Vacuum power tubes, using old valves, and vacuum tube failures


Vacuum Tube Failures

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3CX1500A7 failures

Filament Voltage life

HV arcing peak voltage when mistuned

811 tube history



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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”.      

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
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

manufacturers use a
variety of numbering
systems, most Eimac
tubes use an easily

number = number of
active elements in
the tube

  • 2=diode

letter = envelope

  • nothing
    or  – =

  • C
    = ceramic

letter = anode type

  • nothing
    or – = internal

  • X
    = external

  • W
    = water

  • V
    = vapor

group =  anode
dissipation (actual
dissipation can be
higher than the tube-type number, so
check the books)

letter = Base or
connection type

  • Blank
    = pins or tabs

  • A
    or Z = coaxial
    or pins

  • F
    = flying leads

number = mu, where 1
= lowest mu 
7 = highest mu  

  • 1
    through 5 are
    best suited for
    grid driven

  • 7’s
    are best for
    cathode driven

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

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


vacuum tubes operate
by a thermionic
emission process. In
a vacuum tube, we
commonly call the
electron emitter a
The cathode can be
directly heated or a
Another type is the
indirectly heated,
or heater-cathode
type of electron

cathode, regardless
of sub-category,
uses a material
easily able to
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
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.

Used Cathode

Material Operating
Temp Range
mA per cm^2
2400-2600 2-10 100-1000 Immune
to positive
ions, can be
operated at
highest anode
Instant on.
1800-2000 50-100 700-3000 Much
emission, but
less immune to
positive ions.
Can be
operated at
voltage or
operated with
peak currents
over emission
limits with
little effect
on life.
Oxide-filaments 1000-1100 200-1000 400-3000 Very
high emission
per watt of
power, but
sensitive to
positive ions.
failures occur
from low
operation if
electron cloud
is depleted.
Must be
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

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.

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

If a
cathode material
sensitive to
positive ions is
operated with
excessive HV,
or  has
excessive cathode
current for the
temperature, it will
suffer emission
failure. This is why
tubes or
oxide-cathode tubes
must have long
controlled warm-up
times before any
cathode current is
drawn. We must NEVER
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


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

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
heat the anode. That heat is generally small, although in directly
heated external anode tubes the filament to anode heat transfer can be
hundreds of watts.

In operation with large transmitting tubes, we can ignore the small
filament or grid heat transfer to the anode. Usually, the vast majority
of anode heat comes from kinetic energy of electrons as they strike the
anode. This heating is much like the heating of a resistor, and follows
voltage times current at any instant of time. If, at
any instant of time,
anode-to-cathode voltage accelerating
electrons is 3000 volts, and at that very same instant of time anode current is
200 mA, heat at that instant of time is 3000 *.2 = 600 watts. This is true for
EVERY instant of time during tube operation! The actual
time-averaged heating is the time-integrated value of the continuously varying
anode dissipation
over many thousands of full plate current cycles. The time has to be
long enough to get a good “sample” of heating.

With a typical AB2 class 3-500Z,
running at maximum power with 3000 volts, maximum instantaneous plate-to-cathode voltage is around 5500 volts.
At this voltage, anode current is at zero (or nearly
zero) current. Dissipation at the instant of peak voltage, or near peak
voltage, is 5500 * 0 = 0 watts (plus filament and grid heat transfer).

The same system would have maximum anode current as minimum anode voltage is
reached. Minimum anode voltage might be 500 volts, with 1.2 amperes of
instantaneous current. This would be, at that instant of time, 600
watts of heat (plus filament and grid heat transfer).  The large
area of the anode of the anode has significant thermal mass. The thermal
mass “smoothes” heating out to an average value over time. The actual concern is time-averaged heat over many seconds of time.
Depending on anode style, averaging time can somewhere around 15 to 45 seconds with 3-500Z

This averaging is important, because it limits anode temperature
during overloads. 1000 watts of dissipation for 10 seconds causes no
more heat problems than 500 watts would for 20 seconds, or 250 watts for
40 seconds. At some point heat escaping the anode, in the case of a
3-500 mostly through infrared radiation, places anode temperature in
equilibrium. If thermal equilibrium is reached at a safe temperature,
the tube can handle that power for limitless time.

Dissipation is closely related to anode waveform and anode current
pulse width. Class A amplifiers have steady average anode dissipation,
with anode current producing heat over the entire signal cycle. The
time-integrated, or time-averaged anode dissipation, is steady at some
level. At the other extreme, Class C amplifiers can have much less anode
heat. The anode is close to being either off or on, like a switch in
series with a small resistance. Averaged anode dissipation
is much less for a given signal output power than a more class A amplifier,
since the class A amplifier spends a
long time in an area where dissipation is high.

In an amplifier with
steady current,
anode dissipation is
found quite easily
by multiplying the
accelerating voltage
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.

following picture is
the internally
damaged anode of a
tube. This tube was
operated from a
large power supply
that had a string of
1 ohm 2 watt fault


lower power tubes
use internal anodes.
Internal anode tubes
can use any electron
emitter or grid
although the most
common large
transmitting types
Internal anode tubes
are cooled by
infrared radiation,
by direct thermal
conduction to the
envelope, and
through external
connections via
leads exiting the

anode often serves
three very important
functions in a

  • The
    anode collects
    most of the
    electron current
    from the
    electron emitter
    in the tube

  • The
    anode must
    dissipate heat
    produced from
    kinetic energy
    of electrons
    that strike the

  • The
    anode often
    degasses 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

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
heating is dwarfed
by heating from
electrons smashing into  the anode surface.

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
breakdown voltage
through a

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

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.

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.

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

either from poor
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


commonly associate
external anodes with
expensive tubes
having fragile
grids, long warm-up
time oxide cathodes,
and poorer
reliability of tubes
like the
8874, or

anode tubes, just
like internal
anode tubes, can use
any type of grid and

3CX3000’s, and
3CX10,000’s are
popular tubes using
emitters. They have
very long life,
nearly instant
warm-up, and very
rugged grids.

anode construction
provides three
primary advantages,
all of which center
around size

  • More
    (power) can be
    handled in a
    smaller package

  • Compact
    size allows
    better operation
    at higher

  • Gain
    is higher,
    electrons are
    more focused and
    controlled by
    field of

oxide-cathode tubes
enjoy the last two
advantages. Oxide
cathodes provide a
very compact
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.

construction allows
the control grid’s
electrostatic field
to control emission,
yet keep grid wires
away from the
electron streams.
The grid is placed
very close to the
(thousandth’s of an
inch), while the
anode is much
further away. 

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.

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 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

only disadvantage of
ceramic tubes is
gain and frequency
response is
generally less than
provided by

most common failures
ceramic tubes relate
to old age, and loss
of emission. Ceramic
tubes have very good
shelf life, unlike
large glass


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 and grid
normally operate at
cooler temperatures
than those where
robust thermionic
emission of
electrons occur.
With little or no
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
electron flow as
electrons impact the

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

determines rated
grid dissipation of
a tube by increasing
long-term grid
dissipation while
checking at very
short intervals for
thermionic emission
from the grid.

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

same is not true for
gold-plated grids,
like those in
metal-oxide cathode
tubes. Grids plated
with soft
materials like gold
suffer gradual
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

is why ALL
metal-oxide cathode
tubes should have a
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

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.

dissipation, like
anode dissipation,
is a time-integrated
function of
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

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

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
Metal-oxide tubes
require a gold
overlay, to prevent
contamination of

failures occur from
four primary causes:

  • Poor
    alignment of the
    grid causes some
    areas to
    without overall
    current being

  • Excessive
    grid current in
    tubes causes
    gold to migrate

  • Gold
    plating is not
    always bonded
    resulting in
    sputtering or
    flaking of gold

  • Poor
    welds or
    problems result
    in grids warping
    or moving into
    the cathode

related grid
failures, like anode
failures, are
generally the result
of a series of
somewhat lengthy
abuse spread over
long periods of

following picture is
the grid of a
that was ruined by a
single HV arc. The
only fault
protection was a
string of 1 ohm 2
watt resistors:

Old 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

section below
describes how to
remove gas in old

and Arcing

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
Uncontrolled arcs
are always the
result of gas or
element alignment in
the tube, rather
than excessive
current from

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. 

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.
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

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.

is also subject to
rusting. As odd as
it seems, glass
transmitting tubes
should be stored in
a dry
Glass tubes should
be operated at full
temperature every
few months.

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
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

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

arcing and
overheating, while
pumping down a tube,
are often a normal
part of tube


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.

amateur service,
filaments and
heaters rarely (if
ever) are operated
enough hours to have
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

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
techniques by tube
manufacturers are
critical to tube
life, and sometimes
mistakes are made.

claims are made that
oscillations can
cause filaments to
bend, but there
isn’t anything that
even remotely
supports such

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
dimensions of 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.

Does The Tube Go

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

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

of these conditions
would allow an
internal arc, with
fault current
limited mostly by
external circuit
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!

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
, and the

areas of this


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
problems within the
tube. In amateur
service, most
failures relate to
problems in the

and seal leakage are
troublesome in glass
tubes, since porous
anodes and imperfect
glass-to-metal seals
allow miniscule
amounts of gas to
enter the

insure maximum life
in Amateur Service
using thoriated
tungsten tubes:

  • Keep
    seals below

  • Avoid
    long periods of
    excessive anode
    or grid

  • Avoid
    extended periods
    of non-operation

  • Avoid
    filament voltage
    OVER the

  • Do
    not excessively
    cycle the

  • Do
    not subject the
    tube to shock or

  • Use
    some form of
    limiting in the

insure maximum life
in Amateur Service
using metal-oxide
cathode tubes:

  • Keep
    seals below

  • Avoid
    long periods of
    excessive anode

  • Avoid
    even very short
    periods of
    excessive grid
    dissipation by
    including a fast

  • Avoid
    filament voltage

  • Do
    not excessively
    cycle the

  • Always
    allow full
    warm-up before
    allowing any
    cathode current
    to flow

  • Never
    apply excessive
    high voltage, as
    it may strip or
    poison the

  • Do
    not subject the
    tube to shock or

  • Use
    some form of
    limiting in the


since Aug