Palisadian., 1925-10-09
BY SIDNEY E. FINKELsTElN,
In New York HeraId-Trihune


BY SIDNEY e. FINKELSTElN.
In New York Herald Tribune.
The neutrOdyne may be successfully
refiexed. even as to both stages of
transformer-coupled audio amplifica-
uon. SO that only three tubes are
needed instead Of five. In practice 1t
has been found that this maximum
renexing Of the neutrodyne Is best
achieved by resOrting to the inverse
duplex system. This is designed to
spread the load more evenly over the
tubes The first audio stage is reflex
In the second radio-frequency stage,
while the second audio stage is in the
hrst radio step. Thus, as the first
radio stage handles the lightest radio
load, this same tube Is called upon to
ampi1fy at audio frequencies the final
output. which is by far the greater
AS the same system is used in both
ampiifying tubes, the load Is about
equal in both tubes, thus minimizing
distortion. Indeed, the tone quality
from the receiver shown In the dia-
gram Is delightful.
The best tubes to use for this cir
cult are those requiring a six-volt stor-
age battery. They serve best because
OF their greater ability to stand the
compound amplificatiOn due to the
larger elements.
The set Is one that cannot be ex-
pected to give perfect results instantly.
No such expectation should be cher
ished regarding any home made reflex
Size Of Panel.
1t Is not circuit for the novice to
be tco confident over. as some skill
In the placement OF parts, avoiding of
stray feed-back at radio and audio fire.
quencies is necessary The set, how
ever, may be well constructed on a
by 1S-inch panel, using a Dashboard
about S1s inches deep.
The set, once In good working or
der, will be found on a par with the
standard neutrodyne where four tubes
are employed. The volume Is great
and the seIectivity sufficient to meet
the needs of the day.
The audiO transformers preferred
for this circuit are of the type entirely
inciosed in metal housing. The vari-
able condensers may be instruments
you have around the house now, the
coils being constructed SO that the
broadcast range will be covered
However. if 1t is desired that the diaIs
tune In step, then not only should in
strumentS OF the same maximum ca
pacity be used. but they should be
condensers made by the same manu-
facturer. This necessity arises from
the divergence of actual maximum ca
pacity as among condensers of differ
emf manufacture. Every condenser
rated at COOS mid. has not just that
maximum actually. Sometimes the
variation Is as much as 20 per cent.
However, the best makes of con-
densers are so close to their rated
capacity that no regard need be paid
to the difference. If the same make is
used. so far as coil-winding is con-
cerred.
Construction Of Coils.
In constructing the coils use fiber
or hard rubber tubing, 31h inches
diameter, 4 inches high, the wire be-
ing No. "a double silk covered
throughout. The coils are wound with
the primary at one end, one-half inch
separation being left after the primary
is completed and the secondary being
wound next Tiny parallel holes are
drilled In the tubing for threading the
wire terminals and thus making them
secure. Enough excess wire Is left
at the beginning and end of each wind
Ing to utilize it for actual connection
inside the set, instead of using bus bar
or other such connecting wire for the
purpose. About six inches Of excess
will suffice. What Is not needed may
be cut Off later when the connections
are sOldered
The primary Of the aerial r. F.
transformer. extreme left, has 20
turns. The secondary, shown next to
1t, has 40 turns, untapped. The two
other primaries consist of IS turns
each, hence these interstage coupling
primaries are different from the aerial
primary. Also, the secondaries are
different. in that a tap is taken at the
fifteenth turn. A tiny loop is made
and then grasped between the thumb
and index finger and twisted around
twice. Then the insulation is scraped
off the tap, so that a wire may be !
soldered from the tap to one side of