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connected
in series
with
two
parallel connected will give approx-
imately
250 ohms.
As
light bulbs are highly reactive, the actual
load will be considerably removed from the proper condition
but will serve
to
allow
a
good preliminary checkup.
As the
power amplifier tank circuit
is
directly affected by
the load,
it is
well
to
discount this in the subject
of
loading conditions.
The power amplifier tank
is
usually provided with
a
padding capac-
ity
C33 in
addition
to
the
variable tuning capacity
C32.
The
tank capacity and the coupling capacities C34 and C35 determine
the harmonic suppression,
a
chart
of
recommended values for
various frequencies and common loadings
is
given elsewhere in
this instruction book.
Special loading conditions may require
modification, and frequently the addition
of
a
series condenser.
In
this latter case, the condenser
is
between
the line coil L11
and
meter
Ml.
The
proper loading
of the
transmitter
to
the
particular condition must be admitted
as
more or less on
a
cut
-
and -try basis, adjusting C34 and the taps on coil
1,11;
maintaining
the
final power amplifier in complete resonance will eventually
produce
the
desired results where
the
load
is not
below
30
ohms
or exceeds 300 ohms.
Final current into the line load on the
formula I2R
is,
of course, the
answer
to
the correctness of the
load.
The
efficiency
of the BC
-250GY transmitter
is
70% and
seldom over
75
%.
It
should be remembered that efficiency
is
measured
into
a
known load,.and preferably
a
resistive load
designed for radio frequency operation.
It
should also be re-
membered that
a
radio transmitter
of
standard construction
is
not
normally inefficient
when, of course,
properly adjusted
and
tuned.
Thus, lack of
efficiency in operation can usually always
be attributed
to
lack of proper loading conditions
to
the trans-
mitter itself, which
is
usually indicated by unbalanced
line
currents in the case
of
a
transmission
line, and of course, can
be checked by making sure ground connections
to
the
transmitter
are
well nade directly
to the
radiating system
of
the antenna.
Abnormally high efficiency
is
quite frequently found to be in
the
antenna system.
This may be checked
if
a
dummy antenna
of
equavalent resistance
is
substituted for
the line or antenna.
It
may
be
found that the line current meter
is
measuring reactive
currents, which would give
a
false impression
of
efficiency.
9/11/63
-12-
BC- 250GY
-1
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