The Birmingham age-herald. [volume] (Birmingham, Ala.) 1902-1950, 1922-09-24
PUSH ANDPULL
AMPLE TER SET
IS DESCRIBED
System Works on Principle
Similar to Steam Engine
With Two Valves
ny JACK BINNs

lily JACh BINNS
One of the most unique amplifica-
ton systems is the so-called push
and pull circuit. in which two
vacuum tubes are hooked up to-
gether. The operation of this system
has been constant puzzle to the
novice, especially to those who own
one of the commercial loud-speak-
ing units embodying this principle.
The. commercial type consists of
two stages of application, but uses
three vacuum tubes. Now the puzz-
ling feature of This set is that either
of the two last tubes can be taken
from their sockets and the set will
continue to work. The reason, OF
course, Is that the two tubes are
Joined together in a special manner
in the last stage of application.
At first glance the reason for this
is not at all apparent. but it can be
taken fsr granted that the set works
at greater efficiency and with less
distortion. than in the case of
straight amplflcation. This will be
more readily seen from the analogy
which will be given here.
Vacuum Tube ms a Valve
Before outlining the analogy, how
over, ii would be we'll to point out
once more that the vacuum tube is
primarily a valve opening and clos
ing all the time the set is in opera
ton, permitting current to flow in
in one direction enly. and checking
the flow in the other direction, just
the same as an ordinary valve does
in wataE system.
Now it stands to reason that two
way valve system should be more
efficient than a single valve, and 1t
10 ~
and pulp circuit is based. Perhaps
the best way to understand this is
to take the case of the steam engine
| employing single cylinder which
is driving fly-wheel.
If this engine is equipped with a
single valve it will operate all right.
but it will have to rely upon the
momentum of the flywheel to bring
the piston back to position where
the steam will impart fresh power to
1t when the valve Is open.
How the Two Valves Work
Suppose instead of single valve
at one end of the cylinder we en-
pioy two valves. one at each end. AS
soon as the valve in one end is
open the piston will be pushed out
and the flywheel will make half a
circle in it's rotation, and the piston
! the cylinder.
| In this condition with single
valve the flywhee1 would have to
rush the piston back by means ofeits
own momentum before the valve
could act upon IL again. With the
/ two valves. however. as soon as the
| flywheel has completed It's half cycle.
| the second valve comes into play,
and the flow of steam through 1t
pushes the flywheel around Athe
remaining half cycle until the first
valve can come into play again.
| 1t will be seen from this, there
fore, that there is great increase
in efficiency in the two-valve sys-
tenn, because new power is impart-
ed at critical moment, instead of
power being wasted to keep the
system in operation. The whole
| cycle of operation, of course, can
easily be made automatic in its ac-
| ton.
Cribs Are Opposed
The same is of course true In the
case of the push and pull" vacuum
tube principle of implication. The
two grids of the tubes are Joined to
the same coil in the secondary trans
former, with common lead in the
center of the coil. This means that
the two grids are therefore opposed
to each other.
The consequence is that when one
grid is positive, the other Will be
negative. One will be assisting the
flow of electrons to the plate of the
tube in which it is located. at the
same time that the other is resisting
the flow of electrons. This action Is
therefore somewhat similar to that
Ct the two-valve cylinder in the
steam engine which we have just
considered. Moreover, there will be
less distortion in the telephones. be-
cause the output of the system is
completely controlled and well bal
anced. In addition the efficiency
Is at its highest possible order.
In this system the use of C
battery (cr battery for the purpose
/ OF maintaining a negative bias on the
grid is necessary. In addition 10
this it is best to use transformer
in the plate circuit of the last two
tubes which are Joined together. The
plates of the two tubes VIII be JOIN
ed to the primary of this transformer
in such manner that one plate Is
Joined to one end of the transformer
and the other plate is joined TO
the other end of the transformer.
lead is then taken from the center
of the primary winding and connect
ed to the positive terminal OF the
81 battery.
Using this system in special re-
ceiver, consisting of a regenerative
detector and three stages OF ampiific-
ton. Paul c. Hoernel. an electrical en-
eoineer, has been able TO project
speech and music received by. radio
Sc that it could be heard clearly ana
distinctly at distance or Two miies
in open air. OF course, In TINS con-
nection he used a horn of special
design. about six feet IN length and
almost three feet In width at the
mouth. This set was tried out at
Babylon, L. /.
All you fellows who go to Cen