October 1935 Radio-Craft
[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
Radio-Craft was published from 1929 through 1953. All copyrights are hereby acknowledged. See all articles
The advent of metal-encapsulated vacuum tubes was supposed to
be the death knell for traditional glass tubes. This 1935 article
from Radio-Craft spelled out the many virtues of 'metal' tube
and how in short order their superiority would obviate the need
- even desire - for 'glass' tubes. I'll let you read the article
for the details, but want to make note of an evidently archaic
term used that could potentially be really popular in today's
manufacturing world if duly resurrected - 'quantiquality' (aka
'quanti-quality' or 'quanti quality'). The connotation is a
process of high quantity in conjunction with high quality. The
only references I could easily find to quantiquality was from
late-19th-century newspaper archives. If sometime within the
next few years you start seeing some form of quantiquality appear
in marketing copy and scholarly papers, remember that you heard
it here first!
An Inside Story About Metal Tubes
Interesting details concerning the all-metal tube; also,
recently-introduced, associated types are described.
R. D. Washburne
Metal tubes - being more compact, inherently strong in structure
and having many manufacturing advantages over glass tubes -
fulfill an urgent need in the field of applied electronics.
Fig. 1 - Table I lists the above details.
As the artist has expertly illustrated, Fig. B., the glass
tube design that has held sway for the last 28 years at last
has succumbed to the modern demand for "quantiquality" (a word
coined by Sachs which aptly typifies the modern demand for greater
production and closer tolerances)!
The Glass Prototype
The case against glass tubes is a strong one, but only a
few important points will be mentioned.
That there is little difference between the elements inside
the glass and the metal envelopes is evident by reference to
Fig. F. This view shows that a great amount of material required
in order to secure rigidity within the necessarily large glass
bulb, is not required in a metal tube.
It is necessary to coat the glass bulb with a graphite preparation
in order to prevent the collection of potential charges on the
bulbs, especially at the higher radio frequencies. The metal
envelope eliminates this fault, permitting perfectly satisfactory
operation right on down to below 5 meters! (See the article,
18 Metal Tube All-Wave Superhet," in this issue.)
The metal tube structural requirements previously mentioned,
and some of those hinted at, will be evident by reference to
Fig. 1, which illustrates a typical screen-grid tube; a description
is given in Table 1.
(Additional construction details appeared in the article,
"Now - Metal Tubes," in the June, 1935 issue of Radio-Craft.)
At the same time, the metal envelope is much superior and
more convenient in its function as a metal shield and in reducing
inter-electrode capacity than is the skin-tight shield required
in a good glass-tube arrangement. Present circuit practice demands
that the grid-to-plate capacity of a typical screen-grid tube
be approximately 0.01-mmf., or less. The metal tube reaches
this tolerance with little difficulty.
Fig. A - Left, metal-tube exterior; center,
"X-ray eye" view; right, cross-section. (G.E.)
Fig. B - Going-going - - -!
Fig. C - "Metal-spray" tube.
Fig. D - Metal-glass tube.
Fig. E - Glass-"metal" tube.
X-ray eyes would- be required to see "through" the opaque
metal tube. Therefore, what is more logical than that the X-ray
should be used in checking the internal arrangement of the elements
in defective tubes? This procedure eliminates the need for an
expensive wrecking process in order to check the malformation
of the elements within the normal number of tube rejects. The
internal arrangement of the elements (in screen-grid arrangement)
as thus disclosed by X-ray is illustrated in the central view
in Fig. A (and the cover illustration), and may be compared
with the exterior and cross-section views at the left and right.
X-ray views of the representative types appear in Fig. G. This
photograph and the preceding X-ray view were taken by the General
Electric Company especially for Radio-Craft; (they are interesting
examples of the advanced X-ray photography technique which makes
it possible to obtain such excellent "separation" of the various
metal elements within the steel jacket).
Fig. G - G.E. X-ray photos of metal tubes.
"Other" Metal Tubes
An article discussing the present status of metal tubes would
be incomplete without mention of several new technical developments
in related types. The consumer should acquaint himself with
the following facts in order that due justice may be given to
A Canadian manufacturer, Rogers Radio Tubes, Ltd., has just
introduced a type of tube which at first glance might be confused
with the steel jacket variety. However, as shown in Fig. C,
the envelope is of glass, and only the "octal" (8 prong) base
is of the metal type. The glass bulb is given a metal coating;
from this process is derived the term -"metal-spray" tube. The
metal-spray tube is completely shielded from top to the bottom
of the base; it is furnished in dual-purpose types. The new
interior construction of the tube includes the cementing of
the grid rod to the ceramic top-support, together with expanding
and sealing of the cathode tubing to this ceramic support.
This eliminates the possibility of noise due to vibration,
as well as the possibility of hum from the cathode tubing or
poor emission from the cathode coating.
Fig. F - Illustrating metal vs. glass tube
A second variation has been brought out by Triad Manufacturing
Company, Inc., and is illustrated in Fig. D. These metal-glass
tubes are known as the MG series and are not all-metal, there
being a glass inner sleeve which is used for maintaining the
The MG series at present, includes the following types: 5Z4MG,
6A8MG, 6C5MG, 6D5MG, 6F6MG, 6H6MG. 6F7MG, 6K7MG; the characteristics
of these tubes are said to parallel those of all-metal construction.
There is available in addition the following types, for use
in A.C.-D.C. sets: 25Z5MG and 43MG. with characteristics similar
to those of the types 25Z5 and 43 tubes; there is also available
a type 50A2MG tube, which is a ballast tube in a metal shield
with a tap to supply two No. 40 pilot lights in series, and
type 50B2MG, similar to the 50A2MG except that it is tapped
to supply only a single No. 40 pilot lamp. Both ballast tubes
have an over-all drop of 50 V.
The tubes in the MG series incorporate standard octal bases.
Finally, there is the type of tube which, while it incorporates
an octal or 8-prong base, utilizes a glass envelope and depends
for its shielding upon an external shield. This glass-"metal"
tube described in the August, 1935 issue of Radio-Craft as item
No. 754, is illustrated here in Fig. E. The characteristics
are said to approximate those in the regular metal series. The
latest step towards securing identical characteristics has been
to use a skin-tight shield of improved design. This shield is
illustrated in the Latest in Radio Department of the October
issue as item No. 820. The glass-"metal" series, identified
by the suffix letter G, is a product of Arcturus Radio Tube
It is interesting to speculate on how many manufacturers
of test equipment may have provided switches to take care of
the unavoidable discrepancies that so far have arisen in the
"standardization" process in connection with metal tubes.
We refer specifically to the types 6F5 and 5Z4 tubes.
Metal 6F5 is a high-mu triode, with its plate connected to
No.4 terminal instead of the No. 3 terminal utilized for plate
connection in the other amplifier tubes in the metal series.
This connection to No. 4 terminal eliminated undesirable circuit
Metal 5Z4 is a full-wave rectifier about equivalent to the
glass 80; therefore, its filament potential is 5 V., against
6.3 for the remaining tubes in the metal series. To prevent
damage to the 5 V. filament of the rectifier if plugged into
one of the 6.3 V. sockets, or to the plates of the 6.3 V. tubes
due to high voltage if plugged into the rectifier socket, the
rectifier filament terminal wires are connected instead to terminals
Nos. 2 and 8, instead of Nos. 2 and 7 like the remaining tubes
in the metal series. (We wonder how many technicians are going
to cuss the circuit limitations imposed by reason of the 5Z4
filament being tied to the high-voltage side of the rectifier
Service Men and experimenters who wish to revamp existing
equipment in order to properly test the metal tubes should note
that some analyzers have the No. 1 or ground terminal of the
octal socket grounded to the metal mountings and the wiring
- at the same time, the No.8 terminal is grounded. This puts
the tube shield at the rectifier cathode potential - that is,
350 V., in some instances! (An army mule can't beat the kick
you'd get from that jolt!) Just remember, then, to isolate the
wiring to terminals 1 and 8.
Heating of the metal tubes, as compared with the glass type,
is a popular topic for discussion these days. Do the metal tubes
run hotter than the glass ones of equivalent type? If they do,
and the rise in temperature is due to better conduction by the
steel envelope, is this an indication that the tube will perform
better because the internal elements are being kept more cool?
These and other questions of this nature are still to be answered;
it is of exceptional interest that an article discussing the
subject will appear in a forthcoming issue of Radio-Craft.
Some off-hand claims have been made that the metal tubes
are less sensitive and more noisy than the glass predecessors.
On the contrary, comparative tests between receivers utilizing
similar sets each properly designed, one for the metal tubes
and the other for the glass prototypes, prove that the metal
tubes are more sensitive and less noisy than glass tubes!
Finally. there remains the question of whether the metal
tubes will work at very high frequencies. Well. the writer is
in the position of having heard a commercial metal-tube set
(described elsewhere in this issue) operating on 4 1/2 and 5
meters! It functioned with far greater tone quality, and far
less static at 5 meters than did the same set at the broadcast
It seems, however. that in some circuits the shield of the
metal tube when operated at ultra-high frequencies may exhibit
hand capacity. Whether this is due to lack of conductivity of
the Swedish-iron shield, to close proximity of the internal
elements, or to some other reason has not as yet been definitely
It is even possible that the control-grid terminal atop the
tube, in some high-gain circuits may present sufficient area
to cause inter-tube feedback and circuit oscillation or undue
regeneration. An expedient to obviate this fault is the use
of a top-cap shield, constructed as illustrated in Fig. 2.
Fig. 2 - This metal cap completes the shielding.
Definitely, metal tubes are here to stay. Therefore, the
radio man might just as well start right now to absorb every
available bit of technical data relating to them.
October 26, 2015