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MSA TT P-8357 

6 (nasacro 

6-0 c/<e. ^ ol 


by H, Dolezalek 


WASHINGTON February 1963 

/Published in Archiv fuer Technisches Messen , 
Ho. J, 8334-4? January 1962, pages 19-22/ 

Hans Dolezalek, Diploma Pbysicisi; 
Meteorological Observatory of the German ?feather Services, Aachen 

The report begun in Part I (ATM Ho, J-8334-3j December I96I) is 
continued with the description of systems with electrometer tubes. 

Yery many designs have been developed for the arrangement of 
amplifiers for electrometer tubes of which we shall discuss only tvro 
basic types. 

In all cases concerning continuous measurements with electrometer 
tubes which is not possible without a certain degree of automation, 
utilization of a bridge circuit is the preferred means (5-g)» However, 
if a short-interval measure is visually controlled by the observer who 
can manually regulate input voltages, a straight-line circuit is then 
preferable because of consuming less input current and therefore more 
economical (5-b.). 

g) - Bridge Amplification for Electrometer Tubes . Utilization of a 
l?ilheatstone bridge with two electron tubes and two working resistances 
for direct-current amplification was apparently first proposed by Wold - 
/16_/, This was further developed subseq_uently, including designs for 
bridge circuits with only one electron tube and replacing the others by 
greatly varying types of circuits. After electrometer tubes began to be 
available, they also were employed for such bridge amplifiers. The 
advantages of a bridge amplifier arej 

I. Disturbing influences of input voltage variations are reduced 
or canceled out by tubes sufficiently similar to each other within the 
limits of accuracy so given. If the two tubes are someTirhat different, 
which is the general rule, similarity can be improved by balancing methods. 
If the circuit is correctly designed, zero drift due to ageing and tempera- 
ture variations is reduced. 

II. In raaror cases, a bridge circuit permits compensation of 
disturbances which become mixed with the impulse already through the 
generator or circuit. This is successful if a reactive generator (with 
reactive line) can be employed parallel with the measurement generator 
(with measurement line). This retains advantage I. 

III. If the impulse, is symmetrical to ground and the two components 
of tension to ground are not of interest, the bridge amplifier can be 
used as push-pull amplifier (impulse voltage between the control grids 

of the two tubes). The summation value then appears in the output. This 
also retains advantage I. 

A standard wiring diagram of a bridge amplifier is shown in Pig. 2 
/sic-?~should be Fig. 1 — Tr,J. In principle, both electrometer tubes 
should always be provided with equally high ohmic resistances at the 
control grid — and equally large capacities — because only then is it 
possible to achieve the optimiim of bridge balancing and only then do 
advantages II and III become at all perceptible. As indicator or recorder, 
either a high-ohmic voltmeter or a high-ohmic tension recorder is used 
(cf . Pig. 2) which compares the two anode voltages, or the two working 
resistances are replaced by the two coils of a differential galvanometer. 

Further direct-current amplifier stages which are to be connected 
may, in turn, be designed as bridge amplifiers. 

Proper operation of the bridge depends on the similarity of the 
two electrometer tubes utilized. Unfortunate ly^ the manufacturers of 
electrometer tubes will not make the effort of selecting pairs of tubes 
as similar to each other as possible and leave this to the consumer. 
Utilization of a twin-tube v/^ith a common cathode is obviously the desirable 
arrangement (cf. 6-d}. 

Even selected pairs still have differences which are obviously 
greatest in regard to filament emission, lie are therefore unable to get 
along without so-called "balancing," This consists in varying the filament 
currents in both tubes in the opposite direction by "systematic trial and 
error" until no further change in. the differences of anode voltage occurs 
when the common filament .voltage is changed (cf . Schintlmeister flij for 

Fig, 1, Standard wiring diagram of the bridge amplifier. 
Such balancing is obviously possible accurately only for a single 
operating point. For this, we select about half of the average value of 
the magnitude to be measured subsequently. Balancing similar to that of 
the filament currents can be effected also for the anodes by varying the 
Value of the working resistances also in the opposite direction (potentio- 
meter connected at the upper branch of the bridge). Anode balancing 
detunes the filament balancing made earlier which must then be repeated 

again until both are satisfactory but filament balancii:^ is generally- 
sufficient. If the tubes have space-charge grids, these can be advanta- 
geously used for balancing (cf. 6-a). 

^) Straight Amplification for Electrometer Tubes . If we are forced 
to economize on battery weight which results in reduction of the number 
of tubes, then the straight amplifier is preferable. This eliminates argr 
compensation of input-voltage variations and all other advantages of the 
bridge amplifier so that the straight amplifier should be used in 
principle only when all voltages are continuously controlled and can be 
adjusted manually. In certain cases, it may be used when direct observa- 
tion is not possible, provided that all input voltages are recorded 
sufficiently accurately so that it becomes subsequently possible to decide, 
depending on the necessary accuracy of measurement which parts of the 
recorded data can be utilized. The arrangement should be such that it 
prevents any regeneration effects as far as possible. Fig. 3 /sio — should 
be Fig. 2 — Tr._/ shom^s the wiring diagram of a straight amplifier which 
needs no further explanation. 

Fig, 2, Standard wiring diagram of the straight amplifier. 
Here it will be best to use triodes, not only because this saves 
space-charge grid current and/or grid-screen current but because they 

furnish a higher plate current and can therefor© be used more easily 
without additional amplification. A measuring instrument and a regulating 
potentiometer should always be coupled in the circuit, 

i) Amplifier with "Floating Grid ." Ever since electrometer tubes 
have become available, the "floating grid" method has teen proposed again 
and again. It consists in entirely eliminating a defined grid-leakage 
resistance. In order to properly evaluate the method, we must go back 
to the composition of the ccmponents of the grid current. 

In the operation of electrometer tubes, several extraneous resist- 
ances are inevitably parallel (cf . 4-c) to the "tube-inherent" insulation 
resistance (group 1 in 4-a). They all act jointly and cannot be sepa~ 
rated. If the generator .does not produce any tension or produces a 
constant tension, then the potential of the control grid also remains 
constant as long as the grid current does not change. In other words, 
when tube and parallel resistances are coupled, the grid current is zero. 
In principle, this is possible for any grid potential but is not an 
indication that the grid potential adjusts itself to the zero point of 
the characteristic of the "total grid current" (Fig. 1). On the contrary, 
the grid potential corresponds to the grid bias. This does not change 
fundamentally if we now make the defined grid-leakage resistance larger 
and larger. In principle, it should be smaller than the resistance of 
the electrometer tubes and of the generator and line leakages, etc., 
because it is the only one defined and (relatively) accurately known. 
We sacrifice this advantage of a relatively known leakage resistance if 
we assume E„ as infinite. 

From this point of view, we can say that the floatijag grid arrangement 
does not exist in principle* f/henever an approach toward it is made, we 

only attempt what should always be done when using electrometer tubes for 
maxim^^m efficiency which is to derive the maximum value of the still 
permissible grid-leakage resistance through estimating the tube-inherent 
insulation resistance and yet to rest so low with it that the unknown 
variations of the tube-inherent insulation resistance will not cause 

The difference in character of the grid-current grou-p 1 (4-h) as 
against groups 2 and 3 Jaay already be seen from the fact that groups 2 
and 3 are the product of generators for which a supply of power is 
necessary and available. The latter is the temperature of the filament 
in group 2, and either the liberation of latent energy or of extraneous 
radiation in group 3* These characteristics are therefore also not 
linear. Only one resistance should be entered in the equivalent circuit 
for group 1. 

A criterion for the possible use of the "free grid" method for an 
electrometer tube can be obtained by comparison of the sum of the grid- 
current components 2 and 3 with the grid-current component 1. If 1 is 
relatively large compared to 2 and 3, the tube-inherent insulation 
distance simply replaces the defined grid-leakage resistance. However, 
if one is small, the addition of a defined S„ becomes necessaiy because 
the sum of 2 plus 3 varies greatly and the effect of these variations 
must be eliminated by a smaller grid-leakage resistance. 

From the beginning, authors have warned against the method of the 
floating grid. Kleen and Graff under /8/ point out that inevitable 
changes of the Yolta effect in the tube uncontrollably displace the 
operating point in the course of time and that the trace of the grid- 
current/grid-voltage characteristic (in its temporal variability) must 


be known because the resistance is not linear. Morton [Wj finds that 
the sensitivity of tension of the electrometer tube is less with the 
method of the floating grid. Easmussen [\2j believes the method to be 
erroneous in principle because the effective grid resistance is then not 
constant and only minor amplification occurs in this regard. 

Systems containing floating grids are indicated especially frequently 
when "inverted triodes" are utilized (of. 6-a). 

k) The "Mekapion" Principle . Of the mary circuit designs based on 
electrometer tubes, we want to briefly mention only the Mekapion arrange- 
ment .,/l47. It does happen that stray effects in electrometer tubes 
create an unintended Mekapion effect v/hich is difficult to identify in 
searching for the disturbance ^personal communication from G. Hies, 
diploma engineer). 

6. Techniques, Designs, Models, Demands 

^) Different Electrometer systems and their Manner of Operation . 
o() Triodes with Standard Control Grid . The first electrometer 
tubes were triodes of customaary design, except for more highly insulated 
control grids. Electrometer triodes are still being offered today but 
have a higher grid current in general than other systems which will be 
discussed below. 

A) Inverted Triodes . In principle, the anode voltage of a triode 
can also be connected to the grid coil and be controlled by the anode 
plate J the "penetration factor" of the control plate influences the 
electron flow in the space between cathode and anode grid. 

It is to be expected, in this arrangement, that the component 312 
(4-a) of the grid current is kept away from the control electrode and the 
"grid current" therefore becomes less, f/e can summarize the advantages 

of "the inverted triode as follows (Frommhold [\J)x at the same mutual 
conductance (as in a tetrode, cf. below), there results a smaller control- 
electrode current and the same control-electrode current v/ill produce 
higher mutual conductance. 

^) The "Plation»" The so-called two-plate tube ("plation") is 
based on a principle similar to the inverted triode s the filament is 
located between two plane parallel plates, one of which operates as 
anode and the other as control electrode. 

o) Tetrodes have a positive grid between cathode and control grid. 
This "space-charge grid" corresponds 'simultaneously to several purposes. 
Because it attracts electrons out of the negative space-charge cloud 
ahead of the cathode, it, increases the negative minimtun potential and 
therefore affects the effective potential in the space between the wires 
of the control grid, makes the latter more positive and so increases the 
mutual conductance of the tube. The action of the control grid is now 
somev/hat different than in the triode? it also influences the distri- 
bution of the current to the two positive electrodes. The space-charge 
grid further retains the ions originating on the cathode or in the space 
between cathode and space-charge grid v/hich then do not reach the control 
grid so that the grid current becomes less. Moreover, the space-charge 
grid permits a variety of special systems, e.g., some proposed bridge 
amplifiers are designed with only one tube by utilizing the EG. Buhk [2] 
points out that, due to the dropping characteristic of the space-charge 
grid current (in some ranges), phase-accurate feedback is possible 
already with one tube. 

A special advantage of the space-charge grid is the fact that we 
can largely compensate the inevitable differences between cathodes in 
bridge amplifiers. 


^) £££i2^£2.' •^^ recent years electrometric pentodes have become 
available commercially which obviously permit a relatively high amplifi- 
cation of voltage ( /< ^ 30, sometimes indicated as 25O) and work with 
very low grid currents, in spite of the absence of a space-charge grid. 
They have a specially small plate current, 

^^ transistors . During proof reading, we have become aware of an 

interesting further development. This concerns a transistor in which the 

"grid"- input resistance lies at about 10 ohm. Although it cannot be 

utilized directly for electrometric purposes, this would seem to be a 
promising development. (C, T, Sah, A new semiconductor tetrode, the surface- 
potential controlled transistor. Fairchild Transistor Corp., Palo Alto, 
California, I96I, 14 P«)», 

b) Technology of Electrometer Tubes . Requirements for obtaining 
high insulation of the control grid consist in the selection of insulation 
materials with high specific internal resistance, the treatment of the 
surface of these insulation substances and the geometric reduction of 
leakage (long and narrow leakage paths). This should be supplemented by 
the interposition of grounded metal rings which v^ill prevent the spread 
of undesired surface potentials over the insulator although they will not 
prevent the flow of charge to ground. Frommhold /47 reports on this in 
detail. Best insulation is obtained when the physical realization of the 
control grid circuit is as far distant from other feed circuits as possible 
(e,g., at the top of the bulb or at the point of the sub-miniature bulb) 
and, in some cases, the grid design is surrounded by a glass collar in 
order to prolong the leakage path. 

For the purpose of reducing other grid-current components, various 
other possibilities exist and have been adopted in some cases. We shall 

here restrict ourselves to a simple listing because details can be found 
in Schintlmeister [VhJ and Frommhold fijt extremely high vaouumj low 
cathode temperature, control grids of metal with especially high electron 
output efficiency (coating with gold:)| positive electrodes of substances 
of low order number | reduction of space angle in which photoelectron 
radiation impi3ages on the gridj reduction of space in which extraneous 
radiation may have an ionizing effect. 

Since electrometer tubes are subject to all the disturbances which 
occur in electrometric circuits, positive measures for decreasing material- 
electrical effects are necessary. Most important here are the polariza- 
tion phenomena in the insulating substances. The nature of these phenomena 
which decay r only very slowly ("remanance") is still largely unknown. In 
order to decrease their effect, the ratio: of the conductor surface touching 
the insulation to that of the surface opposite to the vacuum should be as 
small as possible . 

These requirements (to be further complemented in 6-d) to some extent 
contradict each other and are generally not easy to fulfill. However, 
they represent imperatives for eliminating at leas* a part of the "infan- 
tile diseases" which restrict the possibilities of utilization of 
electrometer tubes, 

c) List of Electrometer Tube Models . A listing attempting to be 
complete but probably not successful is contained in Table 1, It generally 
contains values specified "h-^ the manufacturer which have not always been 
checked by us. These values are hard to compare with each other because 
they have been obtained under different operating conditions. In particu- 
lar, a reduction of the anode voltage below 5 ^ ^^^7 produce appreciably 
lower grid currents (at lower mutual conductance) (e.g., T-113 and T-116), 


^) Peinands for Improvemen-t of Elec-fcrometer Tubes . In addi-tion to 
the demands discussed in 6-13^ the principal demand is for the production 
of indiTxdual units of the same tuhe model which are more closely similar. 

The obviously desirable goal would be the possibility of replacing 
one electrometer tube by another one of the same type without making re- 
calibration necessary. This is scarcely possible in highly sensitive 
systems but it should be possible to establish the U^I. characteristic 
so definitely that changes of the plate current of more than a few percent 
no longer occur under othen/^ise equal conditions for individual tubes 

operating with a control range of about 1 V at the control grid« If the 

grid current remains below 5 3C 10 A, it can then be different in 

individual units. 

The great importance inherent in electrometer bridge systems leads 
to a further urgent demand. For several decades, the requirements for 
the production of electrometer twin-tetrodes have teen theoretically- 
known. The point here is that both systems must be and remain very 
similar to each other in operation. This makes it necessary that both 
control grids in the twin-tube are constructed with very high-degree 
insulation. An effort should be made that both anodes receive electrons 
/?/ /German has "Elektroden" = "Electrodes"— Tr._/ from the same parts of 
the cathode fl, 10? 15/» Moreover, the space-charge grids of both 
systems should be constructed separately. Indirect heating would somewhat 
simplify the stabilization problems. If satisfactory twin- tubes do not 
exist, it would be highly desirable to be able to order selected pairs of 
tubes for the bridge system which was generally possible in the past. 

e) Industrial Equipment using Electrometer Tubes . Stange & ¥olfr^^m, 
Berlin S¥-6l, produces apparatus for continuous recording of the four 


atmosph.eric-eleo1;ric basic elemen"ts ("atmospheric-electric station") 
which includes four electrometer-tube bridge amplifiers for operation by 
connection to a network power source* The high-ohmic design of the grid 
at the compensation aggregates of the individual bridges permits compen~ 
sation of disturbing voltages from the atmospheric-electric antennas and 
the measurement circuits, 

"Teraohmmeter" utilizing electrometer-tube bridge systems are 
produced by the company E. Jahre, Berlin ?/-35« They permit measurement 
of very high resistances even at low voltages which is not the case for 
most of the other teraohmmeters, 

Keithley Instruments, Inc., Cleveland (Ohio, USl) furnishes a whole 
line of equipment provided with electrometer tubes which is suitable for 
tube-electrometers, tube-galvanometers, teraohmmeters, etc. for many 
purposes. Types 510> 610, 610-A, 600tA, ..411»-412, 413," 410, 420 contain 
two electrometer tubes in a bridge circuit (but compensation side is not 
high-ohmic) J types 200, 200A, 414> etc., contain only one electrometer 
tube (this listing is not complete). 

Yictoreen Instrument Co., Cleveland, Ohio, UBA, a producer of widely 
employed electrometer tubes, furnishes equipment provided with electro- 
meter tubes, including types YTE-p to 7TE-3. 

The electrometer of the company P. 1. Klein, Tettnang/Bodensee, has 
an electrometer tube as input tube. 

Literature references for the part II were published in the preceding 
issue (J 8334-3, December I96I). 





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