Even with scores of intercontinental
telecommunications satellites orbiting the Earth (not all "birds" are for telecommunications),
there is still a significant need for wired communications cable. Satellites alone
cannot handle the amount of data needing to be exchanged across the globe. At least
once a year I post a headline about a new undersea cable being run between Asia
and Australia, Asia and Africa, Europe and America, etc. I don't recall ever reading
about a subsea cable between Europe or Asia and Australia, or Asia to America -
that would be a significant achievement. Because of the lengths of the cables, signal
amplification at intervals is required, even with optical cables. Because of the
depths at which the cables lie, access for maintenance / repair is extremely difficult
and expensive. Accordingly, in addition to supremely rugged physical construction
of the cable itself, reliability of the amplifiers (relays) must be extremely high.
That nothing but the best design and construction practices humanly possible is imperative to
satisfy such requirements is evidenced by the number of superlatives used to describe
them. No follow-up article has been seen reporting on how successful this 20-year
vacuum tube amplifier actually was. Hopefully it met or exceeded all expectations.
May 1963 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
No Service Needed on This Transatlantic Subsea Cable Telephone
This is what is in the amplifier on the cover.
It's designed to work under water 20 years before repairs.
An amplifier that is expected to work perfectly for at least 20 years without
maintenance or repair is the subject of our cover this month.
The amplifier is the central feature of a new submarine telephone repeater. Installed
20 miles apart along a new transoceanic cable, these repeaters will make it possible
for the cable to carry 128 two-way telephone messages (almost twice as many as on
any present transoceanic cable).
It is possible to service an amplifier submerged under a mile or two of water,
but fantastically difficult and expensive. The unit must first be located, then
brought to the surface for repair. So Western Electric engineers made every effort
to assure that the equipment will function reliably for at least 20 years. The tubes
(W-E 455's) alone go through a process of aging and testing for nine months, during
which time each tube is tested some 2,900 times. Only about one out of every ten
survives the program. Upon completion of the aging and testing period, each tube's
record is scanned by an acceptance committee, and the tube is accepted or rejected
on that record. The committee is able to distinguish between these near-perfect
tubes. The lowest-noise type is used in the first stage, and that with the highest
power output capability in the third. The tubes are shipped in matched groups of
six-lettered A to F - enough for one amplifier.
Other components are equally carefully inspected, and quality control is so detailed
that a semi-automatic data system is required to handle it. Many of the components
- such as mica and paper capacitors - are manufactured to Western Electric's own
tolerances in the Clark, N. J. plant where the repeaters are being constructed.
Others, such as certain resistors, are bought outside and subjected to grilling
tests in the plant. A resistor that costs 30¢ originally may have some $3.00
worth of testing put in on it before it is finally accepted. The complete amplifiers
are valued at $60,000 to $70,000 each.
Components of the complete subsea cable repeater.
Functional diagram of the subsea cable repeater.
All work on the equipment is done in "clean rooms", where scrubbed-up technicians
in special garments work in a moisture and temperature-conditioned, dust-free atmosphere.
The amplifiers are in twin pairs, as shown in the figure and photographs. (The
technician on the cover is inspecting a solder joint on the amplifier, with the
help of a jeweler's loupe.) Three of the six Western Electric 455 A-F tubes are
seen in their plastic housings. They extend down into the shield cans in the section
between the two amplifiers.
The amplifiers are paralleled, to reduce the possibility of breakdown. The signal
normally goes through the two amplifiers in parallel, but should one amplifier fail,
the other one can take over the whole job. Special gas tubes (WE 456 A and 458 A)
prevent possible damage by surges, and if one of the tube heaters opens, a fusible
link is provided to bypass it and insert an equivalent resistance in its place.
Each repeater uses about 400 milliamperes at 45 volts. The 45-volt drop across
the heaters furnishes the plate voltage. Up to 11 ,000 volts can be supplied for
the transoceanic circuit, using a 5,500-volt generator at each end of the cable.
Each repeater is supplied with a quartz crystal set to an individual frequency
for that repeater. The gain of any given repeater can then be measured by sending
a signal of the correct frequency through the cable.
The complete undersea repeater includes, besides the two amplifiers in the center
section, two directional filters - one at each side of the amplifier - and two power
filters, at the ends of the repeater. The DC power and the signals are separated
by the power filter as they enter from the cable. The signals are modulated on two
carriers, the east-west ranging from 116 to 512 kc, and the west-east from 652 to
1052. (Each telephone conversation covers a 3-kc band.) The broadband amplifiers
cover the whole spectrum with equal efficiency, aided by a special inverse feedback
network, not shown in the functional diagram.
A signal from the west, after leaving the power separation filter, goes through
the west high-pass filter, through the parallel amplifiers, through the east high-pass
filter, then on through the filter that unites it with the power supply in the cable
going east. Signals in the opposite direction are on the low band, and reverse the
process, being directed through the low-pass filter into the input of the amplifiers,
and so on through the second low-pass filter to-ward the west cable.
The entire repeater is about 26 inches long and 8 inches in diameter. Cased for
submersion, it is 40 inches long and 13 inches in diameter. Weight is about 500
Posted May 24, 2023