June 1935 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Today as I write this it is New Year's Eve. The year 1935 could
be considered 'metal tube's eve' as a new paradigm was about
to hit the world of high frequency circuit design. Thentofore[sic],
vacuum tubes were almost exclusively encased in a glass envelope.
Metal-encased tubes provided, among others, benefits like better
heat dissipation, smaller physical size, ruggedness, inherent
RF shielding, and lower parasitic values of capacitance and
inductance due to smaller plate areas and shorter lead lengths,
respectively. The highest barrier to widespread adoption, history
would show, was the higher cost of production that made consumer
products more expensive at a time when not every household saw
the need for a radio or, eventually, a television.
Now - Metal Tubes
As first announced by Radio-Craft, last month, the new all-metal
tubes are making their appearance. Many advantages are claimed
for the new tubes, as explained.
Since the brief mention made on page 646 of May 1935 Radio-Craft
of the new metal tubes, definite information has been released
by the General Electric Co. who developed the new method of
manufacture for the RCA Manufacturing Co., Inc.
There are many advantages claimed for the new tubes. They
are smaller in size, being 1 in. in diameter at the largest
point and varying between 5/8-in, to 3 inches high (above the
base or sub-panel). They provide their own shielding by virtue
of the metal shell and this shell is a better heat conductor
and radiator than glass. Because of the method of construction,
the leads are shorter, within the tube, which permits greater
amplification at the higher frequencies (the inter-electrode
capacity is approximately 1/3 of the value for the equivalent
glass tube capacity) and the more effective shielding assures
Ten types of these metal tubes have been developed, to date,
having 6.3 V filaments. Some of these tubes have identical characteristics
with present glass tubes (some of them are shown in the photo
above, along with their glass twins) though several new types
are planned including a double diode and a hexode which is an
improved pentagrid converter.
The new tubes have one more base pin than comparable glass
tubes, since the metal envelope has become a shield, and provision
must therefore be made to ground this envelope. Designers of
the tubes have even taken into consideration ease of inserting
them in their sockets. The contact pins are all the same diameter
and in the center is a longer, larger, insulated keyed pin.
By placing this insulated pin in a hole centrally located in
the socket and rotating the tube until the key slips into its
groove, the tube is quickly and easily inserted. (See "Radio
Trends," by Hugo Gernsback. page 69, August 1934 Radio-Craft!)
The mechanical construction of the new tubes
is evident from these detailed sketches of a screen-grid type.
The 8-pin base and socket have been standardized for all
the metal tubes; when less than 8 pins are needed, they are
simply omitted from the tube base and, of course, the corresponding
contact on the socket is not wired into the circuit. This is
a revolutionary change in tube manufacture.
In appearance the metal tubes are cylindrical in form, some
having a reduced diameter at the top. Others, such as a radio-frequency
amplifier, have a terminal at the top extremity. Each lead-in
wire passes through a tiny bead of special glass that is fused
securely within an alloy eyelet, which in turn is welded to
the metal container, thus assuring a long life vacuum. This
alloy, having substantially the same coefficient of expansion
as glass, is known as "Fernico" and is a combination of iron
(fer) nickel (ni) and cobalt (co). It was developed expressly
for this purpose of a perfect seal in the new tubes.
The inner parts of the tube are first assembled on the steel
end plate or "header;" The shell is placed over the assembly
and welded to the header at its circumference. This welding
is done with Thyratron controlled welders and takes about 1/20-second
to complete: about 20,000 to 30,000 amperes flow through the
metal when the weld is being made.
During evacuation, the occluded gases in the tubes are removed
by heating the entire tube in a gas burner to a red heat. Design
engineers claim that the metal envelope helps to "clean up"
these molecules of gas, acting as a sort of sponge, thus producing
a more complete vacuum.
Elimination of the glass "pinch seal" in which all leads
and supports are concentrated in the glass tubes, allows the
leads to enter the header of the new tube at the proper points
for short direct paths. The metal shields are, of course, much
stronger than glass bulbs, and not subject to breakage, while
the use of short stiff supports results in less mechanical vibration
of the elements.
The familiar metal shield which is necessary with the glass
tube in radio frequency portions of a circuit is no longer required
with the new tube. The metal envelope itself serves as a shield.
And since closer proximity of shield to elements is realized,
the shielding is more effective.
No details are available yet, about the price of the new tubes
compared to equivalent glass types. However, the methods of
making them, and the simplified machines required indicate that
the price will be somewhat lower than glass tubes when production
reaches its full height. Manufacture of the new tubes will start
early in the summer - but it is expected that the first sets
to use them will be the fall models of the G.E. line.
In a preliminary folder issued by RCA Manufacturing Co. Inc.,
six of the new metal tubes are identified with type numbers.
The 6A8 is listed as a pentagrid converter (similar to 6A7)
- the 6C5 is a triode (similar to the 76) - the 6D5 is a power
amplifier triode (equivalent to the 45 but having a 6.3 V.,
0.7 A. filament) - 6H6 is a twin diode (this tube has no equivalent
in glass tubes) - 6J7 is a triple-grid detector-amplifier (somewhat
similar to the 6C6 - though the characteristics are not identical)
and the 6K7 is an R.F. pentode with remote cut-off (similar
to the 6D6 glass tube).
Posted December 31, 2015