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A NEW TYPE OF RADIO VACUUM TUBE

A Thesis Submitted to
The Faculty of
MICHIGAN STATE COLLEGE
of

AGRICULTURE AND APPLIED SCIEICE

By

\.\

Chauncey E. Park Orin D. Dausman

Candidates for the Degree of
Bachelor of Science
in

ELECTRICAL ENGINEERING

June 1925

.1

The use of vacuum tubes in radio transmitting
and receiving sets has increased rapidly in the past
few years. his extend d use has uncovered certain
limitations and disadvantages of the old type of
vacuum tube. For this reason, it eemed advisable,

not only from an educi oint but from an

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economic standpoint, to investigate the possibilities
of the construction and the operation oi a new type
of vacuum tube.

The old style of vacuum tube consists of three
elements, a filament, a plate, and a grid. The filament
is an emmitter of negative electrons. The plate, due
to the high positive potential at which it is charged,
attracts the electrons. The grid is a net—like
structure located between the filament and the plate.

In passing from the filament to the plate, the
electrons must pass through the grid. Due to the

gative charge carried by the electron, its motion
is affected by any electro-static potential in the
vicinity. This property makes it possible, by
changing the potential impress d on the grid, to control
the quantity of electrons passing from filament to plate.

Although this tube is sensitive and amplifies
without distortion, it has several disadvantages.

Some of these disady antag ges are the necessity for

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using 1i ate voltag s, its sensitivity to

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comparatively weak electrostatic fields, its
tendency to build up undesirable osci latory currents
in resonant circuits, an: the necessity for an

extremely high degree of vacuum. The new tube has
been designed With the intent of overcoming hese
disadvantages and making other improvements.

It is a well known fact that an electric current
is affected by the presence of an electromagnetic
field. This fact is made use of in the design of the
new type of tube. The new tuoe, as first prOposed,
was to consist of a filament, two like plates, a
parabolic reflector, and a control solenOid. These
elements were to be arranged as shown in Plate #1,

Fig. #1. The filament F was to be located at the

focus of the parabolic reflector R. The axis of

the parabola was to pass between the two plates, P1 and
P2. The control solenoid was to be placed around

the enclosing tube. In operation, a negative potential
would oe impressed on R, and equal positive potentials

on P1 and P2. In the absence of a magnetic field,

the electron flow would be equally divided between

0

P and P . In the presence f a mannetic field, the
2 L O

1
electron flow would be deflected from one plate to the
other depending upon the direction of the field.

If leads are brought from these plates to Opposite ends

of a solenoid, and if the plate potential is supplied

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

through a center tap on the solenoid, the flux in

the solenoid due to the equally divided plate currents
will be zero. Now if the plate currents are unbalanced,
due to a changing magnetic flux in the tube, flux will
be set up in the solenoid. It is easily seen that

this is the condition obtained in a push-pull ampli-
fier with the old style of tubes.

The greater part of the material used in the
construction of the new tube was obtained from burned
out tubes of the Old style. It was necessary to obtain
non-magnetic material for the elements. If magnetic
material were used for the elements, the magnetic
field set up by the control coil would be concentrated
in the elements and would have little effect in
controling the electron flow. It was found that the
plates of General Electric Co. tubes were of non-
magnetic material and so this material was used for
the reflecter and plates of the first set of elements
constructed. Plate #1, Fig.1 shows the arrangement
of the elements. These elements were made about two
inches in length and of lateral dimensions to fit in

a one inch test tube. The filament was of oxide-
coated platinum obtained from the W stern Electric

type of tube. The base used was the base of an old

Western Electric tube. It was considered undesirable
to actually construct the base because of the
difficulty of making an air tight Joint when fusing
leads through glass.

After some experimenting, it was found that the
best way of fastening the elements to the leads was by
brazing with brass. The leads were depended upon to
support the elements. This was the first real
difficulty encountered. It was found to be impossible
to hold the elements rigidly by this method.

At this time, a new type of construction was
decided upon. The plan of the elements is shown on

Plate #1, Fig.2. The Opposite plates P a P1 and P2

1
& Pé were connected as shown by Fig. 3. The

reflecter was replaced by the the shield shown in

Fig. 4. The filament was suspended in the slot in the
shield.

The plates and shield were constructed of sheet
COpper because it was found that the material obtained
from the old General Electric Co. tubes oxidized very
easily and it was very difficult to remove the oxide.
Also this metal was very brittle. It would probably
not be desirable to use OOpper in a commercial tube

because of its tendency to hold occluded gases.

The elements were made of a size such that they

would just slip inside a one inch test tube. This
gave a rigid support for the plates. The tOp of
the shield and the crossleads between the plates were
fused into a piece of glass which supported them as
well as insulating them from each other. The tOp end
of the filament was connected directly to the shield.
The shield thus served as one filament lead while
the other filament lead was kept apart from the other
elements. The leads from the two sets of plates and
the filament leads were brazed to the leads through
the glass base. The entire group of elements then
appeared as in the accompanying print on Plate #2.

The next problem was to inclose the elements
in a glass tube of the form saown in Fig. 5, Plate #1.
After a great amount of diiiiculty, working with
soft glass, it was found to be necessary to construct
the tube 0: Pyrex glass and fasten this to the soft
glass base with De Khotinsky cement. De Khotinsky
cement is the only compound known which will form an
air tight joint with glass. This completed form of
tube is shown by the print on Plate #2.

Dr. Ewing of the chemistry department mad
possible the use of the vacuum pump owned by that
department. The evacuation of the tube proved to be

a long process. During this process two filaments

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were burned out, making it necessary to remove the
tube from the pump for replacements. at another time,
the mercury pump on the evacuating system became
over heated, which made it necessary to remove the
tube from the pump.

After considerable time had been Spent trying
to reduce the pressure in the tube to such a degree
that ionization would not occur at ordinary plate
voltages with the filament lighted, it was decided
to test the tube while it remained on the pump.

The definite reason for being unable to prevent
ionization is not known. It is thought that some air
may have filtered through the De Khotinsky cement,or
that the cement actually evaporated, thus reducing
the pressure. The gage onthe pump indicated a
pressure of only .0002 millimeters of mercury at

that time.

The only quantitative tests possible to take
of the tube were static tests. The connection
diagram for taking these tests is shown on Plate #4.
Test Connection #1. 0n Plate #2 will be seen the
prints of the tube on the pump with the testing
apparatus in place. The data from the static tests

is given on the following pages.

 

 

 

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In the following data, If : filament current,
Ep = plate voltage, {pl 3 milliamperes in P1,
Ip2 - milliamperes in P2, Rp = resistance in the

plate leads, and 16 : current in the control coil.

If : 1.25 HP 3 2000 Ep 3 90

When IC = O, Ipl - 3.3, and Ip2 . 3.9.

IO Ipl Ipe IDl-Ip2+o6

.3 3.4 3.7 .3

.6 3.4 3.8 .2

.9 3.3 3.6 .3
1.2 3.3 3.5 .4
1.5 3.35 3.6 .35
1.8 3.2 3.5 .15
2.1 3.35 3.5 .45
2.4 3.35 3.5 .45
2.7 3.30 3.4 .50
3-0 3-35 33 ~65

If = 1.25 Rp = 2000 ED . 60

When IO - o, 1pl = 1.95, and Ip2 = 2.3

Is Ipl Ip2 Ipl--D2+o3‘)
.3 1.98 2.23 .10
.6 1.98 2.13 .20
.9 2.00 2.13 .22
1.2 2.01 2.10 .26
1.5 2.05 2.10 .30
1.8 2.05 2.06 . 4
2.1 2.05 2.00 .40
2.4 2.12 1.37 .60
2.7 2.10 1.83 .62
3.0 2.15 1.ao .70

If : 1.25 RD . 2000 Ep - 30

When IO = O, ID - .7, and Ip2 : .92.

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IC Ipl Ip2 Ipl-Ip2+022
.3 .79 .87 .14
.6 .82 .82 .22
.9 .87 .80 .29
1.2 .91 .77 -2_
1.5 .95 .72 - 5
1.8 1.09 -67 -55
2.1 1.01 -63 .60
204 .1006 ’58 ’70
2.7 1.11 .50 .83
3.0 1016 '45 '93
If a 1.25 Rp . 2000 Ep = 20
When lo a 0, Ipl a .35, and Ip2 : .37.
IC Ipl 1:32 Ipl-Ip2+o02
'3 039 033 008
.6 .40 .28 .14
.9 .45 .22 .25
1.2 .49 .13 .33
1.5 .51 .17 .36
1.8 .52 .15 .39
201 056 013 045
2.4 .59 .12 .49
2.7 .60 .08 .54
3.0 .61 .07 .56
If . 1.09 Rp . 2000 ED . 88
When 1c :0, Ipl - .40, and 1p2 : .60-
IC Ipl 1p2 Ipl-Ip2+.20
7
6.1 '33 .10 1.00
5 .o '13 .90
4 .;6 ‘17 .84
3 :65 ’23 '73
2 57 '33 052
1 ’50 .43 .34
O .40 ‘55 017
_l .70 060 CO
-2 .55 .;2 -020
— . ° “031
3 ~21 .80 v

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4 .19 .80 -.41

If . 1.09 ap . 2000 Ep = 50
When 1c .0, Ipl : .27, and Ip2 = .37.
Ic Ipl ng Ipl—Ip2+.1o
605 061 000 O7
502 056 003 063
4. .51 .03 .53
30 .50 013 04/
2. .42 .20 .32
.10 035 030 01.5
00 .27 037 000
'10 021 043 “012
-20 018 04/ -019
‘40 009 030 '031
-502 003 050 -037
“605 000 050 ‘040
If = 1009 Rp I 2000 Ep = 20
when 10 I O, Ipl 3 013, and Ip2 : 013
10 Ipl Ip2 Ipl-Ip2-0
4 .21 .00 .21
3 .23 .01 .22
2 021 '02 019
1 .20 .05 .15
0 .13 .13 .00
"l 010 017 -007
-2 008 .20 -012
-3 003 020 ”017
_4 .OO .20 -.20

Ic Ipl 102 1p1-1p2..45
.75 1 80 3.00
2.8 $.59 2.13 2.22
2.9 3.45 2.73 1.17
2. 3.2 2.87 06
1. 3.08 3.13 '25
0. 2.85 3.30 .00
-1. 2.7 3.27 -.13
-2. 2.5 3.23 - 2o
-3. 2.29 3.20 - 46
—4. 1.98 3.43 -.9';}
-5.1 1.35 3.67 -1.87
~6.3 1.00 3.97 -2.52

If : 1.29 Rp : 10,000 Ep : 40

When 10 - 0. Ipl . 1.28. and 192: 1.33.

6. 1.45 .00 1.50

5.5 2.05 1.03 .67

4. 1.8 .83 1.02
3. 1.7 .90 .85

2. 1.5 1.07 .48

0. 1.28 1.33 .00
-2. 1.13 1.67 -.49
'40 075 1063 ‘0J3
-5.5 .40 1.63 -l.18

If = 1.29 R - 2000 :2 2 20

1711611 IC = O, :‘71 :- 038, and Ip2 : 1.10.

IC Ipl ng Ipl-Ip2—.Ol
3.1 .55 .00 .54
2.9 .59 .07 .51
2. .57 .10 .46
l- .50 .22 .27
O. 033 '37 .00
-l. .28 .43 -.16
-2’ 019 050 “032
~3- .09 .53 —.41
-4.l .04 .53 -.50
~5.l .00 .40 -.54

Since the output of this tube is controlled by
the current in the control coil, it is desirable to
know the relation which exists between the coil
current and the currents in the two plates. Curves
showing this relation for various conditions of
plate voltage, filament current, etc. may be found
on the Gurve Plates #1, #2, #3. The amount of
unbalance between the plate currents is plotted
against the current in the control coil.

Consider Curve Plate #l. The first curve
shows the result of a high degree of ionization of
the residual gas in the tube. This ionization
produced such an unstable condition ih the tube
that tde points determining the curve are widely

scattered. It is interesting to note the point of

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nflection on this curve, and on the next one also.

Ho

This may be explained as follows. Below the point

of inf ection the curvature is due to the approach

of saturation - the entire filament emmission is
being concentrated on one plate. At this point,

the concentration of electrons seems to cause the

gas to become mudh more highly ionized - or it may be
that there are two gases in the tube, and that one
does not ionize until a critical concentration is
reached. This change was accompanied by a sudden
increase in the currents from both plates, and by a
marked change in the illumination of the ionized
vapor. Saturation is again being approached at the upper
end of the second curve.

The third curve indicates a very stable pperating
voltage. The characteristic curve is nearly a straight
line. It appears from this that thirty volts is the
best Operating potential for the tube.

The last curve also shows a very stable condition,
but the shape of the curve indicates that saturation
would be reached at a much lower value of plate
current than on the preceding curve.

The first two curves show the effect of a highly
ionized condition of the gas in the tube. Curve three

was taken just on the verge of ionization. Curve

four might be a reproduction of curve three to a
smaller scale, since Operation below the ionization
point is stable.

Curve Plate #2 differs from the one just
considered in that the leads to the control coil
were reversed to get the maximum deflection in both
directions. These curves were taken at a lower filament
temperature, and consequently do not show the effects
of complete ionization. Filament temperature seems
to have had much more effect on the degree of ionization
than the plate voltages. The fact that the curves are
unbalanced is probably due to lack of symmetry in the
mechanical construction of the tube. The most noticeable
thing about the curves is their resemblence to tne
characteristic curves of the ordinary three electrode
tube.

The first two curves on Curve Plate #3 were
taken with the same plate voltages but with different
resistances in the plate leads. The resistance
served two purposes. It was protection for the
instruments in the circuit and to obtain conditions
that would approach the conditions of dynamic Operation.
The last curve shows the effect of lowering the voltage
on the plates. It has been impossible to find a
suitable eXplanation for the irregularities of the

first curve.

A consideration of the curves indicates that, for
certain voltages at least, straight line amplification
can be expected over a rather large range.

Another thing to determine that is very important
is the ratio of power input to the power output, or
the amplification ratio of the tube. This was found to
be a difficult thing to determine, eSpecially when there
was but little resistance in the plate circuits. One
way was to find the ratio between the output c0pper
loss and the input COpper loss, since no power is involved
except the losses. Take Fig. 2 on Curve Plate #3. It
appears that a maximum current output of about one and
one-half milliamperes is available without a serious
introduction of harmonics. This output calls for a
maximum of six amperes in the control coil of thirty-
four turns, or two hundred four ampere turns in a coil
of three inches in diameter. These values would
correspond to effective alternating currents of
1.05 milliamperes, 4.2 amperes, and 142.8 ampere turns.
With 10,000 ohms in each plate circuit, the effective
A. C. power output would be

.m2 x 20,000 = .022 watts.
For a coil of 34 turns, the current would be 4.2
amperes and the resistance would have to be

.022

I776 ; .00125 ohms.

This would call for a coil of larger than #1 wire, to
have an amplification ratio of one to one. With a coil
of 130 turns the current would be 1.43 amperes. The
resistance for this coil being .022 or .0107 ohms.
This would call for about #6 wire? Ogor a coil of 500
turns, the current would be .286 amperes and the
allowable resistance, L922 or .269 ohms. The length
of wire would be about 158$§eet, and the size about #12.

For 5,000 turns, the current would be .0286 amperes

and the allowable resistance, .022 or 26.9 ohms.

——-—-.._

.000818
This would call for #22 wire. The tube, from this

standpoint would be impractical,since to obtain a
power ratio of five to one would require a coil of about
25,000 turns of #22 wire.

In an effort to find if the tube, in an ionized
condition, would respond to audible frequencies,
apparatus was connected as shown in connection #2,
Plate #4. Using a control coil of 100 turns and with
the buzzer taking an effective direct current of
from .2 ampere to .5 ampere, a clearly audible note
could be heard in the receivers. This indicates

controlled
that currents through ionized gas can beAmagnetically

at audible frequencies.

In general it may be said that both electronic

and ionic currents may be controlled by electro-

magnetic fields. Also in a tube of this type, at
a certain plate voltage, and within a rather wide
range, the controlled currents are a linear function
of the controlling currents.

0n the other hand, it appears that more power
would be required to control the output than the
output itself would amount to. In other words its
amplification would be negative.

It is quite possible that, with a more carefully
constructed tube, a circuit could be designed which

would give the tube an amplification ratio which would

make it commercially practical.

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