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Odd Shapes in Vacuum Tubes
Many vacuum tubes are very attractive for their shapes or even for what of their electrode structures
is visible through the glass. Every detail looks as carefully tailored to the solution of some
problems, such as maximum operating ratings, electrical interface, heat transfer, mechanical
sturdiness, shielding and more. To fully appreciate the ingenuity, the know-how and the careful
workmanship of vacuum tubes, one must first give a look at their data sheet and to their application
circuits. Only when the intended application of a given tube are known, part of the solutions used in
its design and manufacture can be fully understood and properly evaluated.
Since the early vacuum tube applications, thousands of people through the years spent their life in
understanding all the faces of the physics and the chemistry of vacuum tubes. Every interaction of
electrons and ions with electrodes and with electromagnetic fields was thoroughly studied and
experienced. Every suitable materials was characterized for its compatibility with the vacuum
technology and its usefulness in any given application; new processes were defined for each new
material, glass, ceramic, metal, and for any combination of them.
Receiving tubes were optimized for high volume production. Their shapes, and often their internal
structures, changed very slowly through the years: tennis-ball, globe, dome, GT or metal, lock-in,
all glass miniature, 7 and 9 pin, up to compactrons and nuvistors. Special tubes, on the contrary,
were designed to meet specific requirements and followed no rules, but the compliance with the
customer’s requirements. In other words, special tubes were custom designed to have the better
performances in the customer’s system. So we have as many different tubes as different
applications we find: high power broadcast transmission, high-frequency communication,
telephonic relays, for laboratory instrumentation, for avionics equipment as well as tubes for radar
sets.
Some tubes look very similar to their equivalent receiving archetypes. It is the case of ruggedized
military tubes, where accurate selection of materials, well established procedures and aging grant
uniform behavior through their life. It is the case of tubes for mobile operation, where heaters are
redesigned to operate over the extended voltage range of storage batteries. Or it is the case of
computer rated tubes, with special cathode coatings, to grant full emission even after prolonged cut-
off periods.
Odd-shaped tubes can be found in the other sections of this collection, power transmitting,
magnetrons, klystrons and more. Here are listed some additional low-power high frequency tubes
designed to keep the connections to the resonating circuit as direct as possible; sometimes the
resonating circuit is built around the tube itself.
Acorn Tubes
Acorn tubes were introduced around the mid 930s to reduce parasitic parameters, inductance and
capacitance, of the connections. Pins went out radially from the glass press around the body and
sometimes from the top and from the bottom. When the miniature 7 pin bulbs appeared, the small
electrode structures of the acorn tubes were readily moved to the new envelope, without appreciable
decrease in high frequency performances.
Door-knob Tubes
Door-knob tubes, so called for their shape, were designed to operate in the UHF region. Electrode
structures were very small and short pins connected the active elements directly to the resonating
lines. Top, two views of the 316A/VT-191, usable up to 750MHz; at 500MHz it delivered 5W.
Bottom left, a 388A, capable of 50W anode dissipation, 900MHz max operation. Bottom right, a
German equivalent, the RD12Ta, capable of 5W anode dissipation and operating at some hundreds
megahertz.
Rocket Tubes
Sylvania gave them the name ‘rocket’ because of their shape. Rocket tubes were designed to fit an
external coaxial cavity. In the years some shape variations appeared. These triodes were used as
amplifiers or oscillators up to frequencies exceeding 3GHz.
GE UHF cermet low-power tubes
GE offered the most advanced and complete line of low power cermet tubes, usable as amplifiers or
oscillators at frequencies up to 10GHz; also available factory assembled modules, based on these
tubes. The tubes were very small, typically 1 to 2.5 cm. Flat electrodes and ceramic spacers were
used to ensure uniform spacing. A: 6299,
m
110, 3GHz. B: 7077,
m
90; used in Pioneer III space
probe. C, H: 7462, lugged variant of 7077. D: 7486, variant of 7077 for use as amplifier or
oscillator. E,F: 7391, 65mW at 5.4GHz as oscillator. J: Y1610, 7.5GHz oscillator. K and M: two
MCMs, Microwave Circuit Modules, using tubes similar to the J one.
GE UHF modules
Detailed views of some GE modules. Using cermet planar triodes, they were used as stable RF
oscillators at several gigahertz. The quantity of gold all over the surface tells of the care in their
manufacture.