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SQPt 17, 1946.
Filed Oct‘. 31, 1944
my; M04765 ‘
Patented Sept. 17, 1946
Robert F. Holtz, Merchantville, N. J., assignor to
Radio Corporation of America, a corporation of,
Application Gctober 31, 1944, Serial No. 561,280
4 Claims. (Cl. 315—-1l9)
This invention relates to the protection of elec
tron tubes, and more particularly to the protec
tion of such tubes from damage which may be
present invention, and
external circuits
the possibility of abnormal current ?ow in two
associated circuits of a tube; and it is sometimes
undesirable, on grounds oi‘ cost, and more par
indicated generally at 29, for the anode, and the
total impedance of this circuit (including imped
Thus, in
the generation of high frequency heating currents, anode current rises ‘steadily during the
heating cycle; towards the end of the cycle, and .
sometimes in the middle of it,‘ a substantial in
crease of anode current may occur which, if con~
tinned, will cause damage to the tube, and may
at least cause it to lose emission.’ There is also
grid circuit, and this may be either in excess of
or below the normal operating grid current. An
abnormal ?ow of grid current will not always be
re?ected in a change of anode current, particu
larly when the electrical characteristics of the
load arechangin‘g; and it may be overlooked by
It is, therefore, an object of the invention to
provide an improved method of and means for
protecting‘an electron tube from damage which
may be‘ caused by abnormal current ?ow.
Another object is to provide an improved, com
pact‘ and unitary relay for the prevention of ab
normal current ?ow
two associated circuits of
an electron tube.
A further object of the invention is to provide
ance of the circuit within the tube itself) is rep
resented by the ‘block marked Zp. Similarly, the ,
block Zg represents the total impedance of the‘
grid circuit 22 of the tube.
A resistor AC is connected partly in the anode
circuit and partly in the grid circuit; the resistor
is also connected at an intermediate tapping point
B to the cathode l2 by a conductor 2,4. Point B
is so chosen on AC that the voltage drop from A
to B is equal to that from B to C. An example
will illustrate what value to‘select‘for the resistor '
AC and how to determine the point B.
An oscillator in commercial use in high fre
the possibility of abnormal current ?ow in the
An electron tube ii) is represented as having a
cathode l2, a grid It andan anode l6. The anode
circuit iii of the tube includes a source of power,
for this purpose are well-hn-own, it is often neces
sary to afford protection at the sometime against
tion to a tube with more than three electrodes.
While over-current and under-current‘ relays
an unskilled operator.
Figure 2 is a schematic drawing similar to Fig
ure 1, but showing the application of the inven
caused by abnormal current flow in two of its
ticularly of space, to use two relays.
s-ociated circuits and embodying the relay of the
quency industrial heating consists of two RCA
Type 892—R tubes connected in parallel. These
tubes are very expensive, and ‘it'is highly desir
able that they should not be operated ‘at other
than their proper ratings, particularly for any
length of time. In normal operation, D. Generic
current should be of the order of 3.5‘ amps, and
grid current approximately 35,0 m. a. If :a loss of
100 volts can be tolerated in the anode circuit due
to the relay, then BC may have a ‘resistance of
approximately 30 ohms. Since the voltage drop
DD :21 across AB must equal that across BC, the resist
ance of AB will be approximately 3.00 ohms, and
improved means for protecting automatically an
electron tube from damage due to abnormal cur~
the total resistance of AC about 330'ohms.
rent flow which may not be evident to thelop
shunt with the resistor-S AB and BC respectively,
These objects are achieved in one embodiment
of the invention by inserting a resistor in each of
two associated circuits of a tube, in such a man
nor as to procure equal and opposing voltages in
each of the resistors. These voltages depend on
the normal flow of current in the circuits in which
they appear. A relay adapted to sic-energize the
By-pass condensers 32 and 34 are connected in
40 so that the voltage appearing across these two
resistors shall not be a?ected by current of radio
frequency ?owing in either ‘of the circuits £53
or 22.
An actuating winding ‘25 or .a relay 2% is con
nected in parallel with the resistor AC, current
in the secondary circuit to of the relay being em
ployed to cut off the supply of power from the
tube or otherwise control its operation is con
source 26. In practice, the secondary relay circuit
nected with its actuating winding in shunt with
3!} is connected with the various auxiliary controls
the resistors. Any change, therefore, in the ratio 50 or interlocked safety devices which automatically
of current in the two circuits will result in a volt
cut off the supply of power to the anode i 5 when,
for example, any of the doors giving access to the
and actuation of the relay.
apparatus are opened.
Figure l of the accompanying drawing is a
If current in the anode circuit l3 rises to 4
schematic representation of a triode with its as 55 amps, the voltage drop across BC rises to 120
age being impressed on the winding of the relay,
volts. If, therefore, it is desired that anode cur
rent shall not exceed 4 amps, the relay 28 is so
designed as to be actuated when a voltage of 20
volts appears across the Winding 26. In that
event, and if anode current remained unchanged,
the relay would also be actuated if grid current
exceeded 400 m. or fell below 260 m.
Thus, operation of the relay depends on a
variation in the ratio of current normally ?owing
in the two associated circuits of the tube, this in
turn causing a variation in the ratio of the volt
age drops across AB and BC. The circuit illus
trated constitutes abalanced bridge circuit with
the resistors AB and BC forming two adjacent
arms of the bridge, the impedances'Zp and Zg
constituting the other two adjacent arms and the
winding 26 constituting a conjugate arm of the
Many variations of the embodiment described
are within the scope of the invention. It is not
I claim as my invention:
1. A system for protecting an electron tube,
which has external grid and anode circuits, from
damage due to an abnormal‘ flow of current in
either of said circuits, said system comprising
a resistor connected in each of said circuits to
form a balanced bridge circuit on normal flow '
of current in said circuits, said resistors forming
two of the arms of said bridge, and the remain
ing total impedances of each of said circuits form
ing the other two arms of said bridge, means for
adjusting the relative values of said resistors, and
a relay for de-energizing said tube having its ac
tuating winding connected across opposite points
of said bridge, whereby on abnormal flow of cur
rent in either of said external circuits the balance
of said bridge will be disturbed and said relay
actuated to de-energize said tube.
2. Apparatus for protecting an electron tube
limited to the use of an electron tube as an 05- '
from damage due to an abnormal ?ow of current
in external grid and anode circuits, which com
cillator, but may be employed in an ampli?er, or
wherever the ratio of current normally ?owing
in two circuits of a tube is subject to variation.
The invention is equally applicable to tubes which
have more than three electrodes, in which event
theresistors AB and BC may be contained in the
two circuits of the tube whose. current is most
likely to vary. Fig. 2 illustrates the application
of the invention to a tetrode, parts which are
common to Figs. 1 and 2 being designated-by the
same reference numerals. In Fig. 2 the tube IB
prises means for deriving a potential proportional
to current flowing in one of said circuits, means
for deriving a second ‘potential equal in amount
and opposite in direction to said ?rst potential
and proportional to current ?owing in another of
said circuits, means for adjusting the balance
point of said potentials, and means responsive to
a variation in the ratio of 'said potentials for de
energizing said tube.
3. An electron tube having at least a cathode,
an anode and a grid, an external circuit connect
ing said cathode to said anode, 'a resistor in said
circuit of such value as to establish thereacross,
by reason of normal current flow in said circuit,
a voltage drop of predetermined amount, a sec
is shown as having a screen grid electrode 36 in
addition to the three electrodes mentioned in
connection with Fig. 1. A circuit 38 connects this
electrode to the cathode l2 and includes a source
of power 48 for the screen grid. If the circuits
22 and 38 are to be protected from an abnormal
?ow of current, the resistor ABC is connected in
ond external circuit connecting said grid to said ‘
cathode, a resistor in said second circuit of such
7 these two circuits in the manner indicated above.
value as to establish thereacross, by reason of ‘
Other departures from the embodiments illus
norma1 current ?ow in said circuit, a voltage drop
trated in Figs. 1 and 2 may include the use of a
gas-?lled tube or other form of relay in place of
the mechanical relay 28, and the utilization of
current in the secondary relay circuit 30 to give
an alarm or perform some other protective or
equal to that of said ?rst-mentioned resistor,
means for adjusting the relative values of said
i resistors, a capacitor connected in parallel with
each of said resistors, and means responsive to
control operation in place of, or in addition to
de-energization of the tube In.
It should be noted that'the relay will not be
‘actuated if both grid current and anode increase
or decrease simultaneously by such amounts {that
.the balanceof potential across AB and BC is not
su?i'ciently disturbed to impress the required ac
tuating potential on the winding 26. On the
other hand, the arrangement here described is CI in
more effective than a prior art relay if one of
used to create a third voltage which is employed,
in turn, to de-energize the tube.
an inequality of said voltage drops for de-ener
gizing said tube.
4. A unitary device for protecting a generator
of high frequency heating current, which has as
sociated grid and anode circuits, from damage due
to abnormal flow of current in either of said cir
cuits, said device comprising a resistor, a part of
which is so connected in one of said circuits and
the remainder of which is so connected in the
other of said circuits that on normal flow of
current in each of said circuits the voltage drops
across each of said parts of the resistor is sub
the two currents increases while the other de
stantially equal, means for adjusting the relative
There has thus been described a simple method
of and means for protecting an electron tube m values of said parts of said resistors, and a relay .
for dc-energizing said generator and having its
from damage due to an abnormal ?ow of current
actuating winding connected in parallel with said
in its associated circuits. Two voltages represent
ative of current in two of the tube circuits are
balanced, and any unbalance of these voltages is
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