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.
The saga continues... Please bear with me even if you have no
interest in the much-hyped (at the time) history of the advent
of all-metal vacuum tubes in the mid 1930s. They were predicted
to make glass-encased tubes obsolete. It never happened. Believe
it or not, there are a few folks out there (like moi) who like
reading about the history. Posting this on the RF Cafe homepage
helps the search engines find and register it faster. If you
also happen to care, then you might be interested to know this
article was later referenced an
1935 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
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."
Here is the new English "Catkin" alongside
an American type 24, screen-grid tube.
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 rubber mounting.
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.
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.
The entire story of the Catkin is told in
the drawing here. A really "new" tube.
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
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 November 10, 2015