The use of hearing-aid type tubes in portable counting-rate meters and amplifiers


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The use of hearing-aid type tubes in portable counting-rate meters and amplifiers
Series Title:
United States. Atomic Energy Commission. MDDC ;
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10 p. : ill., diagrams ; 23 cm.
Nierman, L
U.S. Atomic Energy Commission
Technical Information Division, Oak Ridge Operations
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Oak Ridge, Tenn
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by L. Nierman.

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University of Florida
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MDDC 725




L. Nierman

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(Work done November 1944 through April 1945)

L. Nierman

The purpose of this report is to make available the general results
of development work done on portable counting-rate meters used with
proportional counters; such portable instruments have hitherto been com-
paratively impractical because of the excessive weight of the electronic
equipment. By adaptation of standard circuits to the low-power tubes
used in hearing-aids and incorporation of components now available, a
great reduction in size and weight has been achieved. This general re-
port will be followed by detailed reports on specific instruments using
the circuits and materials described here.


Figure 1 is'the schematic diagram of a six-tube model, consisting
of a four-stage amplifier and a pulse-height selector. The amplifier
consists of a pair of degenerative two-stage amplifiers. The overall gain
of the amplifier, from grid of the input stage to grid of the pulse-height
selector, is a little less than 200. In frequency response, the amplifier
is flat to well beyond 200 kc., thus providing sufficient pulse discrimina-
tion for the separation of an alpha purlse-height distribution from an in-
tense beta pulse-height distribution. The negative feedback network con-
sists of a voltage divider between the plate of the second tube and ground,
impressing on the cathode of the first tube of each two-stage amplifier a
portion of the output of that two-stage amplifier. The tubes used, Raytheon
CK505- AX, are designed for operation with the filaments of two tubes in
series across a li volt cell. It will be noted that the circuit used requires
three A batteries for the four-stage amplifier, the total A drain being 90
milliamperes at 1 .4 volts: one-third of the filament power delivered to
the amplifier is wasted in resistors placed in series with filaments. A
two-stage amplifier using 1L4 tubes gives substantially the same per-
formance and requires only slightly more filament power, but the B bat-
tery current drain is considerably in excess of that required for the cir-
cuit of Figure 1. In attempting to reduce the number of filament batteries
and to eliminate the power loss due to voltage-dropping resistors in series
with the filaments, a number of other feedback circuits (wherein the fila-
ments could be operated at the same A. C. potential) were tried, includ-
ing common cathode resistors in the odd stages, plate-grid feedback over

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three stages (instead of plate-cathode feedback over two stages as in
Figure 1) and feedback to screen grids. None of these could be made to
give the desired performance and at the same time be reasonably stable.
It was likewise found impractical to replace the cathode resistors with
chokes or to achieve the desired broadband amplification by video-type
compensating coupling networks.

The pulse height selector is an adaptation of the basic circuit de-
veloped by Wendell Bradley for standard vacuum tubes and used in numer-
ous applications as a pulse-former, pulse-height selector and counting-
rate meter. The last tube is normally cut off. The circuit is so dcoigned
that any signal at the input to the pair which is less than a predetermined
level will produce no appreciable plate current in the last tube. However,
a negative pulse at the input which is greater than the predetermined le 'el
will produce in the last tube a rectangular current pulse whose amplitude
and duration is unaffected by the shape or amplitude of the input pulse.
Thus the D. C. component of the plate current in the second tube is pro-
portional to the number of pulses above the desired height appearing at
the input, and the number of these pulses is measured by placing a meter
in the plate circuit, with an appropriate series resistor and shunting con-
denser to give the desired speed of response to statistical fluctuations.

The chief requirements in adapting this pulse-height selector cir-
cuit to use with extremely low-power tubes are first, a sensitivity suffici-
ent to make it operate properly with the limited amplifier gain which is
available, and second, the production of current pulses of sufficient ampli-
tude to drive available D'Arsonval D. C. meter movements.

The portion of the random input pulses which will not be counted
because of the resolving time of the circuit may be calculated. Let these
losses, equal to unity minus the ratio of the counts registered to the actual
number of input pulses, be represented by L. Let T represent the resolv-
ing time of the circuit, i.e., the period during 4hich the circuit has not
sufficiently recovered from the effects of a previous pulse to respond to
another pulse, and let t represent the average time between input pulses.
t is equal to 1/R, where R is the average rate of occurrence of input pulses
random in time, R being expressed in input pulses per unit time. Then L
may be approximated closely at T/t, or RT. (See Strong, Procedures in
Experimental Physics. 1938. p. 95). In the circuits discussed, i.e.,
pulse-height selecting counting-rate Meters, for any given size of input
pulse, the resolving time is proportional to the length of the pulse formed,
or T is equal to kd, where d is the duration of each output pulse and k is
a constant depending on the tubes and parameters used in the circuit. Thus
L = Rkd. But i = Rid, where i is the average current as read on a meter
and I is the current which is drawn during the rectangular pulse. Thus

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L = ki/I, or the portion of counts lost is equal to the product of the ratio
of resolving time to pulse length and the ratio of average (D.C) current
to peak current. It may be seen that with standard radio type tubes, a
fairly insensitive meter can be used without greatly impairing linearity
of calibration.

Let us apply the above analysis to the design of the present pulse-
height selecting counting-rate meter. It must first be noted that k is not
a constant independent of pulse height, but becomes smaller as the input
pulse size is increased; in other words, when the circuit has been adjust-
ed to respond to given amplitude of input pulse, and it is fed with pulses
slightly above that amplitude, it will have a resolving time of k1 times the
duration of the output pulse: if the size of the pulses is increased the re-
solving time will be k2 times the duration of the output pulse (which will
be unaffected), k2 being smaller than kl. In the present case, the pulse-
height selector will respond to a pulse of about 0.3 volt amplitude. When
pulses of this size are impressed, k is about 6. With pulses of ten times
this amplitude, k is down to as low as-2, the circuit being "forced" to
respond to new pulses although it has not completely recovered from the
previous pulse. This effect may be observed with an oscilloscope at the
grid of the last tube. Measurement of the resolving time for any input
pulse amplitude is easily made by using a pulse generator input with phones
on the output. As the frequency of the input is increased, the frequency
of the output will be the same as the input frequency up to a point where
it suddenly drops to one-half the pulse generator frequency; at a further
point, the frequency of the output will drop to one-third the pulse generator
frequency. The resolving rime for any given input pulse amplitude is the
period of the lowest input frequency at which the output frequency falls.

With relatively equal pulse amplitudes at the input, as with GM tubes.
it is possible, by use of the above equation, to predict the linearity of this
type of counting-rate meter with considerable accuracy. It is to be noted
that if, as in most cases, k, the ratio of resolving time to pulse length, is
a constant, the non-linearity will be the same for all ranges, where a range
switch is provided. As an example, suppose a particular circuit has a
resolving time three times the pulse length for the pulse size to be count-
ed and further suppose the peak current delivered during the output pulses
is 1 milliampere. Then it appears that if a 50 microampere meter is used,
15% of the counts will be missed when the meter reads full scale. (It
must, however, be noted that the above analysis cannot be used with ac-
curacy for losses over 20% or so because of the approximation involved
in the original derivation.)

In the design of the present instrument, involving pulses varying in
amplitude by a factor of 50 or more, the computation can not be made with

MDDC 725


any exactness. Nevertheless,it is clear from the above discussion that
in order to keep the non-inearity negligible with a 20 microampere move-
ment, the most sensitive obtainable for such uses, the current during the
pulses must be of the order of 1 milliampere. This consideration, together
with the sensitivity requirement, led to the present design. To achieve
high sensitivity without danger of oscillation, it is necessary to bias the
last tube somewhat beyond cut-off and have high gain in the first tube of
the pair. It will be observed that the latter is an ordinary degenerative
amplifier till the input signal reaches such amplitude as to effect the
"triggering" action. The latter occurs when the incremental current in
the cathode resistor caused by current flow in the last tube is sufficient
to overcome the decrement in current in the input tube of the pair caused
by the negative signal pulse, thus making the total effect of the cathode
resistor regenerative, rather than degenerative. High peak current is
obtained by using a bias battery to cut off the last tube in the absence of
signal, instead of relying wholly on the voltage drop which appears across
the cathode resistor as a result of normal plate current in the input tube
of the pair. This device allows, the use of a much smaller cathode resistor
than would otherwise be required, thus increasing the gain of the first
tube and decreasing the load on the last tube. The counting-rate meter
has no perceptible non-linearity over the full scale.

A model of this instrument, after calibration with a pulse generator.
was used with a boron-walled -eutron counter at various distances from
a radium-beryllium source. The counter was surrounded by a paraffin
moderator. When readings (averaging by eye) of the counting-rate meter
were compared with runs with a standard proportional amplifie'- and scaler,
the results agreed within 3%.

The model built is incorporated in a 6 inch cubical chassis, exclud-
ing high voltage batteries for the counter and B batteries, both of which
are intended to be carried in a knapsack. It weighs 6- pounds, including
tne A batteries, which are in the chassis.


It was found that the standards of accuracy required in the survey
work for which these instruments are primarily designed are such as to
make possible the use of a much lighter and simpler unit, requiring only
one adjustment in addition to the range switch and the on-off switch. It is
remarkably free of microphonic effects. A schematic diagram of this
circuit is shown in Figure 2.

The amplifier is a two-stage resistance* coupled nondegenerative

MDDC 725


design. To discriminate against pickup and microphonics the grid and
screen circuit time constants are such as to produce an amplifier with
no flat region in its frequency response curve, which reaches a maximum
in the vicinity of 12,000 cycles, where the gain is about 400, and is down
to 0.707 of its peak value at 4500 cycles and 35,000 cycles.

The pulse-height selector is essentially the same as that used in
the instrument previously described except that the bias batteries are
eliminated. The non-linearity thus introduced is, however, negligible in
comparison with the error limits within which the instrument is designed
to make measurements.

This circuit, unlike the one previously described, is not intended
to count all the pulses produced by the proportional counter. The plan
for use of this instrument is to provide a convenient calibration sample
to be slipped onto the counter at intervals of time (not yet determined)
in the field, thus eliminating much of the error due to drifts in the batter-
ies or circuit. In initial calibration of the instrument, a considerable
percentage of counts will be deliberately lost. The calibration will be
held constant by adjusting the single control until the standard sample
gives the same reading as upon initial calibration. When this 'can no long-
er be done without bringing in spurious counts which appear in the absence
of radiation, the batteries must be replaced. It is expected that the bat-
teries incorporated in the present design will last about two weeks with
the instrument in use six to eight hours daily.

The model, currently under laboratory test is incorporated in an
aluminum chassis 5" x 41 x 31", weighing less than three pounds, includ-
ing all batteries except the high voltage for the counter, but excluding the
meter, which is in a separate case 2 3/4" x 31", connected to the circuit
chassis by microphone cable. It is expected that this model will shortly
be in the field, together with appropriate probes and light-weight high
voltage batteries, used as a portable proportional alpha rate-meter. Later
a portable slow neutron rate-meter will be constructed. The present
general report will be followed by subsequent reports on these instruments
when all features of their design are complete.


Before designing the circuits described above a survey was made
of low filament-power tubes available, since most of the bulk of the port-
able vacuum tube power supply is in the A batteries. A number of brands
and types of hearing-aid tubes were considered and tested. Of these,
those made by Raytheon are the most satisfactory. Zenith and Western
Electric tubes, comparable in performance, are no longer manufactured.

MDDC 725


although samples were made available for testing. Hytron tubes require
far more filament power than the others mentioned. It appears that a
number of the hearing aids formerly made with other tubes have been
redesigned for use with the Raytheon tubes. The electrometer tubes made
by Victoreen also may be adapted advantageously to certain A. C. applica-
tions. In the following summary of the tubes found most suitable, the fila-
ment voltage is designated on the basis of 1.5 volt cell operation, but in
all cases there is relatively little change in emission as the cell runs
down to 1.2 volts. All tubes are filament type.

Raytheon CK505AX Pentode. 30 mils. at 0.75 volts. Gm with 45 v. on
plate and screen 175 micromhos. Maximum cur-
rent as diode (suppressor internally connected to
filament) approximately 1.5 mils.

Raytheon CK509AX Triode. 30 mils. at 0.75 volts. Amplification
factor 20. Plate resistance 150,000 ohms.

Raytheon CK510AX Double tetrode 50 mils. at 0.75' volts. Designed
for use as double triode with virtual cathode formed
at first grid, which is common to both sections.
Amplification factor of each second grid 20. High
plate resistance, of the order of 1/2 megohm. Low
plate currents.

Raytheon CK512AX Pentode 20 mils. at 0.75 volt Gm with plate and
screen at 45 volts. 120.

Raytheon CK502AX Power pentodes. 30 and 50 mils at 1.5 volts. Gm
503AX up to 500. A CK-507AX has been incorporated in
506AX a BG survey instrument model built by the Health
507AX Division to drive a small speaker in place of the
usual headphones.

Victoreen VE32 Electrometer triode. 10 mils. at 1.5 volts. Amplifi-
cation factor 1.75. Maximum plate current as diode
approximately 2 mils.

Victoreen VE124 Electrometer tetrode. 10 mils. at 1.5 volts. When
connected as high-mu triode, amplification factor
is 17, with sharp cut-off characteristic. Maximum
plate current as diode approximately 2 mils. Be-
cause of the relatively high peak current, a pair
of triode-connected VE124 tubes make an excellent
counting rate meter where high sensitivity is not

MDDC 725


required, as in the Mark 1, Model 31A GM tube
portable meter.

All of the above tubes are made without bases, tinned leads being
brought directly out of the envelope and soldered into the circuit. Some
of the Raytheon tubes are available with bases under the same numbers
without the designation AX.

Tube-checking is done here by applying standard voltages to the
electrodes and observing the plate current. A simple tube-checker in-
corporating a 1 mil. meter has been constructed; a switch inserts l1 volts
bias on the control grid for a rough measurement of transconductance.
The tubes are inserted on a screw-type terminal board.


In mounting circuits in small spaces, it is necessary to utilize
smaller components than those appearing in most general electronics
stockrooms. In fixed resistors, 1/2 watt sizes such as those made by
Allen-Bradley and Erie will ordinarily fulfill the requirements. (See
Landsman's Reports CP2746 and CP2861 for voltage and temperature
coefficients, life tests and other data on various brands of resistors). In
variable resistors, the midget rheostats and potentiometers made by most
manufacturers may be found sufficiently compact. But it is well to note
that there are available smaller units made especially for hearing-aid
and instrument use, such as the Centralab NS series and the Stackpole
Type PSM.

In choosing components, the greatest saving of space is to be ac-
complished by selection of appropriate condensers. In capacities greater
than 0.001 microfarad, the tubular paper condensers ordinarily used, rated
at 400 volts, are extremely bulky. The midget paper condensers manu-
factured by Dumont Electric Company as their number PlN, rated at 150
volts, occupy a small portion of the volume required for the usual paper
condensers. They have proven particularly useful in 0.1 and 1 microfarad
sizes. Small 150 volt bathtub condensers are made by Mallory. Although
the usual small mica condensers will in most cases be adequate for capaci-
ties less than 0.001 microfarad, the button-type silver-micas made by
Centralab may be considered.

Concerning batteries, it has heretofore been the custom to use flash-
light cells for filament power, although hearing-aid type B batteries have
commonly been employed. Flashlight cells, designed for high-drain service
over short periods, will be found to give much less life per unit weight on
low drains than the hearing-aid batteries which are designed to supply

MDDC 725


filament power. The plug-in feature oi many of the hearing-aid batteries
also simplifies battery replacement.

It should be pointed out that many of the coaxial cables in general
use, such as RG26/U, are poorly suited for portable applications as probe
cables because of their lack of flexibility. For low-current high voltage
applications, it is possible to procure cable of smaller diameter, and
thus much more flexible, without sacrificing low capacity. Thus RG6/U
cable has been used at 2500 volts with a proportional counter without
corona effect or other spurious counts. At lower voltages smaller di-
ameter cables may be used. These include RG 58/U and RG62/U.

-9- MDDC 725




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