Two Relics from The Vacuum Tube Days – The Acorn Tube and The Lighthouse Tube

As the years passed and radio communications engineers tried to produce transmitters and receivers that would work effectively at higher frequencies. The interelectrode capacitance was the largest limiting factor for the upper frequency a vacuum tube would work. So through the years various designs of tubes were made in an effort to reduce the interelectrode capacitance.

As you probably know anytime there is are conductors separated by insulation

Interelectrode Capacitance illustrated

there is capacitance formed by this configuration. The cathode, the grid or grids, and the plate are all separated by a vacuum which is an insulator. This is what is meant by interelectrode capacitance. As the frequency of operation is increased the capacitive reactance of the interelectrode capacitance decreases thus reducing the tubes ability to amplify.

During the late 1930s a small triode was constructed which was shaped much like an acorn. The leads, which also produce problems as the frequency increases because they too have capacitance, came out sideways from the tube thus reducing the lead capacity and the small size reduced the interelectrode capacity. Later tetrodes and pentodes acorn tubes were added to the list.

The 955 was one triode acorn tube which the data published on it said the tube would work from 4 to 600 MCs (MHz) but there were circuits using these tubes operating in the 900 MCs.

Acorn Tube

The acorn tubes were used in VHF and UHF circuit design up until the late 1940s.

An other interesting tube design that could work at a little higher power then the acorn tube, about 150 milliwatts, and  much higher frequency, 3.3 gigacycles (gigahertz), was the lighthouse tube. The lighthouse tube made its debouch in the 1940 and continued to be used in new radio design up until the early 1970s.

Typical Lighthouse Tube

The name lighthouse tube is, like the acorn tube, derived from its appearance. The lighthouse tube has the filament and cathode near the base of the tube which fit into an octal socket. The grid sat above the cathode and was accessed by a ring around the tube. Finally the plate was at the top of the tube and was connected to a plate cap. Each succeeding section of the tube was smaller in diameter thus giving an appearance that was similar to a lighthouse.

Most of the radios that made their way into ham radio operations was via amateur radio operators purchasing military surplus units which for several years after WWII were readily available for very low cost.





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