<s>
A	O
magic	B-General_Concept
eye	I-General_Concept
tube	I-General_Concept
or	O
tuning	B-General_Concept
indicator	I-General_Concept
,	O
in	O
technical	O
literature	O
called	O
an	O
electron-ray	B-General_Concept
indicator	I-General_Concept
tube	I-General_Concept
,	O
is	O
a	O
vacuum	O
tube	O
which	O
gives	O
a	O
visual	O
indication	O
of	O
the	O
amplitude	O
of	O
an	O
electronic	O
signal	O
,	O
such	O
as	O
an	O
audio	O
output	O
,	O
radio-frequency	O
signal	O
strength	O
,	O
or	O
other	O
functions	O
.	O
</s>
<s>
The	O
magic	B-General_Concept
eye	I-General_Concept
(	O
also	O
called	O
a	O
cat	B-General_Concept
's	I-General_Concept
eye	I-General_Concept
,	O
or	O
tuning	B-General_Concept
eye	I-General_Concept
in	O
North	O
America	O
)	O
is	O
a	O
specific	O
type	O
of	O
such	O
a	O
tube	O
with	O
a	O
circular	O
display	O
similar	O
to	O
the	O
EM34	B-General_Concept
illustrated	O
.	O
</s>
<s>
Its	O
first	O
broad	O
application	O
was	O
as	O
a	O
tuning	B-General_Concept
indicator	I-General_Concept
in	O
radio	O
receivers	O
,	O
to	O
give	O
an	O
indication	O
of	O
the	O
relative	O
strength	O
of	O
the	O
received	O
radio	O
signal	O
,	O
to	O
show	O
when	O
a	O
radio	O
station	O
was	O
properly	O
tuned	O
in	O
.	O
</s>
<s>
The	O
magic	B-General_Concept
eye	I-General_Concept
tube	I-General_Concept
was	O
the	O
first	O
in	O
a	O
line	O
of	O
development	O
of	O
cathode	O
ray	O
type	O
tuning	B-General_Concept
indicators	I-General_Concept
developed	O
as	O
a	O
cheaper	O
alternative	O
to	O
needle	O
movement	O
meters	O
.	O
</s>
<s>
Tuning	B-General_Concept
indicator	I-General_Concept
tubes	O
were	O
used	O
in	O
vacuum	O
tube	O
receivers	O
from	O
around	O
1936	O
to	O
1980	O
,	O
before	O
vacuum	O
tubes	O
were	O
replaced	O
by	O
transistors	O
in	O
radios	O
.	O
</s>
<s>
An	O
earlier	O
tuning	O
aid	O
which	O
the	O
magic	B-General_Concept
eye	I-General_Concept
replaced	O
was	O
the	O
"	O
tuneon	O
"	O
neon	O
lamp	O
.	O
</s>
<s>
The	O
magic	B-General_Concept
eye	I-General_Concept
tube	I-General_Concept
(	O
or	O
valve	O
)	O
for	O
tuning	O
radio	O
receivers	O
was	O
invented	O
in	O
1932	O
by	O
Allen	O
B	O
.	O
DuMont	O
(	O
who	O
spent	O
most	O
of	O
the	O
1930s	O
improving	O
the	O
lifetime	O
of	O
cathode	B-Device
ray	I-Device
tubes	I-Device
,	O
and	O
ultimately	O
formed	O
the	O
DuMont	O
Television	O
Network	O
)	O
.	O
</s>
<s>
The	O
RCA	B-General_Concept
6E5	I-General_Concept
from	O
1935	O
was	O
the	O
first	O
commercial	O
tube	O
.	O
</s>
<s>
The	O
earlier	O
types	O
were	O
end-viewed	O
(	O
see	O
the	O
EM34	B-General_Concept
)	O
,	O
usually	O
with	O
an	O
octal	O
or	O
side-contact	O
base	O
.	O
</s>
<s>
Later	O
developments	O
featured	O
a	O
smaller	O
side-viewed	O
noval	O
B9A	B-General_Concept
based	O
all-glass	O
type	O
with	O
either	O
a	O
fan	O
type	O
display	O
or	O
a	O
band	O
display	O
(	O
see	O
the	O
EM84	B-General_Concept
)	O
.	O
</s>
<s>
This	O
design	O
prompted	O
the	O
contemporary	O
advertisers	O
to	O
coin	O
the	O
term	O
magic	B-General_Concept
eye	I-General_Concept
,	O
a	O
term	O
still	O
used	O
.	O
</s>
<s>
There	O
was	O
also	O
a	O
sub-miniature	O
version	O
with	O
wire	O
ends	O
(	O
Mullard	O
DM70/DM71	O
,	O
Mazda	O
1M1/1M3	O
,	O
GEC/Marconi	O
Y25	O
)	O
intended	O
for	O
battery	O
operation	O
,	O
used	O
in	O
one	O
Ever	O
Ready	O
AM/FM	O
battery	O
receiver	O
with	O
push-pull	O
output	O
,	O
as	O
well	O
as	O
a	O
small	O
number	O
of	O
AM/FM	O
mains	O
receivers	O
,	O
which	O
lit	O
the	O
valve	O
from	O
the	O
6.3V	O
heater	O
supply	O
via	O
a	O
220	O
ohm	O
resistor	O
or	O
from	O
the	O
audio	O
output	O
valve	O
's	O
cathode	O
bias	O
.	O
</s>
<s>
Some	O
reel-to-reel	O
tape	O
recorders	O
also	O
used	O
the	O
DM70/DM71	O
to	O
indicate	O
recording	O
level	O
,	O
including	O
a	O
transistorized	O
model	O
with	O
the	O
valve	O
lit	O
from	O
the	O
bias-oscillator	O
voltage	O
.	O
</s>
<s>
The	O
function	O
of	O
a	O
magic	B-General_Concept
eye	I-General_Concept
can	O
be	O
achieved	O
with	O
modern	O
semiconductor	O
circuitry	O
and	O
optoelectronic	O
displays	O
.	O
</s>
<s>
The	O
high	O
voltages	O
(	O
100	O
volts	O
or	O
more	O
)	O
required	O
by	O
these	O
tubes	O
are	O
not	O
present	O
in	O
modern	O
devices	O
,	O
so	O
the	O
magic	B-General_Concept
eye	I-General_Concept
tube	I-General_Concept
is	O
now	O
obsolete	O
.	O
</s>
<s>
A	O
magic	B-General_Concept
eye	I-General_Concept
tube	I-General_Concept
is	O
a	O
miniature	O
cathode	B-Device
ray	I-Device
tube	I-Device
,	O
usually	O
with	O
a	O
built-in	O
triode	O
signal	O
amplifier	O
.	O
</s>
<s>
It	O
usually	O
glows	O
bright	O
green	O
,	O
(	O
occasionally	O
yellow	O
in	O
some	O
very	O
old	O
types	O
,	O
e.g.	O
,	O
EM4	B-General_Concept
)	O
and	O
the	O
glowing	O
ends	O
grow	O
to	O
meet	O
in	O
the	O
middle	O
as	O
the	O
voltage	O
on	O
a	O
control	O
grid	O
increases	O
.	O
</s>
<s>
the	O
AGC	O
line	O
in	O
an	O
AM	O
superheterodyne	O
receiver	O
,	O
or	O
the	O
limiter	O
stage	O
or	O
FM	B-Algorithm
detector	O
in	O
an	O
FM	B-Algorithm
receiver	O
.	O
</s>
<s>
The	O
purpose	O
of	O
magic	B-General_Concept
eye	I-General_Concept
tubes	I-General_Concept
in	O
radio	O
sets	O
is	O
to	O
help	O
with	O
accurate	O
tuning	O
to	O
a	O
station	O
;	O
the	O
tube	O
makes	O
peaks	O
in	O
signal	O
strength	O
more	O
obvious	O
by	O
producing	O
a	O
visual	O
indication	O
,	O
which	O
is	O
better	O
than	O
using	O
the	O
ear	O
alone	O
.	O
</s>
<s>
The	O
tuning	B-General_Concept
eye	I-General_Concept
was	O
driven	O
by	O
the	O
AGC	O
voltage	O
rather	O
than	O
the	O
audio	O
signal	O
.	O
</s>
<s>
When	O
,	O
in	O
the	O
early	O
1950s	O
,	O
FM	B-Algorithm
radio	O
sets	O
were	O
made	O
available	O
on	O
the	O
UK	O
market	O
,	O
many	O
different	O
types	O
of	O
magic	B-General_Concept
eye	I-General_Concept
tubes	I-General_Concept
were	O
made	O
available	O
,	O
with	O
differing	O
displays	O
,	O
but	O
they	O
all	O
worked	O
the	O
same	O
way	O
.	O
</s>
<s>
Some	O
had	O
a	O
separate	O
small	O
display	O
to	O
light	O
up	O
indicating	O
a	O
stereo	O
signal	O
on	O
FM	B-Algorithm
.	O
</s>
<s>
The	O
British	O
Leak	O
company	O
used	O
an	O
EM84	B-General_Concept
indicator	O
as	O
a	O
very	O
precise	O
tuning-indicator	O
in	O
their	O
Troughline	O
FM	B-Algorithm
tuner	O
series	O
,	O
by	O
mixing	O
the	O
AGC	O
voltages	O
from	O
the	O
two	O
limiter	O
valve	O
grids	O
at	O
the	O
indicator	O
sensing-grid	O
.	O
</s>
<s>
In	O
USA	O
made	O
radios	O
,	O
the	O
first	O
type	O
issued	O
was	O
the	O
type	O
6E5	B-General_Concept
single	O
pie	O
shaped	O
image	O
,	O
introduced	O
by	O
RCA	O
and	O
used	O
in	O
their	O
1936	O
line	O
of	O
radios	O
.	O
</s>
<s>
Other	O
radio	O
makers	O
originally	O
used	O
the	O
6E5	B-General_Concept
as	O
well	O
until	O
,	O
soon	O
after	O
,	O
the	O
less	O
sensitive	O
type	O
6G5	B-General_Concept
was	O
introduced	O
.	O
</s>
<s>
Also	O
,	O
a	O
type	O
6AB5	B-General_Concept
aka	O
6N5	B-General_Concept
tube	O
with	O
lower	O
plate	O
voltage	O
was	O
introduced	O
for	O
series	O
filament	O
radios	O
.	O
</s>
<s>
Type	O
number	O
6U5	B-General_Concept
was	O
similar	O
to	O
the	O
6G5	B-General_Concept
but	O
had	O
a	O
straight	O
glass	O
envelope	O
.	O
</s>
<s>
Zenith	O
Radio	O
used	O
a	O
type	O
6T5	B-General_Concept
in	O
their	O
1938	O
model	O
year	O
radios	O
with	O
"	O
Target	B-General_Concept
tuning	I-General_Concept
"	O
indicator	O
(	O
resembling	O
a	O
camera	O
iris	O
)	O
,	O
but	O
was	O
abandoned	O
after	O
a	O
year	O
already	O
,	O
with	O
Ken-Rad	O
manufacturing	O
a	O
replacement	O
type	O
.	O
</s>
<s>
Several	O
other	O
"	O
eye	B-General_Concept
tubes	I-General_Concept
"	O
were	O
introduced	O
in	O
USA	O
radios	O
and	O
also	O
used	O
in	O
test	O
equipment	O
and	O
audio	O
gear	O
,	O
including	O
the	O
octal-based	O
types	O
6AF6GT	B-General_Concept
,	O
6AD6GT	B-General_Concept
and	O
1629	B-General_Concept
.	O
</s>
<s>
The	O
latter	O
was	O
an	O
industrial	O
type	O
with	O
12	O
volt	O
filament	O
looking	O
identical	O
to	O
type	O
6E5	B-General_Concept
.	O
</s>
<s>
Later	O
USA	O
made	O
audio	O
gear	O
used	O
European	O
tubes	O
like	O
EM80	B-General_Concept
(	O
equivalent	O
to	O
6BR5	B-General_Concept
)	O
,	O
EM81	B-General_Concept
(	O
6DA5	B-General_Concept
)	O
,	O
EM84	B-General_Concept
(	O
6FG6	B-General_Concept
)	O
,	O
EM85	B-General_Concept
(	O
6DG7	B-General_Concept
)	O
or	O
EM87	B-General_Concept
(	O
6HU6	B-General_Concept
)	O
.	O
</s>
<s>
Magic	B-General_Concept
eye	I-General_Concept
tubes	I-General_Concept
were	O
used	O
as	O
the	O
recording	O
level	O
indicator	O
for	O
tape	O
recorders	O
(	O
for	O
example	O
in	O
the	O
)	O
,	O
and	O
it	O
is	O
also	O
possible	O
to	O
use	O
them	O
(	O
in	O
a	O
specially	O
adapted	O
circuit	O
)	O
as	O
a	O
means	O
of	O
rough	O
frequency	O
comparison	O
as	O
a	O
simpler	O
alternative	O
to	O
Lissajous	O
figures	O
.	O
</s>
<s>
A	O
magic	B-General_Concept
eye	I-General_Concept
tube	I-General_Concept
acts	O
as	O
an	O
inexpensive	O
uncalibrated	O
(	O
and	O
not	O
necessarily	O
linear	O
)	O
voltage	O
indicator	O
,	O
and	O
can	O
be	O
used	O
wherever	O
an	O
indication	O
of	O
voltage	O
is	O
needed	O
,	O
saving	O
the	O
cost	O
of	O
a	O
more	O
accurate	O
calibrated	O
meter	O
.	O
</s>
<s>
The	O
magic	B-General_Concept
eye	I-General_Concept
tube	I-General_Concept
also	O
appears	O
on	O
the	O
cover	O
of	O
My	O
Morning	O
Jacket	O
's	O
2011	O
album	O
Circuital	O
.	O
</s>
