New-York tribune., 1922-09-24
Push and Pull
Annplifier Set
Is Described
System Works on Principle
Similar 10 Steam Engine
Which Has Its Cylinder
Fitted With Two Valves
Has Greatest Efficiency
Grids Are Opposed TO Each
Osher SO That Tubes Are
Working All of the Time

One of the most unique amplification
systems is the so-called push and
pull" circuit, in which two vacuum
tubes are hooked up together The
operation of this system has been a
constant puzzle to the novice, especi-
ally to those who own one of the COM
mercial loud-speaking units embodying
! this principle.
| The commercial type consists of two
! stages of amplitication, but uses three
vacuum tubes. Now the puzzling fea-
ture of this set is that either Of the
two last tubes can be taken from it's
| socket and the set will continue to
| work. The reason, of course, is that
the two tubes are Joined together in a
special maner in the last stage Of
amplifi cation.
At first glance the reason for this
is not at all apparent, but it can be
taken for granted that the set works
at greater efficiency and with less dis
tortion, than in the case OF straight
| amplification. This will be more read
| lily seen from the analogy which will
! be given here.
Vacuum Tube Is Valve
| Before outlining the analogy, how
ever, it would be we'll to point out
once more that the vacuum tube is
primarily valve opening and closing
all the time the set is in operation,
permitting current to flow in one direc-
tion only, and checking the flow in the
other direction, just the same as an
| ordinary valve does in water system.
! Now 1t stands to reason that two
way valve system should be more eff
clent than single valve, and it is on
| this rpoSOnipt hat stp,,cL o,,J o,,Ii',
circuit is based. Perhaps the best way
to understand this is to take the case
Of the steam engine employing a single
cylinder which is driving a fywheel.
| III this engine is equipped with a
single valve it will operate all right,
but 1T will have to rely upon the mo
mentum of the flywheel to bring the
piston back to a position where the
steam will impart fresh power to it
when the valve is open.
How the Two Valves Work
Suppose instead of a single valve at
one end of, the cylinder we employ two
valves, one at each end. AS soon as
| the valve in one end Is open the pis-
ton will be pushed out and the ify-
wheel will make half circle in its
rotation and the piston head will be
at the other end Of the cylinder
In this condition with a single valve
the flywheel would have to push the
piston back by means of its own mo-
mentum before the valve could act
/ upon it again. With the two valves,
however, as soon as the fiywheel has
completed its half cycle, the second
valve comes into play, and the flow of
steam through it pushes the fywheel
around the remaining half cycle until
the first valve can come into play
again.
1t will be seen from this, therefore.
that there is great increase in eff
cheney in the two-valve system, be-
cause new power is impaired at
critical moment, instead of power be-
ing wasted to keep the system in op-
GRAFTON. The whole cycle of opera
tion, of course, can easily be made
automatic in its action.
Grids Are Opposed
The same is of course true in the
case of the push and pull" vacuum
tube principle of amplifcation. The
two grids of the tubes are Joined to
| the same coil in the secondary of
! transformer, with common lead in the
center of the coil. This means that
| the two grds are therefore opposed to
each other.
The consequence is that when one
grid is positive, the other will be neg
ative. One will be assisting the flow
of electrons to the plate of the tube
in which it is located at the same
1 time that the other is resisting the
flow of electrons. This action is there
lore somewhat similar to that of the
| two-valve cylinder in the steam engine
which we have just considered. More
over, there will be less distortion in
the telephones, because the output of
the system is completely controlled
and we'll balanced. In addition the
efficiency is at its highest possible
order.
In this system the use of &c" bat
terry Cor a battery for the purpose of
| maintaining negative bias on the
grid is necessary. In addition to this
! 1T IS best to use a transformer in the
! plate circuit of the last two tubes
which are Joined together The plates
OF the two tubes will be joined to the
primary OF this transformer in such
| manner-that one plate is Joined to one
! end of the transformer and the otner
! plate Is Joined to the other end of the
! transformer. A lead is then taken
from the center OF he primary winding
and connected to the positive terminal
of the "B' battery.
! Using this system in a special re-
| ceiver, consisting of a regenerative de-
! tector and three stages of amplifica-
tion, Paul c. Hoernel, an electrical en-
gineer, has been able to project speech
and music received by radio so that it
could be heard clearly and distinctly
at distance of two miles in open air.
| Of course, in this connection he used
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. T.
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