New-York tribune., 1922-11-12
Push and Pull
Set With Two
Transformers
How TO Utilize Famous Sys-
tenn i'm Most Efficient Man
ner With Ordinary Stand
ard Audio Equipment
Both Connected in Series
Will Reduce Distortion by
Permitting Arnplifiers 10
Function Continuously

There is probably no system en-
pIcyed in radio reception that has
esuscd as much keen interest as the
push-pull" ampiifier. This has been
clearly demonstrated by the large mum
hcr OF inquiries that followed the
original descriptive story published in
these columns. The unexpected in
tercst is undoubtedly due to the desire
on the part of all radio fans to obtain
clearer reproduction on their loud
speakers, and particularly to eliminate
any distortion.
In the first place, OF course, il must
be clearly understood that the push
pulp system is patented one, and in
its ordinary commercial form has
specially constructed and designed
trangformers, as well as loud
speaker, built to fit in with the rest
o. the equipment. For the radio fan
no makes his own the question there
iore is how he can utilize this system
in the most efficient manner with the
afnaratus available to him.
Before describing this it may be as
we'll to outline the manner in which
the ''push-pull" system operates. 1t
IS system whereby two vacuum tubes
are joined together in one stage of
smplifcation III such manner that one
is assisting while the other is inopera
tive. 1t can be compared to a two
cylinder engine, which certainly has
better efficiency than the single cyl
inder engine. In the latter the fly
wheel has to complete ite cycle on it's
own momentum before it can again
obtain the necessary energy from the
sirgle cylinder to send it around once
more, whereas in the two-cylinder
another push from the second cylinder
~ the moment that the first cylinder
is imperative.
Tube Action Constant
Now, in the "push-pull" amplifer
system the two grids Of the two tubes
are Joined to the same amplifying
transformer in such manner that they
are opposed to each other, and the
consequence is that when one grid is
positive the other will be negative. In
other words, one win be assisting the
flow of electrons to the plate of the
Nube in which it is located at the same
time that the other is resisting the
aow of the electrons. In this manner
constant tube action is maintained
throughout, with the result that the
possibility of distortion is completely
eliminated.
To successfully employ this system
1t is necessary to have an audio-fre-
quency transformer, which should have
q middle tap in its secondary and an
other transformer with middle tap In
its primary. AS such transformers are
not being manufactured and because
of the difficulty of opening UP the wind
nf o SfnnCo T1 ~ G and
finding the exact center, system of
using two audio-frcquency transform
ers joined together is recommended
and shown in the accompanying dia-
gram.
The increased impedance in the plate
circuits of the two tubes caused by
using two transformers in series will
not interfere with the successful opera
ton of this set, and the advantages
Viii more than offset any defect.
Can Re Used ON All Sets
In the diagram a simple receiver is
shown connected to the two stage
amplifier, but, Of course, any Of the
Hsandard type Of receiving tuners can
he successfully employed by making
the usual connections.
1t should be remembered that high
ratio transformers are to be. avoided
because, beyond certain limit, the
Transformer OF very high ratio defeats
is own object.
Careful attention to the details of
the diagram should insure success with
his system. The plate circuit of the
detector tube is joined in series will,
he two primaries of the first two
transformers, which are in series with
each other.
In the case of regenerative circuits
The ticker coil Cor in the case of the
Ewo-variometer type Of receiver the
eiate variometer, will be placed in
series between the plate of the detector
tube and the first of the two trans
Iormer primaries. In the regenerative
circuit it will be necessary to place the
ilxed condenser across the two trans.
former primaries. This condenser
mould be Of about .0Q25 mid.
Two Grids to Secondary
The grid of each of the two "push-
puil' tubes will be Joined to one end
OF the transformer secondary. At the
point where these two secondaries are
ioined in series lead will be taken
60 the common connection of the entire
circuit. The diagram clearly shows the
femaining connections.
Although not shown in the diagram
the best results can be obtained by
inaintaiing a negative bias on the grids
of the amplifying tubes. The voltage
OF the G batteries necessary for this
purpose will depend upon the type of
tube that is used as an amplifier and
will have to be determined by expert
aient. A small 1@-volt fiashlight bat
CRY, which can be obtained in the five
and-ten-cent stores, will serve this pur-
pose very we'll.
Since the original description of the
apush-pulp' system was published /
haye received a number Of letters ask
ng whether or not this could be an
Dlied to radio-frequency ampliiication
IN the same manner. The answer to
his question is, NO." for the reason
hat in radio-frequency amplification
he vacuum tube is operating under diff
{erent conditions--on currents of very
high frequencies that have not been
ectified, and consequently are not sub
jcct to the possibility of distortion, as
IN the case of currents at audible fre-
fueneies.