We use Optical Character Recognition (OCR) during our scanning and processing workflow to make the content of each page searchable. You can view the automatically generated text below as well as copy and paste individual pieces of text to quote in your own work.
Text recognition is never 100% accurate. Many parts of the scanned page may not be reflected in the OCR text output, including: images, page layout, certain fonts or handwriting.
How to Build a One-Dial Reflex
727
FRONT VIEW OF THE PANEL
With the one-tube reflex unit, the problem can be solved by the addition of another tube as an audio-frequency amplifier. It has proved highly impractical to add another audio-frequency tube to the two-tube circuit. One transformer and one tube alone have been unable to handle the output of the preceding two tubes without much distortion. So push-pull amplification has been a life saver — or shall we call it a volume saver?
Push-pull amplification is a necessity when additional volume is desired.
The "how and why" behind push-pull amplification is not half as intricate as some people seem to believe. It differs from the usual straight audio, at first glance, in that two transformers and two tubes are used for each stage. The first transformer is the coupling transformer which divides the output of the prccecding tube evenly between the grids of the two push-pull tubes. The second transformer collects the outputs of the two push-pull tubes and passes the total energy on to the loud speaker.
The term "push-pull" is used because the grids of the two tubes in each stage of pushpull amplification are charged with opposite polarity at any instant. While one grid is positive, the other is negative. Any tendency to distort in one tube is counteracted by the other tube.
The first transformer performs the double duty of dividing the input current between the
two tubes and of delivering it to these tubes in such a manner that the two grids will be oppositely charged. It does this by means of a split winding. While the primary winding is one continuous winding coming out to two binding posts in the usual manner, the secondary is tapped at its central point and is therefore brought to three binding posts. The binding post leading to the center tap is connected to the negative A battery through the C battery. The other two binding posts are attached to the grids of the two tubes.
Conversely, the second or output transformer of the push-pull stage of amplification has a tapped primary and a conventional secondary. In this transformer the primary winding is brought to three binding posts and the secondary winding to two. In this case the central primary tap is connected to the B battery while the other two posts are connected one to each plate. The two posts of the secondary are connected to the speaker just as the two posts of the first transformer were connected to the plate and B battery of the last audio-frequency tube.
THE LAYOUT OF THE AMPLIFIER
THE layout of the push-pull unit can be made rather flexible. If space permits, the two tubes can be placed one in front of the other. The two transformers should be put one on each side of the tubes. This makes the wiring short and direct. It also reduces the
[<-— 3'-' -->j<— -5" >j<— -3^--H<— 3%-"-->K 4
Ji'-*— 4V--«
~ z^
}>v rv
i
* •
\,rj
m
-i
^
^
f
\
1* K W
-&* ^&-*^. ~4^' ~$~
7 " --^x ' Z'' HOLE-" ~—
u-i—
t
.+..-& * *'' <K"" ~NO 18 HOLE— -*4
^ i
•U _ o>» /' —
5"- -Jr
<
FIG. 4 The panel layout. The photograph above shows the appearance of the finished panel