Input Circuit - Henry Radio 3K Classic Mark II Operating And Maintenance Manual

CLASS-C OPERAT ION Although specifically design-
ed for linear amplifier service, the 3CX1200A7 may be
operated as a class-C power arnplifier or oscillator or
as a plate-modulated radio -frequency power amplifer
The zero-bias characteristic of the 3CX1200A7 can be
used to advantage in class-C amplifiers operating at
plate voltages of 4000 volts or below by employing only
grid- resistor bias If driving power fails. plate dissipation
is then kept to a low value because the tube will be
operating at the normal stalic zero-bias conditions
FILAMENT OPERATION The rated frlament voltage
for the 3CX 1200A 7 is 7 5 volts Filament voltage. as
measured at the socket, Must be maintained within the
range of 7 81 to / 13 volts to obtain maximum tube life
For best tube hfe ihe rnrush c:urrent to the fdament
should be limited to two limes normal current during
turn-on
This will minimize thermal stress on the
thorrated-tungsten filarnew wire. which can cause
internal tube geometry changes with repeated cycling
HIGH VOLTAGE Operating voltage tor the
3XC 1200A 7 can be deadly. so the equipment must be
designed properly and operating precautions must be
followed Design equipment so that no one can come
in contact with high voltages All equipment must
include safety enciosures for high voltage circuits and
terminals, with interlock switches to open the primary
circuits of the power supply and to discharge high
voltage capacitors whenever access doors are opened
Interlock switches must not be bypassed or "cheated"
to allow operation with access doors open Remember
that HIGH VOLTAGE CAN KILL
INT ERMODULATION DISTORTION Typical operat-
rng conditions with distortion values included are the
result of data taken during actual operation at 2
megahertz Intermodulation values listed are those
measured at the full peak envelope power noted
INTERELECTRO[)E CAPACITANCE The actual rnter-
nal interelectrode capacitance of a tube is influenced
by many variables in most applications such as stray
capacitance to the chassrs, capacitance added by the
socket used, stray capacitance between the tube
termrnals, and wiring effects. To control the actual
capacitance values within the tube as the key com -
ponent involved, the industry and military services use
a standard test procedure as described in Electronic
Industries Association Standard RS-191 This requires
the use of specially constructed test fixlures which
effectively shield all external tube leads from each
other and eliminate any capacitanco reading to
"ground " The test is performed on a cold tube Other
factors being equal, controlling internal tube capaci-
tance in this way normally assures good interchange
ability of tubes over a period of time. even if the tube is
made by different manufacturers The capacitance
values shown in the manufacturer's technical data. or
test specifications. normally are lakon in accordance
with Standard RS-191
The equipment designer is therefore. Cautioned to
make allowance for the actual capacitance values
which will exist in any normal application Mc:asurc^
ments should be taken with thesoe:ktrt and mounting
which represent approximate final layout it capacitance
values are highly significant in the design
INPUT CIRCUIT
When the 3CX 1200A/ is operated
as a grounded-grid rf amplifier. the use of a resonant
tank in the cathode circuit is recornmended in order to
obtain greatest linearity and power Output I or best
results with a single-ended amplifier it is Suggested
that the cathode tank circuit operate at a 0 of two or
more
SPECIAL APPLICAT IONS If it is desired to opera lF^
this tube under conditions widely different from those
given here, write to the Varian Electron Device Group
Sales Office or the Product Manager. Varian EIMAC.
1678 Pioneer Rd. Salt Lake City. UT 84104, for
information and recommendalion
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