Western electric 297 a brochure

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SECTION AB46. 297A
Issue 1, October 1962
A.T.& T. Co. Standard

BELL SYSTEM PRACTICES
Transmission Engineering and Data.
Electron Tube Data

Weary/'11 Electric
297A Vacuum Tube

Classification-Three element, argon filled, fl :

This tube 15 a. rectifier of low internal impedance 1 .
may be controlled by the grid. It 15 intended for use i , rcuits as a relay or trigger-action
device. A few of us other possible uses are: as a contro" - requency oscillator giving a square
waveform, as a voltmeter or volume level-indicator, or as a source of sweep-voltage for a linear
time axis.

, starting of the conduction cycle

Dimensions-The dimensiw' .

diagrams are given in Figures 1 and 2. The overall
dimensions are:

Maximum length. . . .

. "-e employs a standard four-pin thrust type base suitable for use in
-,-' similar socket. The arrangement of electrode connections to the base

termi
T i. w be mounted in either a vertical or horizontal position, although the vertical
position ' ' le. If mounted in a horizontal position the plane of the filament, which is indi-

cated 1n re 2, should be vertical.

Filament Rating
Filament voltage ............................................ 1.75 volts
Nominal filament current ................. 0.350 ampere
Required filament heating time ............................... 2 seconds

FILE: Thyratron Section

© American Telephone and Telegraph Company 1962

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297A

The filament of this tube is designed to operate on a voltage basis. The voltage should be
maintained to within 5% of its rated value (1.75 volts). Operation of the filament above the upper
limit will definitely reduce the life of the tube, while a decrease below the lower limit may cause
immediate failure.

Sufficient time should always be allowed for the cathode temperature to reach its normal
operating value before anode current is drawn. If filament transformers with good regulation
are used this time is 2 seconds. Failure to allow sufficient time may result in immediate failure.
If instantaneous anode currents less than 10 milliamperes are desired with anode voltages less than
50 volts, anode current may be drawn simultaneously with the application of filament voltage;
but approximately 2 seconds will be required for the anode current to reach its final value.

Operating Conditions
Approximate tube voltage drop ........................ 20 volts
Maximum instantaneous anode current. , , . . . . . , . . , . . . . . . 60 milliamperes
Maximum average anode current ....................... 10 milliamperes
Maximum time of averaging anode current ............... 0.5 second
Maximum peak voltage between anode and grid ........... 250 volts
Maximum instantaneous grid current .............. 10 milliamperes
Operating ambient temperature range ..................... -20° to +50° C.
Nominal deionization time ............................... 100 microseconds

The characteristics of the 297A tube are such that, for any given positive anode potential,
there is a critical grid potential. If the grid is held more negative than this value and the tube is
nonwconducting, the anode current will remain zero. If it is made less negative, the tube becomes
conducting, and the anode current assumes a value determined by the applied anode potential
and the impedance in the anode circuit. When the tube is conducting, the tube voltage drop is
practically independent of the value of both the anode current and the grid potential. To extinguish
the discharge and reestablish control by the grid, the anode potential must be reduced to zero or

made negative for a period at least as long as the dcionization time (100 microseconds).

A typical curve relating the critical grid potential to the anode potential is shown in Figure 3.
This characteristic may vary from tube to tube and during the life of a given tube.

The maximum anode current is specified in terms of an instantaneous value (60 milliamperes)
and an average value (10 milliamperes), with a maximum period of averaging of 0.5 second. These
are maximum limitations and should not be exceeded.

Sufficient-resistance must always be included in the grid circuit to limit the negative grid poten-
tial to 10 volts when anode current is flowing. Failure to observe this precaution will result in
short tube life.

Typical Circuits

The tube may be used in a variety of circuits. Two general types are common. One use of
the tube is to produce a saw-toothed, current wave. The circuit for this application is shown in
Figure 4. The resistance R should, ordinarily, be at least 100,000 ohms, and the product RC
(C in farads) approximately equal to the desired fundamental period.

The second general use for the tube is as a relay. In this application the anode may be supplied
from either alternating or direct current. When supplied from direct current, the circuit, Figure 5,
possesses a lock-in" feature, since the anode potential must be removed momentarily in order to
restore the tube to the non-conducting condition. When supplied from alternating current, the
circuit possesses no lock-in feature, but the average anode current may be controlled by the
relative phase of grid and anode potentials. The schematic circuit for this application is shown
in Figure 6. Figure 7 is a simplified circuit employing a photoelectric cell in place of the resistance,

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