8 November 2017
THE HAM‘S WIRELESS WORKBENCH ; BY WARD SILVER N0AX
This was the era of radioactivity, relativity,
electromagnetic waves, and the quantization of energy
— literally everything changed as our understanding of
the physical world was revolutionized by a stream of
discoveries and inventions. Today, however, the vacuum
tube is largely extinct in electronics except for a few
applications where it still works very well.
What Can a Tube Do Best?
Audiophiles and musicians — particularly guitarists —
are well-acquainted with the technology; “tube amps” are
still sought-after equipment. The sound of a vintage electric
guitar blasting through an equally vintage tube “head”
(turned up to 11, of course!) might well be the clarion of
the 20th century’s latter half. Tubes still hold sway on stage
and in an audiophile’s rack; not because of their fidelity,
but because of the way they lack it!
A tube’s non-linearities color the amplified signal
in ways that are pleasing to the human ear. Similarly,
instruments themselves developed over the centuries to
produce a sonic palette that humans like. The tube is well-suited to reproduce it, and so instruments and tubes and
amplifiers all co-evolved into producing sound with spectral
qualities we have grown to expect. Solid-state may have
better looking specs on paper, but the ear knows what it
wants! Expect tube amplifiers to be with us for some time
Along with audio amplifiers, hams have relied on tubes
since the 1920s when AM broadcast receivers created a
huge market for them. Prices dropped and hams popped
tubes into receivers and transmitters as quickly as new
types were developed. Transmit power climbed from the
early five watt designs to 100 watters.
By the onset of WWII, kilowatt “rock-crushers” were
on the air. Military surplus flooded the market in the 1950s
leading to the desktop-kilowatt. The tube was king!
Like the dinosaurs, tubes have not been outpaced by
bigger devices but by tiny transistors, beginning with fragile
point-contact devices and relatively crude (by today’s
standards) junction diodes. The transistors kept getting
bigger and faster until tubes are only found in today’s
highest-powered applications where transistors just can’t
For hams, “steam radio” that glows in the dark is still
the norm. However, with new pallet-style devices coming
on the market, the tube “linear” is probably having its last
great hurrah before becoming a legacy technology along
with the modulation transformer and swinging choke.
Still, if you need to switch high voltage and high
current, withstand crippling transients, or generate RF
power in the megawatt range, you’ll be talking to the tube
The foundation of all tubes is thermionic emission.
Basically, that means heating a material (usually an oxide-coated wire or cylinder) to where it is so hot that free
electrons can leave the atoms to which they are normally
bound. These free electrons can move into a surrounding
vacuum where they are free to travel in response to electric
fields. This effect was originally discovered by Frederick
Guthrie in 1873 and then again by Thomas Edison a few
years later. However, it wasn’t until the turn of the century
that the effect was put to work.
Each tube has at least two elements: the cathode
from which the electrons are emitted; and the anode
or plate to which the electrons travel. A positive voltage
is applied from the cathode to the plate so that the
negatively-charged electrons move from one to the other.
This is electronic current as opposed to conventional
current which you are probably more familiar with. (See
Tube or not tube? Hams are still on the best of terms
with “hollow state” electronics!
ou might think the vacuum tube has burned itself out of electronics, but
its filaments are still burning brightly in certain quarters. The very first
electronic device capable of amplifying a signal — the vacuum tube or “valve”
as it is known in the United Kingdom — was invented by John Ambrose
Fleming (1905) and then enhanced by Lee de Forest (1906) during some heady
years in physics.