5U4 and 6AL5 Vacuum tube Diodes

5U4 and 6AL5 Vacuum Tube Diodes

The diode seems to be such a simple device people may wounder how much could be said about the diode.  Well the answer is a lot.

Books could be written about the diode, large books.  Not only could books be written about them but books are written about them. has several pages of books about diodes.  I browsed through a few of them I saw one book in paperback edition was only $750, that must be some book.  To see more just inter diode in the Amazon search box on the right and see for yourself.

A simple definition the diode is an electronic device which allows electrons to flow in one direction with much less resistance then in the other.  It could be called an electron check valve.  A check valve is a valve which allows fluid to flow in one direction but will not allow it to flow back.  The valves in our heart are check valves.

When speaking of the diode we usually think it started with Thomas Edison’s discovery in 1880 which he patented in 1883 but the real discovery of the diode goes back further.  Edison only rediscovered thermionic diodes.  The thermionic diode effect was found by Frederick Guthrie in 1873 but Guthrie was not credited with the fine because Edison patented it.

Then the normal thinking is that the semiconductor diode came many years later but the year after Guthrie found the thermionic diode Karl Ferdinand Braun discovered the crystal diode effect which was used in early crystal radios (Though it would be another 32 years before it would be used in radios, 1874 was the year Guglielmo Marconi was born).  “Cat’s whisker” crystal detector used in those early radios was a natural solid state diode.  The “crystal” was usually made of galena, a naturally occurring sulfide ore of lead, there were several other minerals that were also used as crystals such as iron pyrite (“fool’s gold”),  molybdenite (MoS2), silicon carbide, zincite, bornite, and carborundum.

A small metal wire called a “cats whisker was moved around the surface of the crystal, which was mounted in a metal container, until a sensitive point was found.  That sensitive point was nothing more then a naturally occurring PN junction.  If you carefully examine a modern clear glass solid-state diode you can see the cats whisker is still used in some diodes.

The diode has many uses in electronics.They are used as rectifiers, modulators and demodulates, amplifiers, oscillators, capacitors and voltage regulators to name a few.  A magnetron, the heart of a microwave oven, is a diode.

Tomorrow I will post a discussion on a specialized diode called a tunnel diode.  If you are not familiar with the tunnel diode I think you might find it interesting.

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