were determined from the pulse rates obtained from the

dark-current pulse rate of this phototube when used with the test

trigger tube. As seen from equation 5, this requires knowledge of

the absolute sensitivity of the test trigger tube. This sensitivity is the product of the absolute sensitivity of the standard

trigger tube and the relative sensitivity of the test trigger

tube, both of which had been determined.

Since the ultimate purpose of the transmissometer set is to

obtain a measure of the visual range, the effect on this measure

of the leakage current of the trigger tube or the dark current

of the phototube must be considered. The leakage currents of

trigger tubes and the dark currents of phototubes can be converted

to transmission errors by dividing each by the current necessary

to produce a 100% transmission reading.

Since I c = Ip + I D

- 1^ , (4)

if Ip is the photoelectric current which flows when the transmission is unity, then

Ic^o = V1 ®

+ *D Ao - I L Ao

t’= t + At0

- At L

or

where t® is the indicated transmission

t is the true transmission and

Atg and Atj^ are the errors produced by the dark current

and leakage current respectively.

For a 750-foot baseline transmissometer with tubes of average

sensitivity, I 0 is approximately 0.2 microampere. The incremental transmission errors, Atp and AtL ® were computed from the

dark current of each phototube and the leakage current of each

trigger tube, assuming a current of 0,2 microampere for I 0

.

5„ TEST RESULTS

The data obtained are summarized in table I, Test Results

for Type WL-759 Trigger Tubes, and in table II, Test Results for

Type WL-919 Phototubes. For the purpose of identification the

tube number of each tuoe as listed in the first column of the

tables has been engraved into the base of the corresponding tube.

6 0 DISCUSSION

It is difficult to give specific criteria which can be applied

by rote in determining whether a particular trigger tube or phototube is satisfactory for transmissometer use. The effect of

incremental errors in the transmission measurement resulting from

the leakage current of the trigger tube and the dark current of

the phototube upon the intended application of the transmissometer

should be considered.

6.1 Effects of Incremental Transmission Errors.

The maximum permissible values of leakage and dark currents

can be determined by computing the effect of the transmission

errors produced by these currents on the indicated visual range.

Figure 1 is a plot of indicated visual range against the true

visual range when incremental errors in transmission measurements

are superimposed on transmission readings. The family of curves

for the plus-increments is applicable to most phototube dark

currents and to the trigger-tube leakage current from the plate

to starter anode. This condition is a relatively rare occurrence.

The minus-increments are comparable to the usual starter anode-toground leakage currents of the trigger tubes. (Errors arising

from incorrect readings or instrument inaccuracy may, of course,

be of either sign.) Note that even very small increments of
(
=10=

either sign will cause large errors in the indicated visual range

in periods of very clear weather or in very dense fogs. The

effect of these errors will be considered in determining the

maximum permissible leakage or dark currents.

6.2

Selection of Trigger Tubes.

6.2.1 Sensitivity. Experience has indicated that trigger tubes

with relative sensitivities lower than 0.90 are not useful for

transraissometers with baselines longer than 500 feet except when

they can be paired with phototubes of unusually high sensitivity.

6.2.2 Stability. The particular application will determine

whether or not a tube rated as S (stable) or V (very stable) is

necessary. In the majority of cases, S (stable) tubes are

adequate. There are few transmissometer applications for which

U (unstable) tubes can be utilized. In general, unstable trigger

tubes can be utilized only in circuits with exceptionally long

time constants, in applications in which the time integral of flux

is required and the period of integration is sufficiently long so

that the irregularities in the pulse rate do not affect the result.

6.2.3 Plate-Voltage Plateau. It is essential that the platevoltage plateau of each tube be sufficient to include the range of

voltages normally found in service. A minimum range of from 245

vdc to 275 vdc is required because of the expected variations in

the characteristics of the voltage-regulator tubes used in the

receiver power-supply unit.

6.2.4 Leakage Current. The maximum permissible leakage current

may be found by using the curves of figure 1 and the transmission

error corresponding to this current to determine the effect of

the leakage current on the indicated visual range. For most

operational purposes a leakage current of 0.00002 microampere,

producing a transmission error of 0.001, is permissible.

6.3 Individual Selection of Phototubes.

6.3.1 Sensitivity. Experience in service has indicated that

phototubes with sensitivities lower than 0.9 are not useful in

most transmissometer applications. When possible, it is more

desirable to use phototubes with higher sensitivity than to

reduce the capacitance of the sensitivity capacitor of the photopulse unit below 20 ppf.
’
- 11 =

6,3.2 Dark Current. The maximum permissible dark current may be

found from figure 1 by a method similar to that used in finding

the maximum permissible leakage current of trigger tubes. As with

the trigger tubes, for most operational purposes a dark current

of 0.0002 microampere, corresponding to a transmission error of

0.001 and a "dark" pulse rate of one pulse in 15 seconds is the

maximum permissible.

It should be noted that if the value of C1201 is adjusted

to the maximum capacitance permitted by the sensitivity of the

phototube, it is permissible to use nigh-sensitivity phototubes

which have a somewhat higher dark current than that of tubes of

average sensitivity. If C1201 is adjusted, Ig is then approximately equal to 0.2 times the relative sensitivity of the tube.

Consequently, the permissible dark current is also equal to the

product of the tube sensitivity and the permissible dark current

for tubes of average sensitivity.

6.4 Pairing Tubes.

It is frequently desirable to use phototubes of high sensitivity with trigger tubes of low sensitivity, and conversely, in

order to reduce the variations in sensitivity from receiver to

receiver and to reduce the number of tubes to be discarded.

Similarly, since the errors produced by the phototube dark current

and the trigger-tube leakage current are of opposite sign, a pairing of tubes with errors of approximately the same magnitude

is desirable. However, the degree of compensation for leakage

and dark currents which can be obtained in this manner is limited.

\
.

*

;?! I

^

*
- 12 -

Table I Characteristics of Type WL-759 Trigger Tubes

Tube

Relative

Sensi- Firing Plate-Voltage

Leakaae Current

Equivalent

No. tivity Voltaae Plateau Ratina Transmission

volts max volts/

min volts

WB-1 1.00 160 >300/240 V 0.00003

WB-2 l o 20 165 >300/184 V 0.0002

WB-3 1.08 160 295/200 F >0.0005

WB-4 1.00 165 280/190 S 0.0003

WB-5 1.16 149 290/178 S 0.0003

WB-6 1.04 171 285/165 S 0.0004

WB-7 1.26 170 297/160 u >0.0005

WB-8 1.14 168 290/200 u >0.0005

WB-9 1.04 165 295/195 u >0.0005

WB-10 1.40 160 285/190 F >0.0005

WB-11 0.84 183 300/190 S 0.0002

WB-12 1.04 153 295/185 V 0.00004

WB-13 1.04 160 300/225 u »0. 0005

WB-14 1.04 154 274/200 F >0.0005

WB-1

5

1.16 145 290/180 S 0.0004

WB-1

6

1.18 165 295/180 S 0.0004

WB-1

7

0.90 179 300/180 u >0.0005

WB-18 1.22 160 300/220 V 0.0001

WB-1

9

0. 86 173 300/225 V 0.0002

WB-20 0.90 175 300/209 V -0.00002

WB-21 1.64 148 300/205 V -0.00008

V: Very good

Si Satisfactory

F: Fair

U: Unsatisfactory

Stability
.

• ..

.
-13-

Table II

Characteristics of Type WL-919 Phototubes

Tube NOo Relative Sensitivity Dark Current

(Equivalent Transmission)

WB-A 1 0 36 0.0008

WB-B 1.40 0.0014

WB-C 1.46 0. 000?

WB-D 1.34 0.0006

WB-E 1.48 0.0015

WB-F 1.68 0.0017

WB-G 1.32 0.0008

WB-H 0.90 0.0006

IVB-I 0.90 0.0063

WB-J 1.34 0.0007

WB-K 1.48 0.0024

WB-L 1.38 0.0017

WB-M 1.40 0.0016

WB-N 1.56 0.0010

WB-0 1.52 0.0024

WB-P 0.94 0.0008

WB-Q 1.20 0.0018

WB-R 1.08 0.0020

WB-S 1.06 0.0017

WB-T 1.11 0.0012

WB-0 1.22 0.0063

WB~V 1.36 0.0049

WB-W 1.40 0.0040

WB-X 1.00 0.0027

WB-Y 1.32 0.0027

WB-Z 1.12 0.0027

WB-AA 1.32 0.0011

WB-AB 1.18 0.0011

WB-AC 1.22 0.0023

WB-AD 1,04 0.0007

WB-AE 0.98 0.0015
'
INDICATED

VISUAL

RANGE

(MILES)

I

THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants

and properties of materials; the development of methods and instruments for testing materials,

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publications should be addressed to the Office of Scientific Publications, National Bureau of

Standards, Washington 25, D. C.
*

\