Western electric 338 a brochure

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BELL SYSTEM PRACTICES SECTION A346. 338A
Transmission Engineering and Data Issue 1, October 1962
Electron Tube Data A. T.& T. Co. Standard

Wafer/7 flair/2
338A Vacuum Tube

Classification-Three element, argon filled ,,
cathode

It is primarily a rectifier of
the relative instantaneous grid a
relay or trigger-action dev'c
oscillator giving a squar
sweep-voltage for a linear t1

we 05 as a variable-voltage rectifier.

Dimensions '63 ime sions and outline diagrams are given in Figures 1 and 2. The overall

dimensions are: W
mum length. ................................ 4%

Diameter ........................................ 1%

Moun g-This vacuum tube employs a standard five-pin thrust type base suitable for use in
a Western Electric 141A or similar socket. The arrangement of electrode connections to the base
terminals is shown in Figure 2.

It may be mounted in either a vertical or horizontal position, although the vertical position
is preferable.

FILE: THYRATRON SECTION

(g American Telephone and Telegraph Company 1962

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338A

Heater Rating
Heater potential ..................... . . . 10.0 volts
Nominal heater current ............. . . . . . 0.5 ampere
Required heating time ................. 60 seconds

The heater element of this tube is designed to operate on a voltage basis from a direct or
alternating current supply. The voltage should be maintained to within 5% of its rated value
(10 volts). Operation of the heater element 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. Failure to allow sufficient time may result in
immediate failure.

Operating Conditions

Approximate tube voltage drop . ......... . . . . 15 volts

Max. peak voltage between anode and grid . . . . 325 volts

Max. instantaneous anode current . . . ....... . . . . 0.600 ampere
Max. average anode current ...... . . . . ............ 0.100 ampere
Max. time of averaging anode current. . . . . . ...... 5 seconds
Max. instantaneous grid current ....... . . . ...... 0.010 ampere
Max. voltage between heater and cathode. . ...... . 50 volts
Operating ambient temperature range. . . . . . . . . -20° to +50°C.
Normal deionization time ....................... 1000 microseconds

The characteristics of the 338A tube are such that, for any given anode potential, there is a
critical grid potential. It the grid is held more negative than this value and the tube is non-con-
ducting, the anode current will remain zero. If it is made less negative, the current will assume a
value determined by the applied potential and the resistance in the anode circuit. To extinguish
the discharge and return the current to zero, the positive anode potential must be removed.
When current is flowing a visible discharge occurs in the tube. Under this condition, the tube
voltage drop is practically independent of the value of both the anode current and the grid
potential. A protective resistance should always be included in the circuit to limit the anode
current to the rated values. A typical curve relating the critical grid potential to the anode
potential is shown in Figure 3. This charactersitic may vary from tube to tube and during the
life of a given tube.

Sufficient resistance must always be included in the g-n'd circuit to limit the negative grid po-
tential 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 adapted to the application of thyratrons. Two
general types are common. One use of the tube is to produce a sawatoothed, 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 device. 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? is a simplified circuit employing a photoelectric cell in place of the
resistance, R, used in the phase shifting device in Figure 6. The photoelectric cell, however, is
equivalent to a variable resistance in the sense that the current passed will depend upon the
amount of light falling upon it.

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