The Holly chieftain., 1924-01-10


simplicity Of operation in a multi
stage radio frequency amplifier have
caused many a broadcast enthusiast
no end of trouble, according to r. F.
Humphreys in the New York Evening
Mail. OF course, the greatest selectiv-
,lty obtainable in an amplifier Of this
type. consisting Of two or more stages,
ls afforded by the use of tuned or semi
tuned transformers as interstage coup
lings. On the other hand, this method
Of linking up the successive tubes of
a radio frequency amplifier involves
such multiplicity Of tuning controls
and is generally so selectivs that 1t
lacks all operating flexibility, and
therefore is of no practical value
either to the novice or to the listener
In who abhors "knob winding sets" Gas
they have appropriately been called).
Permanent Tuning.
Primary and secondary windlngs of
a tuned radio frequency transformer
are commonly shunted by small varia-
BILE condensers of equal capacity,
which must be progressively rotated in
unison to the left or right In the
course Of hunting station. 1t is
necessary that the primary and sec
ondary circuits Of this transformer
always be In tune, or very nearly SO.
II the signals amplifed by the first
tube are to be passed on to the see
ond, etc. The semi-tuned transformer
comprises one tuned winding, usually
the secondary. and this is controlled
by small variable condenser. Trans
formers Of the foregoing types have
air cores.
With the advent of the so-called
"untuned radio frequency transform
er," an instrument designed to respond
over a band OF wave lengths with
varying degrees of efficiency has come
into widespread use in the broadcast
receiving field. These transformers
differ essentially In their operation
from those previously described in
that they require no tuning ADJUST
ments and lack selectiyity. Some
transformers Of this order have a
working range Of from ITO to 45O
meters, while others possess range
of from approximately 200 to $50
meters.
The band Of wave lengths over
which interstage radio frequency coup
>lings Of this type will respond de-
pends on their construction entirely.
The core used in certain efficient
transformers Of this character con-
sists of thin laminae of silicon steel.
There are also air core untuned radio
frequency transformers in popular use
for broadcast reception, but while
these are generally more efficient
than those Of the steel core type at
some particular wave length, their
working range is considerably limited.
Instruments of this type may operate
efficiently over wave band Of $50
meters. giving their strongest response
at wave length at about the middle
Of this band.
Tuned Impedanee.
Tuned impedance, as form Of in
terstage radio frequency coupling, has
come into vogue with some promi-
nence during the past, year. 1t affords
selectivity with simplicity of control
and takes the form Of variometer
or a coil comprising n few turns of
wire with a low capacity variable
condenser in shunt. This coupling is
adjusted to the wave length desired
by manipulation Of the controlling
knob. The windings Of the variome-
ter or the coil, depending on which is
used, are common to the output cir
cult OF one tube and the input circuit
Of the tube which follows. Great Sc
lectivity is obtained with the tuned
lmpedance by employing 1t consistent
ly throughout the amplifier and be-
tween the last amplifier tube and the
detector.
The circuit herewith illustrated en-
braces two stages of radio frequency
amplif1cation and a detector. The
output circuits of the first and second
stages Of the ampllfier are equipped
with IN untuned iron core transform
er and tuned impedance respective
ly. Since the antenna tuning portion
Of this circuit does not afford very
selective tuning, as is true of most
single coil tuners when used In this
position, the matter of minimizing in
terference between broadcast pro-
grams is left TO the interstage radio
frequency couplings which follow.
In the foregoing text it was pointed
out that high selectlvity may be OBJ
tanned between successive stages of
amplification by employing tuned or
semi-tuned transformers or by the use
OF tuned impedances. From the point
Of installation and operation, perhaps
the most easily adopted of these is the