August 1933 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
This 1933 Radio-Craft
magazine article is typical of the much-hyped (at the time) all-metal
vacuum tubes in the mid 1930s. Their British proponents predicted glass-encased tubes
would quickly become obsolete once everyone realized how the added expense of
metal encasement would easily be offset by the many advantages offered by metal
tubes. Like so many grand new innovations with fantastic promises, this one never
quite panned out. Metal tubes have/had their place in certain applications, but
turned out not to be the panacea hoped for. Undeniably, superior noise immunity
and greater ruggedness and thus reliability are features difficult to replicate
in glass tubes, but not all tubes needed such perfection. If you are interested
in the history of metal tubes, then check out this article which was later referenced
piece where it says, "Several years ago, an English company started to manufacture
wholly metallic tubes in which practically no glass was used except in connection
with certain parts of the sealing (Radio-Craft, August 1933)."
A New English All-Metal Tube
The entire story of the Catkin is told in the drawing here. A
really "new" tube.
By Robert Hertzberg
While American tube manufacturers have been content to produce "new" tubes by
adding grids and plates to old ones in wild confusion, the British have really done
something by eliminating 95% of the glass used in tube construction. The result
is stronger and far more uniform tubes than have hitherto been available.
The current sensation in European radio circles is the new "Catkin" all-metal
tube, which promises to revolutionize the tube manufacturing industry and to solve
a number of vexing problems of set design, construction, and operation. While glass
has not been eliminated entirely, the predominance of metal warrants the use of
the expression "all-metal."
In brief, the Catkin tube uses a copper cylinder, or container, in place of the
customary glass envelope, this container being the plate electrode. The other electrodes,
i.e., cathode and grids, are mounted within the cylinder in their usual relationships
to form triodes, tetrodes, and pentodes. As shown in the accompanying illustrations,
the bottom end of the copper container is sealed vacuum-tight to a short glass member,
through which the connection wires emerge and also through which the air is exhausted.
The entire lower end of this assembly is supported in the base by a built-in circular
While this type of construction has been used for many years in high-power transmitting
tubes, its application by the British General Electric and Marconiphone companies
to the receiving field is worthy of commendation. The name "Catkin" is a coined
word based on the laboratory slang word "cat" for "Cooled Anode Transmitters," a
"catkin" thus being a diminutive "cat." In transmitting work, where the plate power
dissipation is very great, the copper cylinder is cooled by water circulating around
it in a jacket. In the new Catkins the mere air circulation is sufficient to bring
the overall operating temperature well below that of vacuum type tubes, wherein
the very vacuum between the plate and the surface of the glass bulb makes the problem
of heat radiation very difficult of solution.
Here is the new English "Catkin" alongside an American type 24,
Vastly greater rigidity of internal construction is possible with the Catkins
than with glass tubes because the electrode structure can be braced firmly at both
ends by means of insulating spacers that actually touch the inner surface of the
copper "plate" cylinder. Not only does this arrangement practically eliminate microphonic
effects, but it also permits a degree of uniformity in manufacture sadly lacking
in conventional tubes; particularly tubes with a number of critically spaced grids.
In fact, uniformity of characteristics is the main merit claimed for the Catkins,
the tubes, electrically, being the general equivalents of standard British types.
The general-purpose triode and the output pentode of the Catkins series do not
require an external cover or shield, and full advantage is taken of the effective
cooling action of the exposed "plate." These tubes have a conventional base with
apparently nothing but a stubby piece of copper sticking out of the middle. In the
R.F. tetrodes an external shield of familiar appearance is employed. This is of
the same diameter as the base and is permanently attached to it; no separate tube
shields, as we know them in America, are needed.
An incidental departure in construction is the elimination of the usual pressed
glass bead in which the support wires for the electrodes are sealed; instead, the
Catkins use mica, the assembly at this point being braced by a steel clamp.
Providing the metal-to-glass vacuum seal proves satisfactory, it is easy to see
that the Catkins will enjoy widespread popularity and application. The manufacturers
claim they can be dropped six feet on to a concrete floor with but small risk of
either mechanical or electrical damage. The admittedly superior internal electrode
bracing and the built-in rubber mounting should at least do away with the terribly
annoying microphonic howling due to loudspeaker reaction, and should make the tubes
last longer than usual. The greatly reduced overall size is also an important factor.
For portable and mobile radio installations of many kinds, the Catkins possess
obvious advantages: tubes of this kind would give American manufacturers of auto-radio
receivers a wonderful sales "talking point" and would enable them to keep their
sets sold with fewer service worries; it is no secret that some of the new trick-combination
tubes are altogether too critical for bouncy automobile service. For airplane use,
something of the sort is certainly needed to stand the terrific shock of repeated
At the time this issue of Radio-Craft went to press (the middle of June) no Catkins
were available in the United States, and none are expected, except, perhaps, as
samples. If any American manufacturers grab the idea and turn out some tubes, for
experimental purposes, if nothing else, we will be glad to herald their efforts
in this magazine.
Posted January 9, 2024
(updated from original
post on 11/10/2015)