Western electric 257 a brochure
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Page 1
257A
BELL SYSTEM PRACTICES SECTION AB46.305
Transmission Engineering and Data Issue 1, September 1936
Vacuum Tube Data A T & T Co Standard
Western Electric
257A Vacuum Tube
Classification-Small, filamentary triode
The 257A tube is similar to the 231D tube except that the grid terminal of the 257A tube is
at the top of the bulb. An important feature of the 257A tube is its low filament power consumption.
Applications
Audio-frequency and intermediate-frequency amplifier.
Detector.
Oscillator.
Dimensions-Outline diagrams showing dimensions of the tube and base, and the arrangement
of the electrode connections to the base terminals are given in Figures 1 and 2.
Base-Small, four-pin, thrust base. Small metal cap grid terminal at the top of the bulb. One
base contact pin is unconnected.
Socket-Standard four-contact type such as the Western Electric 1433 socket.
Copyright 1936. Western Electric Company, Incorporated
Page 2
Mountings Positions-Either vertical or horizontal. If mounted in a horizontal position the
plane of the filament, which is indicated in Figure 2, should be vertical.
Average Direct Interelectrode Capacitanees
Grid to plate. . .. .. ,,,,, .. ................ .. .. .. 3.0 ppf.
Grid to filament . . . . ............... . . . . . . 1.8 upf.
Plate to filament ...... . . . . . . . . .................... 2.3 upf.
Filament Rating
Filament current. . . . .. . ...... .. ... ..... .. 0.060 ampere, d.c.
Nominal filament voltage ,,,,,, . . . .............. . . . 3.1 volts
The filament of this tube is designed to operate on a current basis and should be operated at as
near the rated current as is practicable.
Characteristics-Plate current characteristics of a typical 257A tube are shown in Figure 3 as
functions of grid voltage for several values of plate voltage. The grid and plate voltages are meas
sured from the negative end of the filament. Corresponding amplification factor, plate resistance,
and transconductance Characteristics are given in Figures 4, 5 and 6, respectively. Plate current
characteristics as functions of plate voltage are shown in Figure 7 for several values of grid voltage.
Operating Conditions and Output-Permissible operating plate and grid voltages are in-
cluded within the area, ABCD. in Figure 3. Amplification factor, plate resistance, transconductance,
and performance data are given in the table on page 3 for a number of typical operating conditions
represented by selected points within this area. The less severe operating conditions should be
selected in preference to maximum operating conditions wherever possible. The life of the tube
at maximum conditions may be shorter than at less severe conditions.
The performance data include the fundamental power or voltage output and the second and
third harmonic levels for the indicated values of load resistance. The fundamental output is given
in terms of the power, Pm. in milliwatts for values of load resistance, R, equal to and double the
value of the plate resistance, rp, and in terms of the voltage, Epm, in peak volts for values of load
resistance five times the plate resistance. The second and third harmonic levels, Fzm and Fm,
are given in decibels below the fundamental in each case. The peak value of the sinusoidal input
voltage, Elm, is numerically equal to the grid bias for each operating condition. For a smaller input
voltage, E3, the fundamental power and voltage output and the harmonic levels are given approxi-
mately by the following relations:
gm
E3
F3=F3m + 40 logic
[2]