Radio Broadcast - 1925, February - 113 Pages ... - VacuumTubeEra

:
crystal
!
toward
: this
What Reflex Means 675
iripur
OUTPUT
FIG. 7
The usual push-pull audio-frequency amplifier
circuit. A special case of the push-pull circuit is
discussed in the accompanying article
resistance into the various circuits, either
through the transformer winding or by the
detector. This makes tuning broad,
and in the crystal case it makes the tendency
oscillation somewhat greater. For
reason, potentiometers are used to make
the amplifier grids positive a bad practice at
best. Every adjustment of the crystal changes
the resistance in the circuit, and necessitates
readjustment of other parts of the set. Under
certain conditions better signals may be obtained
by removing the crystal contact, a sure
3ign that something is wrong. Often a point
may be found that gives loud signals which if
used in an ordinary crystal circuit would not
be sensitive at all. This is due to the semiregeneration
which occurs in the circuit and is
caused by the crystal resistance.
THE PROPER TRANSFORMERS
OROVIDED the ratio is not too high and
\ the resistance not too great, any good
audio transformer may be used in reflex circuits.
Here as in any other audio amplifier
circuit, only the best transformers should be
used if one is interested in quality as well as
quantity. Much depends upon the proper
value of by-pass condensers, and the poor
quality emanating from some reflexes may be
traced to these small components.
For example, a small condenser placed
across the secondary winding of an amplifying
transformer has the same effect as a large
condenser shunted across the primary. The
result is that high frequencies find a ready
path through this effective primary capacity,
and they naturally refuse to bother going
through the transformer. The average transformer
is a poor device anyway beyond 3000
cycles and when shunted by a condenser of
too large a capacity it misses most of the high
notes.
The coming year promises much in the way
of good transformers. The writer has seen one
new coil that will probably be on the market
by the time this is
being read and doubtless
others are being perfected. The overall amplification
of this transformer and a uv-2oi-A
tube is far beyond that attained at the present
time in the point of equality of amplification
over the audio band. The future of radio
seems to point toward better and better reproduction,
a future that will be present as
soon as more nearly perfect transformers are
for sale.
PUSH-PULL REFLEX HOKUM
POURING the past year, the writer has
I' seen several articles on how to reflex a
push-pull amplifier. Enormous amplification
is claimed, as one might suppose from getting
two tubes to act as four. Who couldn't get
signals loud enough to stop the clock with a
four stage amplifier, one of which is
push-pull
What is
wrong with this scheme
Fig. 7 is the conventional push-pull amplifier.
Between the B batteries and the output
winding are two "XV which should normally
be connected together. Now the great advantage
of the push-pull amplifier connected
as shown, lies in the fact that all of the distortion
due to overloading is balanced out, appearing
only at X and not in the output. 'Jf
one placed his receivers at X he would get all
of the distorting harmonics and none of the
fundamentals.
Fig. 8 is one of the reflex schemes. The
normal output is fed back into the input as
IfiPUT
FIG. 8
A reflex scheme which has been exploited to some
extent which contains a serious technical flaw explained
in the article
shown and the receivers are placed where the
distortion is greatest. Another scheme is to
interchange the output coil and the receivers,
thereby sending the distortion around again.
In either case the amplifier will probably howl,
and should if it does not, for here is a straight
case of feeding the output back into the input
without the usual ceremony of changing frequencies!
Other ideas, fully as unnecessary,
haveTappeared for reflexing the push-pull,
transformer arrangement as if it were not
valuable enough by itself.