Evening star., 1924-04-16

Although hundreds of thousands of
dry cell vacuum tubes are now in
use by radio enthusiasts, it is safe
to say that only a very small percen-
tage OF the users know anything at
all about their process of manufac-
ture.
These little tubes, because of their
convenience and low operating cost,
added tremendously to the interest in
broadcasting. and gave to the great
general public An opportunity to OBJ
tain tube receivers. Before the COM
ing of radiotrons WD-11 and UV-1ss
the greater number of receiving sets
used the crystal detector and thus
were limited in range.
Great Care Needed.
Great care Is necessary in the man
ufacture of these tubes, which despite
the fragile character Of the material
are sturdy bits OF apparatus. well
adapted to stand fair handling and
give constant service during long
periods.
There are thirteen steps or proc
esses of assembly before the tube,
completed shape. There is also a test
made after each assembly and still
further tests after the tube is com
pleted. These tests are so severe
that once a tube has passed through
them II is rarely returned because
of failure in operation.
The raw material from which the
tube is assembled consists of the fol-
lowing: A glass blaAk, thin glass
stem, a short tube of glass, the fila
ment, the plate and grid. The plates
are shaped from rectangular bits of
metal and the grid wires are wound
into the spiral form they take in the
completed tube.
SpinninE a Flare.
The first step In the assembly
process is spinning a flare on the end
Of the short glass tube. This tube is
then called the flare.
|, Next is the inserting of the five
wires in the flare. Looking at the
WD-11 one can see five wires on the
inner unit though there are but four
leads at the base. The fifth wire is a
blind insert to act as a. support for
|, the plate. The five wires are inserted
in the end of the glass tube opposite
shape seen in the completed tube, thus
holding firmly in place the wires. The
glass tube with it's wires inserted is
now called the press.
Five Wires to Cut.
The five wires are next cut to their
proper lengths so that the plate. grid
and fil2ment can be mounted. These
units are spot elded in place by girl
operators. each of whom is a highly
skilled worker. The filament used in
the WD-11 is a metal wire coated
with oxides.
Now the glass blank is tubulated
or in other words the thin glass tube
is placed on its end after q, small
hole has been melted in the glass
blank with gas flame.
Next the press is sealed to the bot
tom of the blank. In this process
the flare is held tightly to the bottom
OF the blank and fame melts the
glass sufficiently so that they weld
together.
Az the long glass stem is placed
on the blank for the purpose of ex-
bausting the tube this process is the
next in order. The long glass stem
is inserted into q piece of rubber
tubing which leads directly to the
pumps, which are two in number
An oil pump and a mercury vapor
pump are used to exhaust the tube
down to the required vacuum Be
| fore the pumps are turned on g
covering which serves as an oven is
pulled down over them and they are
subjected to a high Temperature &c
drive gases from the glass walls and
metal parts
Vacuum Tested.
Then the pumps are turned on and
the tubes exhausted to a very low
pressure, which Is at a much higher