TUNING-EYE
TUBES
1. Background
Today's electronics professional would be surprised to learn how expensive a
voltmeter was before the 1960s. A good-quality d'Arsonval meter movement
would have cost $5 to $10, a large sum especially for a tuning meter in a
consumer-type radio receiver. Professional and amateur equipment used such
mechanical meters routinely; they were rugged and accurate. But for a
common table or console radio, they were costly and difficult to drive with
simple tube circuitry. So, in the early 1930s an alternative appeared.
It was a tiny, low-voltage electrostatic CRT made just to give a visual
indication of proper station tuning. Being a tube, it was both easy to drive
from a high resistance and it could be mass-produced on conventional tube
production lines, thus lowering the cost. Plus, it gave off its own light
and didn't need a pilot lamp. This "eye" tube was sometimes called a "cat's
eye" because its glowing pattern was round and resembled a cat's silted
pupil. One can usually tell a top-of-the-line radio from the 30s or 40s,
because it will have a cat's eye tube on its front panel. Cheaper radios had
no tuning indicator at all; the eye was a valuable convenience feature.
Today an eye tube could be used for its original purpose, in a radio
receiver. With a small interface circuit, an eye could even be fitted to a
solid-state radio or FM tuner. But here, we will concentrate on its use as
an audio level indicator. Eye tubes haven't been used in electronics for a
LONG time, so eyes on new audio equipment are a distinctive novelty. Cary
Audio has an eye tube in one of its single-ended amplifiers, and there's a
VAC preamp with eyes. And in 1996, VTV reviewed Magnum-Dynalab's top-line FM
tuner, which uses a rare EM1 eye tube as a tuning indicator.
A typical eye tube has a hot cathode and a "grid" or control electrode, plus
a "target" which is the accelerating anode. A positive 250 volts or so is
placed on the target, and the cathode is then grounded or placed at a
negative voltage. Then, by varying the DC voltage on the control electrode,
the glowing area on the target (usually green phosphor, same as that found
in oscilloscope CRTs) will show a wedge of darkness that varies in angle
with the voltage.
2. Types
The originating types are the 6E5 family. This includes the 6E5, 2E5, 6G5,
6T5, 6U5, 6AB5 and 6N5. All have more-or-less similar characteristics and
use the same 6-pin basing, so they are pretty much interchangeable (except
the 2E5 with its 2.5 volt heater). The 6E5 was introduced by RCA in 1935,
and was a very popular item, although I suspect it was actually manufactured
by GE under contract, like many RCA tubes over the years.
Each member of this family includes an amplifying triode, which is used to
increase the sensitivity of the eye. The 6E5 uses a plain triode, while the
6G5 has a variable-mu triode to let it handle larger voltage swings. The
6AB5 and 6N5 were intended for battery radios, having lower filament
consumption. The rare 6T5, introduced by Arcturus and Sylvania in 1937, has
an annular ring shadow around its center, rather than the conventional angle shadow. It
was not successful for some reason, and is now a very hot collector's item.
It seems that around the same time the 6E5 was introduced, STC in the UK
demonstrated their "Tunograph" tuning indicator valve. This device was
apparently cruder than the 6E5, with a simple electron gun, two deflecting
plates, and a target/anode with phosphor. The gun cast a small spot on the
screen, which was moved to the side by varying the deflection voltage and
viewed through the side of the envelope. Apparently this device was not a big success,
very few radios used one.
Unfortunately, none of these early indicators are good choices for new
design, for a very simple reason: scarcity. Production was halted long ago
on all of these types. 6E5s were used in nearly all deluxe radios of the 30s
and 40s, plus in some radios and hi-fi equipment of the 50s; and they wear
out so they must be replaced regularly. The sheer number of old radios in
collector's hands has made the demand for these types of eye tube constant, and this has
made fresh ones valuable and hard to find.
A similar round pattern is found in the types 6AD6, 6AF6 and 1629. The 6AD6
and 6AF6 are essentially similar. They are not at all like the 6E5 types,
because they have no internal amplifier triode. Instead, they have TWO
control grids, each controlling the opening angle of its own dark wedge
independently of the other. This was useful for things such as FM radios
where one side might show signal strength and the other would show multipath.
The 1629 is an oddity which is only found in WWII military equipment. Its
base is octal, unlike the previous types, and it has a 12.6 volt heater
rather than the usual 6.3v heater. It has a 6E5 design, with a target plus a
driver triode, and it must be connected a bit differently from the others.
There are NOS surplus 1629s still available, so they would be a good,
inexpensive choice for a new application. (Indeed, a Canadian company called
Mapletree Audio Design has been marketing a stereo audio meter using 1629s;
and it seems that Cary Audio's top-line 845 SE amplifier contains a 1629. If
this continues, even 1629s will become scarce and costly.)
A very cheap and easy-to-get eye tube is the 6AL7. General Electric
introduced it around 1948, for use in hi-fi FM tuners. It was one of the
first eye tubes to break with convention; its display pattern was square,
not round. In fact, its "display screen" is divided into three rectangular
bar-graph indicators. Two on top, side by side, with a double-wide third bar
below. The purpose here was to use the lower bar to show IF limiter current,
as a meter for maximum signal strength, then use one upper bar to show
center-of-channel tuning. See AUDIO ANTHOLOGY, Vol. 1, page 22 for an
article about applying the 6AL7. This tube was not popular, I've only seen
it in some circa-1950 FM tuners such as Browning's RJ series plus in a
couple of circa-1950 TV sets. Yet GE made scads of 6AL7s, heaven knows why;
and many are still with us today. The 6AL7 is rated to take up to 365 volts
on its target, much more than most eye tubes. Thus, it might find use in a
tube power amp which has fairly high B+ available. I have used it as a
simple level indicator in two of my single-ended triode amps to date, plus a
few of my custom vacuum-tube synthesizers. The small size, octal base and
low-voltage drive make it easy to use.
In the 1950s, miniature European eye tubes hit the American market. Most
were developed by Telefunken in the 1950-1956 time period. The EM8x series
was all 9-pin miniature, and less expensive than domestic types. If you own
a Dynaco FM-1 or FM-3 tuner, or certain FM tuners made by companies like
McIntosh, Fisher, Pilot or Lafayette, then you have been using one of these
Euro tuning indicators. The most common are the two-bar EM84 and EM87. They
also appear in many European-made radios of the late 50s. The two bars
lengthen as the grid voltage decreases until they meet. These two types look
similar but have different characteristics and phosphor color (all of these
tubes have green P3-like phosphor except the EM87, which is blue-green).
More unusual are the EM80 and EM81. They have patterns in a half-oval shape
with a wedge in the middle, which closes as the grid becomes more negative.
They differ only in polarity, the EM80 having a glowing wedge and the EM81
having a dark wedge. I have seen an EM80 in an old Pilot FM tuner, and it is
found in some German radios as well. The rare EM85 looks like an EM81, but
with a smaller display.
If you own an old Tandberg or other European tape recorder, chances are it
uses EAM86 eye tubes for signal monitors. They are similar to EM84s, except
the bars are horizontal to the base (wrapping around the tube barrel
partly), rather than vertical.
The EMM801 is used in the Dyna FM-3 tuner. Its display is unique, with a
pair of EM84 two-bar displays side by side. These can be controlled
separately. The older EM83 was similar, though its two bars were
single-ended. And the EM82/E82M had two bars with triangular shapes, which
shadowed from the center to the outer edge. The latter tubes are rare--I
have never seen one.
The EM80, 81, 84 or 87 might be suitable for use in a new design. They are
very common, old stocks abound, and their prices are reasonable (for now).
In fact, I understand that Russian-made EM80s are readily available from
Russian surplus tube dealers, under the original number 6E1P (6E1n). The
Soviet tube industry also made an EM84 (6E2n) and a 6E5 with an octal base
(6E5c).The EMM801 is an exception; it currently goes for $100, and that is
increasing (it always was much more expensive than the other EM types,
since it's much more complex).
There are a number of scarce and/or expensive Euro eye tubes that we won't
mention here, mainly because I haven't been able to get any of them to
experiment with or because they are overly costly. The EM85, for instance,
is mentioned in tube manuals but is very hard to get. I once saw an EFM11
and I regret not buying it; that one has an ordinary round eye, but an extra
pentode built in, plus the obscure prewar "German octal" base.
And there are the EM1 through EM5 (all having prewar side-contact bases), UM34,
UM4, UM80, VME4, EFM1, VFT4, FT4 and a slew of others that are very rarely seen in the
USA. Most had the 6E5-type display,while the EM71/72 had a round eye display that was
offset from the center
axis of the tube. Most radio collectors know of the EM1 with its four-bladed
shadow pattern. In fact, the AM1, EM3, EM5, EM31, EM35, EM11, UM11, UM35 all
had a four-blade shadow. Some types in the UM series have yellow, and even
red, phosphor.
The EM35 is a replacement for the 6U5, that's in the GE tube manual. The
final (1994) issue of the VADE MECUM has more information on European eye
tubes on pages 388-394, including drawings of their shadow patterns.
For that matter, the GE manual speaks of the U.S. type 6355. I have seen one
in the hands of a tube collector; it is a 7-pin miniature with a small round
eye on the tube end. 6355s would be great for collecting purposes.
NEW Information: According
to Mr. Heath from Boca Raton, FL, the 6355 eye tube was used in a Heathkit GR-21 FM radio
from the early 1960s. Apparently this tube was used as a stereo indicator in the tube
portable radio with an auxilliary speaker.
Before eye tubes, special long-electrode neon lamps were developed for
tuning indicator use. Some prewar European radios used them. The Philips
4662 is an example--it had a long cathode, a short anode, and a trigger
electrode. Varying the trigger voltage (when AC was applied to anode and
cathode) caused the length of the glow discharge to vary. Similar devices
were made by M-OV. This design apparently was not very successful.
There were even a few eye tubes intended for use in tube battery radios. I
have a DM70/1M3; it's a subminiature tube with 8 wire leads, only 4 of which
are used. Its filament is for 1.4 volts and its target is meant for 45v to
90v. The pattern is an glowing green exclamation
mark about 3/4" high. The DM71/1N3 is similar but with short wire pins. The
DM160 is similar to the DM70 but barely half as large.
The remaining stocks of DM70s and DM71s might be useful in modern battery
gear or in solid-state equipment, as their prices are reasonable and
supplies are good. Scarcer is the DM160, also called 6977. This is my
candidate for the world's smallest glass electronic tube--barely 3/4" long
and as wide as two matchsticks. Philips, and their US division Amperex,
tried to market the 6977 in the late 1960s as a logic monitor for digital
computers. Unfortunately some radio collectors have found that it can be
substituted for01As in radios, by soldering it into a 4-pin UX base. It makes a very nice
low-mu triode. Supplies of the 6977 are expected to dry up quickly. There
was a DM21, a large device with an octal base and a 2-bladed round eye. It
had a low-power filament like the other DMs, for use on 1.5v battery power.
Japanese companies made a few of their own unique eye tubes. I have a type
6G-E12A, which was pulled out of a junked Pioneer hi-fi tuner. It is totally
unavailable in the USA, and I can't even find data or pinouts for it. Still,
a 6G-E12 might make an unusual audio meter. It has an octal base and its eye
pattern is rectangular, with two open wedges which are controlled
separately. It was often used in binaural AM-FM tuners. And there are some
other Japanese types out there, such as the 6ME10. It looks like a 6355 but
with a black phenolic base. None of these types are available in this
country in NOS form. Besides, the irony of using a Japanese eye tube is a
bit much, since Japan is the country that first produced cheap mechanical
tuning meters in quantity. They practically made eye tubes obsolete.
3. Using them for audio
Overall, I'd say the EM84 and 87 are capable of the greatest brightness, along with the
6AF6 and 6AD6. Note that most of the miniature EMs are side-view, while all the American
types are viewed from the end. Also, the American ones are usually much larger than the
EMs.The 6AD6/6AF6s give bright displays but require control-electrode
voltages of more than 50 volts, as they do not have internal driver triodes.
I leave out the 6E5 family types because of scarcity, and I can't even get a 6355 so I
don't know any more about it. If you MUST use a 6E5 type, it can be wired up
much like the 1629: the 1-meg resistor goes to pin 2, the 33k goes to pin 4,
+12v to pin 5, Vin to pin 3 and 6.3v filament power to pins 1 and 6. The 33k
resistor can be shorted out for some more brightness, at the cost of some of
the tube's lifetime.
In Table 1 is how their displays vary with Vin. Note how similar
they are, with the exception of the 6AL7 which responds in exactly
the opposite polarity of the others. I've left out the DM70 since it's a
very simple device to apply, I leave it up to you to
figure out its response. Hint: target (pin 8) goes directly to +45-90v,
filament (pins 4 and 5) to 1.4v, and input to pin 1.
4. EXIT
The eye tube is quite a unique gadget. It is very distinctive, in an
age of LED bar-graphs and vacuum fluorescent displays. Since almost no one
under the age of 30 has ever seen an eye tube, it has the power of an
obscure but non-obsolescent technology. And if you put one on your homebrew
tube amp or preamp, it is guaranteed to grab attention for your work.
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TABLE 1. EYE TUBE RESPONSES TO VIN, VOLTS DC,
WITH TARGET VOLTAGE=150V:
(all are approximate, vary from sample to sample,
and are dependent on the target voltage)
1629 EM80 EM84 EM87
FULL OPEN 13.75 13.0 16.4 12.0
INTERSECT 6.5 5.4 2.5 1.6
OVERLAP 3.5 1.9 0.2 0
6AL7 LOWER UPPER
SMALLEST BAR -5.0 -4.5
HALF BAR 0 2.0
FULL BAR 3.0 4.0
