Vacuum power tubes, using old valves, and vacuum tube failures
Vacuum Tube Failures
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HV arcing peak voltage when mistuned
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Power
Grid Tubes
Power grid vacuum tubes are used in amplification systems where
grid to cathode voltage controls plate (anode) current. The system can be linear
or non-linear, it can be an amplifier or a regulator. Essentially the tube acts
like a variable resistance in series with a diode, the cathode to anode
“resistance” controlled mostly by the grids. The tetrode or pentode, when
operated normally, is a “stiffer” resistance with regard to anode-cathode
voltage. Triodes are quite a bit “looser”, the anode-cathode voltage having
somewhat more influence on anode-cathode “resistance”.
Power-grid
vacuum tubes cover a
wide range of
applications. A
wide variety of
styles have evolved
over the years.
Every tube design is a
compromise of some
type, designed to
better fit a few specific applications or improve certain characteristics. For
example, there are
dozens of grid and
cathode (electron
emitter) designs.
There are a wide
variety of anode
styles and types.
Even the heaters or
filaments have
many shapes, materials,
and characteristics.
Transmitting tubes typically operate at fairly high voltages.
1kV is about the very least we find in modest power amplifiers, while high power
amplifiers can run at 20 kV or more. The normal peak anode voltage, in a typical
class AB amplifier, is nearly twice the dc anode
voltage. This means the tube has to withstand several times the maximum dc
voltage applied to the stage. I consider a 3-500Z tube good when the anode holds
off at least 8kV in a static leakage test. This requires a very hard vacuum. The
slightest amount of gas will significantly reduce breakdown voltage.
The big enemy of large glass envelope vacuum tubes that
have not been regularly operated is gas.
This is especially true with graphite anode tubes and tubes with large pins. The porous graphite
tends to slowly release trapped gas over time. Glass envelope transmitting tubes generally use a Kovar coated wire or pin. The Kovar coating allows bonding to a special composition glass. Tubes with large diameter Kovar coated pins protruding through bonding compatible (borosilicate) glass, such as 4-250A through 4-1000A style envelopes, often develop storage failure from gas ingress through seals. Kovar seals are sensitive to humidity, and most transmitting tubes only getter or degas during actual operation. This is why new old stock glass transmitting tubes are very high risk for gas-arcing. As pin diameter increases seal leakage becomes more problematic. An 811A is fairly low risk, while a 3-1000Z is fairly high seal leakage risk.
I was at a flea market with a friend in the 1970’s. My friend purchased brand new in box 3-400 tubes with an old code date, I purchased the fresh used pull outs with a similar manufacturing code date. When we tried the tubes, his NOS tubes were gassy. My pull out tubes were perfect. NEVER buy old glass tubes without some agreed warranty!
The big enemy of
high gain power grid
tubes (or valves)
using metal oxide
cathodes is grid
current and
excessive cathode
current, or low cathode temperatures.
The common
problems of both types will be discussed in more detail below.
No one likes to waste money, or see parts go bad. In an effort
to help others have longer tube life, occasionally claims that “running tubes easy will
increase tube life” appear. That might or might be true, because it depends on the tube
type and what is “hard” and what is “easy”. There are many cases where running
“easy” can shorten tube life, and some cases where running “hard” can damage a
tube in seconds.
Bad information is everywhere, and sometimes accepted as fact.
The ARRL, as careful as they sometimes try to be, has published more than a few
incorrect articles about amplifier and tube life. It is easy to see how this
happens, because few people keep meaningful statistics, and few seem to
intimately understand vacuum tubes and amplifiers. it is a dying science, like
general electronics knowledge, because few are interested in why things happen,
and some of the curious never actually measure what they claim to be measuring.
Tube Types and Life (overview)
A thoriated tungsten filament tube can be run “hard”, to the point
of complete filament-cathode emission saturation, and the life will be no shorter
or longer than when run easy, provided the elements being bombarded by electrons
or the envelope does not overheat and suffer permanent thermal damage. We can
lower filament voltage in a thoriated tungsten tube, and provided it doesn’t
get contaminated from prolonged operation at excessively low voltage, all that
happens is peak clipping.
A metal oxide cathode tube can quickly suffer damage if operated that way. This
is why they sometimes have to start on a timer that prevents current before the
cathode is fully heated, which sometimes can be the warm-up time of a rectifier
tube and other tubes in the system! Lower the filament voltage in a metal oxide
cathode tube too low, and you can ruin it in seconds!
Within emission and thermal damage limits, tubes basically do not wear any
faster or slower if just idled or operated. It isn’t like a mechanical engine,
where high RPM operation greatly increases wear by mechanically loading internal
parts increasing friction. As a matter of fact, too cold is often much worse
than hot.
There was an RCA transmitter that used a 4-400A in a driver stage, and the tubes
had chronic failures. This was because the anode never showed color. At Eimac’s
suggestion, RCA changed to a lower dissipation tube in that tube family and life
became normal. The anode ran a little red, and life increased.
The single most common problem today with tubes is like with anything else, poor
quality control of materials (ingredients), unsanitary assembly, and poor
conditioning, testing, or aging. Most tubes come from the same few places, and
just have different external labeling or “features”, while the insides are the
same with the same problems.
The days of 20-year tubes are pretty much gone, and won’t come back by any
change made after the assembly process. The common premature failure problems
cannot be weeded out, operated out, or adjusted out. Poor operating, where an
anode is melted, is controllable. Tube labels, retailers, or other things we do,
unless grossly wrong, make no overall difference at all for most tubes.
Eimac Numbering
System
While
manufacturers use a
variety of numbering
systems, most Eimac
tubes use an easily
decoded
system.
First
number = number of
active elements in
the tube
-
2=diode
3=triode
4=tetrode
5=pentode
First
letter = envelope
type
-
nothing
or – =
glass -
C
= ceramic
Second
letter = anode type
-
nothing
or – = internal -
X
= external -
W
= water -
V
= vapor
Number
group = anode
dissipation (actual
dissipation can be
higher than the tube-type number, so
check the books)
Last
letter = Base or
connection type
-
Blank
= pins or tabs -
A
or Z = coaxial
or pins -
F
= flying leads
Last
number = mu, where 1
= lowest mu
through
7 = highest mu
-
1
through 5 are
best suited for
grid driven
applications -
7’s
are best for
cathode driven
From
this we see a
3CX3000F7 is a
triode with ceramic
envelope and
external anode, has
approximately 3000
watts dissipation
(in fact dissipation is
4000w), has flying
leads, and highest
mu of the 3CX3000 group (most suitable
for grounded
grid).
In
addition, some tubes
have additional
letters inserted in
the type number.
Examples are the
4PR1000 or
3CPX800A7. The P
generally signifies
pulse rating, and
the R more rugged
construction.
Electron
Emitters
Power-grid
vacuum tubes operate
by a thermionic
emission process. In
a vacuum tube, we
commonly call the
electron emitter a
“cathode”.
The cathode can be
directly heated or a
filament-cathode.
Another type is the
indirectly heated,
or heater-cathode
type of electron
emitter.
The
cathode, regardless
of sub-category,
uses a material
easily able to
“give-up”
electrons with heat.
The cathode is
heated in a nearly
perfect vacuum to
temperatures from around
1000K (we would see
that as a red glow)
to as high as 2600K (a
yellowish-white
glow). This heat
supplies energy to
loosely bound
electrons at the
cathode’s surface,
causing some
electrons to escape
the cathode material. These
electrons form a
cloud suspended
around the cathode.
Commonly
Used Cathode
Materials
| Material | Operating Temp Range K |
Emission mA/watt |
Emission mA per cm^2 |
Special Features |
| Pure Tungsten |
2400-2600 | 2-10 | 100-1000 | Immune to positive ions, can be operated at highest anode voltages. Instant on. |
| Thoriated Tungsten |
1800-2000 | 50-100 | 700-3000 | Much higher emission, but less immune to positive ions. Can be operated at reduced filament voltage or operated with peak currents over emission limits with little effect on life. Instant on. |
| Oxide-filaments | 1000-1100 | 200-1000 | 400-3000 | Very high emission per watt of filament power, but extremely sensitive to positive ions. Emission failures occur from low temperature operation if electron cloud is depleted. Must be thoroughly warmed up before any current is drawn, but warm-up time is short. |
| Oxide-cathodes | 1000-1100 | 10-200 | 1000-3000 | High emission in small cathode. Long warm-up time. High peak emission for pulse applications. |
Positive
ions are formed
through ionization
of residual
gasses. High
anode voltages speed
formation of
positive ions. The
space charge
(electron cloud)
surrounding the
cathode repels
positive ions, and
keeps the ions from
poisoning the
cathode. When
positive ions
contaminate the
cathode, electron
emission is reduced.
Tungsten
has the highest
melting temperature
of any metal, above
3600K. Tungsten has
the ability to
operate at high
temperatures for
many hours without
evaporation of
materials becoming a
major problem.
Tungsten is one of
the few materials
able to withstand
total depletion of
the electron cloud
in a vacuum tube.
Tungsten emitters
are not damaged by
stripping away of
electron clouds, and
the resulting
cathode bombardment
by positive ions.
This immunity to
damage means
emission life is not
shortened by
excessive current,
operation before
full temperature is
reached, or low
filament
voltage.
If a
cathode material
sensitive to
positive ions is
operated with
excessive HV,
or has
excessive cathode
current for the
cathode
operating
temperature, it will
suffer emission
failure. This is why
indirectly-heated
tubes or
oxide-cathode tubes
must have long
controlled warm-up
times before any
cathode current is
drawn. We must NEVER
operate
the 3CX1500A7/ 8877,
3CX800A7, or other
oxide-cathode tubes
at reduced heater
voltage! This
would allow the
cathode’s protective
electron cloud to be
depleted, and
nothing would
prevent positive
ions from striking
and poisoning the
cathode.
Anodes
The
anode is operated
with a positive
potential. This
creates a strong
electric field to the more negatively charged cathode. This electric potential
difference causes an electrostatic (truly static at any frozen instant of time,
but changing over time dynamically) field that
attracts electrons
from the cathode.
The grid or grids set up a static field surrounding the grid
wires. This field, because it is between the anode and cathode, changes the
field difference between the anode and cathode. It really becomes a multi-part
system, with the grids changing the potential difference the cathode “sees”.
Cathode electrons from a cloud formed around the cathode are
either accelerated or
slowed as they pass
through the opening
in the grids.
Cathode electrons eventually strike
the anode at
reasonably high
velocity. Since the
electrons have mass
and speed, they have
kinetic energy. This
energy produces
heat, as well as
dislodging other
electrons from the
anode. The amount of
heat is proportional
to the velocity and
number of electrons.
The dc electrical power converted
to heat is anode
voltage times anode
current, at any
frozen instant of
time.
The
actual plate
dissipation, or plate heating, is the
time-integrated product of anode
current times voltage over a long period of small samples of time.
| Handbooks tell us anode dissipation is plate input power minus RF output power. While that method does not include everything, it is a way to reasonably estimate anode dissipation.
In large high-power tubes, elements like the filament and screen grid In operation with large transmitting tubes, we can ignore the small With a typical AB2 class 3-500Z, The same system would have maximum anode current as minimum anode voltage is This averaging is important, because it limits anode temperature Dissipation is closely related to anode waveform and anode current |
In an amplifier with
steady current,
anode dissipation is
found quite easily
by multiplying the
accelerating voltage
(anode-cathode
potential) times
anode current. In an
amplifier system,
dissipation is a
complex function of
the constantly
changing anode
voltage and current.
In such systems, a
very close
approximation of
anode dissipation is
given by multiplying
average anode
current by average
anode voltage, and
deducting the useful
power extracted by
the load.
The
following picture is
the internally
damaged anode of a
YC-156/3CPX5000A7
tube. This tube was
operated from a
large power supply
that had a string of
1 ohm 2 watt fault
resistors:
Internal
Anodes
Many
lower power tubes
use internal anodes.
Internal anode tubes
can use any electron
emitter or grid
configuration,
although the most
common large
transmitting types
use
thoriated-tungsten
filament/cathodes.
Internal anode tubes
are cooled by
infrared radiation,
by direct thermal
conduction to the
envelope, and
through external
connections via
leads exiting the
envelope.
The
anode often serves
three very important
functions in a
transmitting
tube:
-
The
anode collects
most of the
electron current
from the
electron emitter
in the tube
(cathode) -
The
anode must
dissipate heat
produced from
kinetic energy
of electrons
that strike the
anode -
The
anode often
degasses the
tube
The
material used in the
anode varies with
tube type and
manufacturer. Most
transmitting tubes
use molybdenum,
tantalum, graphite
(carbon), or other
high temperature
materials in the
anode. The material
must be mechanically
and electrically
stable, even while
operating with very
high
temperatures.
In
transmitting tubes,
virtually all anode
heating comes from
the kinetic energy
of electrons
striking the anode.
The actual
resistance of
materials in the
anode (and other
elements in the
tube) is very low,
and the relatively
small amount
of 
heating is dwarfed
by heating from
electrons smashing into the anode surface.
Higher
power internal anode
tubes almost always
have a gettering
material coated
directly on the
anode. The getter
acts as a sponge,
soaking up any gas
molecules inside the
tube. This is
necessary because
any gas creates
positive ions,
reducing tube life.
Not only that, even
the slightest amount
of gas greatly
reduces
breakdown voltage
through a
vacuum.
Note:
Peak anode voltage
in a properly tuned
and properly driven
amplifier approaches
two times the dc
anode potential, and
under conditions of
mistuning, load
faults, or excessive
drive anode voltage
can be several times
the dc anode
potential. It is
important that any
tube have an anode
breakdown voltage at
least four or more
times the expected
highest dc anode
potential, or
occasional anode to
grid (and cathode)
arcing may
occur.
The
most common
gettering material
is zirconium. For
example, zirconium
is used on the
outside of graphite
or molybdenum anodes
in 3-500Z and other
tubes. It is the
dull gray powdery or
grainy texture
coating you see on
the surface of
3-500Z, 811A, and
572B anodes.
Zirconium
getters best at
about 1000 degrees
C, this is why large
metal anode
transmitting tubes
like the 4-400A,
4-1000A, and 3-500Z
must be operated
with a dull red to
red anode color.
Zirconium also
releases some gasses
and absorbs other
gasses at various
temperatures. The
varying temperature
across the length of
the anode (and as
the anode heats and
cools) allows the
gettering agent to
absorb a wide
variety of gasses.
The
quickest way to ruin
a 3-500Z, or other
glass power grid
tube, is to never
show anode color
over a prolonged
period of time!
Storing a 3-500Z for
many years without
operation almost
guarantees a
flash-over will
occur at the first
application of high
peak anode
voltage.
Gas,
either from poor
manufacturing
processing,
out-gassing from
elements, or seal
leakage is the
primary failure mode
of glass tubes.
Secondary to
high-vacuum (gas)
arcs are problems
like bad welds or
grid or cathode
materials that fall
out of alignment due
to thermal or
mechanical
stresses.
External
Anodes
We
commonly associate
external anodes with
expensive tubes
having fragile
low-dissipation
grids, long warm-up
time oxide cathodes,
and poorer
reliability of tubes
like the
3CX1500A7/8877,
8874, or
3CPX5000A7.
External
anode tubes, just
like internal
anode tubes, can use
any type of grid and
cathode
structure.
3CX1200’s,
3CX3000’s, and
3CX10,000’s are
popular tubes using
thoriated-tungsten
emitters. They have
very long life,
nearly instant
warm-up, and very
rugged grids.
External
anode construction
provides three
primary advantages,
all of which center
around size
reduction:
-
More
dissipation
(power) can be
handled in a
smaller package -
Compact
size allows
better operation
at higher
frequencies -
Gain
is higher,
because
electrons are
more focused and
better
controlled by
the
electrostatic
field of
grids
Only
oxide-cathode tubes
enjoy the last two
advantages. Oxide
cathodes provide a
very compact
high-current
emitter. Normally
cathode oxide is
deposited in bands
or rings, and grid
wires are aligned
directly over the
gaps in oxide bands.
The 8877, for
example, has fairly
large diameter
cathode (over
1″). More than
100 concentric bands
of oxide are
deposited on the
cathode, and a grid
wire is aligned just
outside the area of
each emission band.
This
construction allows
the control grid’s
electrostatic field
to control emission,
yet keep grid wires
away from the
primary
cathode-to-anode
electron streams.
The grid is placed
very close to the
cathode
(thousandth’s of an
inch), while the
anode is much
further away.
The
result is very low
grid current (grid
intercept) from wire
placement out of the
electron stream, and
very high gain from
the very high ratio
of electric fields
(from distant anode
and very close grid)
reaching the cathode
electron cloud.
Unfortunately
the very same things
that create very
high gain also cause
manufacturing and
potential operating
problems. The close
spacing of cathode
and grid increases
the chance of
grid-cathode shorts.
Critical placement
of grid wires
outside of the many
cathode bands makes
the grid susceptible
to alignment
problems. If every
grid wire, out of
hundreds of wires,
is not perfectly
aligned outside the
electron stream
electrons will
impact one or more
wires and cause gold
to migrate off the
grid. The mechanism
is much like water
evaporation, rather
than a catastrophic
event like boiling
or melting. The gold
will slowly
evaporate and
redeposit elsewhere
in the tube, either
causing arcs,
shorts, or poisoning
of cathode oxide.
This is the most
common failure of
metal-oxide cathode
tubes.
Ceramic
tubes with thoriated
tungsten electron
emitters have much
longer life and
higher reliability
than any other type
of power grid tube,
as a general
rule. They
combine the best of
both worlds, having
the low seal leakage
and low out-gassing
typical of ceramic
external anode tubes
and the rugged
wider-spaced high
dissipation
grids.
The
only disadvantage of
thoriated-tungsten
ceramic tubes is
gain and frequency
response is
generally less than
provided by
oxide-cathode
tubes.
The
most common failures
in
thoriated-tungsten
ceramic tubes relate
to old age, and loss
of emission. Ceramic
tubes have very good
shelf life, unlike
large glass
tubes.
Grids
Grid
control the movement
of electrons inside
the tube through
electric fields
surrounding the grid
wires. The electric
field extends beyond
the many wires
making up the grid,
almost completely
dominating the high
electric field
created by the
anode.
The
anode and grid
normally operate at
cooler temperatures
than those where
robust thermionic
emission of
electrons occur.
With little or no
thermionic
emission, the
electric potential
difference (strong
electric field
gradient) between
the anode and grid(s)
results in minimal
current flow
(electron movement)
between anode and
grid. Most of the
current that does
flow between anode
and grid occurs
because of stay gas
molecules creating
ions or electrons
dislodged from the
anode by kinetic
energy of
cathode-to-anode
electron flow as
electrons impact the
anode.
Elevated
grid temperatures,
primarily caused by
kinetic energy of
cathode electrons
striking the grid,
may eventually
reach magnitudes
where the grid
actually starts to
show thermionic
emission. Many
people assume the
grid dissipate
rating of a tube is
a value set by grid
failure, but that is
incorrect. The
dissipation rating
is actually a power
related value,
indicating the power
required to heat the
grid enough to start
thermionic
emission.
Eimac
determines rated
grid dissipation of
a tube by increasing
long-term grid
dissipation while
periodically
checking at very
short intervals for
thermionic emission
from the grid.
In
actuality, tungsten
grids in tubes like
3-500Z’s can be
operated at
temperatures where
they show color
without permanent
damage. While tube
operation at such
temperatures is
compromised, the
grid and tube often
suffers no permanent
damage.
The
same is not true for
gold-plated grids,
like those in
metal-oxide cathode
tubes. Grids plated
with soft
poorly-bound
materials like gold
suffer gradual
long-term
deterioration even
with moderately low
levels of grid
current. Heat the
grid of a
metal-oxide cathode
tube to temperatures
even well below
incandescence, and
the result is
instant irreversible
failure.
This
is why ALL
metal-oxide cathode
tubes should have a
fast-acting
electronic grid
protection systems,
and why fuses and
worse yet resistors
intended as fuses
offer no protection
at all to grids in
metal-oxide cathode
amplifiers.
Just
as anode dissipation
can not be
determined by simply
multiplying Ip times
Ep in a working
amplifier, control
grid dissipation can
not be determined by
simply multiplying
voltage by current
in a sub-class 2
amplifier (i.e.
AB2). Beware of any
article or author
who tells you RF
grid voltage can
simply be multiplied
by average grid
current to determine
grid dissipation.
Grid
dissipation, like
anode dissipation,
is a time-integrated
function of
instantaneous
dissipation
throughout the RF
cycle. Computer
models have made the
difficult task of
calculating grid
current simple and
accurate. Short of
that you need to do
an actual complex
analysis of the grid
system, such as in a
Chaffee
analysis.
In
grid driven stages,
assuming total power
applied at the grid
is accurately known,
grid dissipation is
determined by
deducting bias power
from RF grid power.
This is very similar
to the short-form
determination of
anode dissipation
from anode power and
RF power output.
This does not easily
work in
cathode-driven PA’s,
because drive power
is mixed with output
power.
Grids
are normally
supported only at
one or two places,
and use very small
wires. Normally the
base material
forming the grid is
selected to be very
hard, and to have
minimal movement
over wide
temperature changes.
The normal base
metal in high power
tubes is tungsten,
just as used in
filaments.
Metal-oxide tubes
require a gold
overlay, to prevent
contamination of
materials.
Grid
failures occur from
four primary causes:
-
Poor
alignment of the
grid causes some
areas to
“hot
spot”
without overall
current being
high -
Excessive
grid current in
metal-oxide
tubes causes
gold to migrate -
Gold
plating is not
always bonded
properly,
resulting in
sputtering or
flaking of gold -
Poor
welds or
material
problems result
in grids warping
or moving into
the cathode
Non
tube-manufacturer
related grid
failures, like anode
failures, are
generally the result
of a series of
somewhat lengthy
abuse spread over
long periods of
time.
The
following picture is
the grid of a
YC156/3CPX5000A7
that was ruined by a
single HV arc. The
only fault
protection was a
string of 1 ohm 2
watt resistors:
Using
Old Tubes
Tubes
that have “set
up” for a while
often collect gas.
This gas either
comes from slow
leakage through tube
seals or outgassing
of tube elements.
This is the number
one problem with old
vacuum tubes.
Another cause of
leakage currents are
deposits on the
insulation inside
the tube envelope.
This can come from
long filament hours
on metal oxide
cathode tubes.
Cathode materials
can migrate to the
insulation and form
a high resistance
path.
The
section below
describes how to
remove gas in old
tubes.
Gettering
and Arcing
Gettering
is very important,
since even miniscule
amounts of gas will
cause a
low-resistance arc
from anode to grid
or cathode. The
normal results of
such arcs are blown
grid chokes,
collapsed anode
chokes, damaged
meter shunts, and
other problems cause
by high fault
currents. While a
few people blame
high fault currents
on parasitics, it is
actually impossible
for a parasitic to
create such arcs.
All the anode and
grid can do is
deplete the electron
cloud from the area
of the cathode, and
the available
current even with a
parasitic is limited
by the available
emission.
Uncontrolled arcs
are always the
result of gas or
element alignment in
the tube, rather
than excessive
current from
oscillations.
Anode
systems should have
series-resistance to
limit peak current
in the event of a
tube arc or failure.
That series
resistance should
always be in the
anode lead between
the filter
capacitors and RF
choke. A diode clamp
should be installed
to protect meters,
especially the grid
meter since the grid
is in the normal
path of any internal
tube arc.
Ceramic
tubes, because of
low anode operating
temperatures, have
the gettering agent
applied to the
cathode or filament
assembly. This is
the only area inside
the tube that heats
enough to activate
most common
gettering materials.
Ceramic
tubes without
internal flaws or
broken seals can
generally be
gettered by running
the filament at
rated voltage for an
extended period of
time before
application of any
high voltage. The
normal time for
gettering is between
one hour and one
full day. If the
tube does not getter
within a day it is
most likely never
going to be restored
to a operational
relatively pure
vacuum.
Glass
internal anode tubes
generally have the
gettering material
coated on anodes,
which must be
operated a high
temperatures to
activate the getter.
Glass tubes have a
propensity for seal
leakage and element
out-gassing, both of
which lead to a
short self life for
large tubes. It isn’t
the glass that leaks
gas, but rather a
Kovar alloy used to
bond the glass to
the metal protruding
through the envelope.
Kovar
is also subject to
rusting. As odd as
it seems, glass
transmitting tubes
should be stored in
a dry
location.
Glass tubes should
be operated at full
temperature every
few months.
Under
some conditions a
glass tube can be
restored to
operation by running
low anode voltages
and positive bias on
the grid. This will
sometimes allow full
operating anode
temperatures to be
reached, and the
tube can be
“cooked”
for several hours.
I’ve had about a 50%
success rate
restoring old
3-500Z’s that have
sat for years
without use. Even
though they
initially arced
severely at full
voltage, by cooking
them at low voltage
and positive grid
bias to show anode
color vacuum was
restored.
Most
important, an arc by
itself will break
down and getter gas
inside the tube.
This is why an
amplifier with a
gassy tube will
sometimes operate
without problem
after a sudden tube
arc.
Intentional
arcing and
overheating, while
pumping down a tube,
are often a normal
part of tube
manufacturing
processes.
Filaments
Filament
life in vacuum
tubes, like filament
life in light bulbs
is a complex
function of
operating hours, hot
and cold cycles
(thermal shock),
mechanical shock,
and temperature.
Other than avoiding
physical shocks to
the tube, or cycling
the filament off and
on needlessly,
temperature is the
only variable we can
control. We should
be sure the filament
is operated at or
below the maximum
recommended voltage.
In
amateur service,
filaments and
heaters rarely (if
ever) are operated
enough hours to have
operating-time
related failures.
Most tube filament
failures are
mechanical failures,
more related to
constant on-and-off
cycling of the tube
than wear from
extended hours of
conservative
operation.
Mechanical
issues such as
material or assembly
defects can combine
with close element
spacings inside the
tube, and cause
failures. With
spacings in
thousands of inches,
even the slightest
change in physical
shape or positioning
filaments or
cathodes can create
a grid-to-filament
(or cathode) shorts.
Material quality and
manufacturing
techniques by tube
manufacturers are
critical to tube
life, and sometimes
mistakes are made.
Some
claims are made that
oscillations can
cause filaments to
bend, but there
isn’t anything that
even remotely
supports such
claims.
For
example, the normal
peak filament
current of a
sine-wave powered
3-500Z filament is
about 1.414 times
15-amperes, or 21
amperes. The total
force on the
filament helice is
11 grams distributed
over a 7 cm area for
this 21-ampere
current in the
typical helical
structural
dimensions of the
filament.
The
small additional
force of a fully
saturated emission
condition (which
requires around 1000
volts of positive
grid voltage)
results in only 30%
increase in total
bending force,
hardly significant
in any filament
robust enough to
last years in normal
operation. Not only
can the peak current
not reach fatal
levels though any
type of emission,
the grid-cathode
potential is not
able to reach levels
required to
saturate filaments.
Why
Does The Tube Go
Bang?
In
order for any tube
under any condition
to draw more than
the peak saturated
current, operating
potential has to
exceed the breakdown
voltage of the
tube.
There
are two reasons
excessive current
might flow between
elements in a tube:
-
The
peak anode
supply voltage
exceeds the
hold-off voltage
of the tube,
causing a good
tube to arc -
The
tube may be
defective either
through gas or
an incorrectly
located anode or
grid
Either
of these conditions
would allow an
internal arc, with
fault current
limited mostly by
external circuit
resistances.
Oscillations, like
normal drive power,
can only take a
healthy tube up to
the saturated
emission limits.
Even then it would
take hundreds or
thousands of
grid-cathode volts
to saturate the
emission, an
impossible condition
unless someone
drives a 3-500Z
mistakenly with a
4CX5000 or some
other large PA!
There
is a special
condition where
tubes can fail, and
that is where
excessively high
voltage components
are used in tank
systems. Under load
fault conditions,
when nothing absorbs
energy supplied to
the tank and tank
components do not
saturate or
breakdown, the tube
peak anode voltage
can reach levels
where an arc occurs
in a healthy
tube. See the load
faults and tuning
section, and the
practical
demonstration
areas of this
website.
Summary
Tubes
are relatively
robust components,
but they are one of
the primary points
of failure in any
electronic device
that uses tubes.
While many of us
would like to
believe we can make
small circuit
changes to prolong
tube life, the fact
is most tube
failures are related
to material or
construction
problems within the
tube. In amateur
service, most
failures relate to
manufacturing
problems in the
tube.
Out-gassing
and seal leakage are
particularly
troublesome in glass
tubes, since porous
anodes and imperfect
glass-to-metal seals
allow miniscule
amounts of gas to
enter the
vacuum.
To
insure maximum life
in Amateur Service
using thoriated
tungsten tubes:
-
Keep
seals below
rated
temperature -
Avoid
long periods of
excessive anode
or grid
dissipation -
Avoid
extended periods
of non-operation -
Avoid
filament voltage
OVER the
manufacturer’s
rating -
Do
not excessively
cycle the
filament -
Do
not subject the
tube to shock or
vibration -
Use
some form of
reliable
fault-current
limiting in the
anode
To
insure maximum life
in Amateur Service
using metal-oxide
cathode tubes:
-
Keep
seals below
rated
temperature -
Avoid
long periods of
excessive anode
dissipation -
Avoid
even very short
periods of
excessive grid
dissipation by
including a fast
electronic
grid-trip
circuit -
Avoid
filament voltage
OVER or UNDER
the
manufacturer’s
rating -
Do
not excessively
cycle the
filament -
Always
allow full
warm-up before
allowing any
cathode current
to flow -
Never
apply excessive
high voltage, as
it may strip or
poison the
cathode -
Do
not subject the
tube to shock or
vibration -
Use
some form of
reliable
fault-current
limiting in the
anode
since Aug
2003