Power tubes in amateur service.
About vacuum power tubes.
By Matt Erickson KK5DR
The info I am putting forth here comes from engineers at CPI/EIMAC,
Rockwell/Collins, and my years of experience with RF power tubes. The data I
am publishing here, is supported in the technical article "Care and feeding of
power grid tubes".
Fact: There is no such thing as a perfectly sealed vacuum tube. All tubes leak.
Fact: Over time, molecules of air pass through these "imperfect" seals,
contaminating the inside of the tube.
Fact: A tube that sits on the shelf collects a relatively large amount of air inside
over time.
Fact: The longer the tube sits inactive, the more air is accumulated.
Fact: The larger the tube, the greater the leakage.
Fact: The high level of vacuum, and removal of contaminant molecules is
maintained by something known as the, "getter", which is usually a form of "rare
earth minerals", that absorb the air, when heated by the filament in the tube.
Fact: Over-heating of tube seals, can compromise the seal, and cause rapid tube
failure. Tube seals can also be damaged by high current start-up in-rush, which
can crack seals by mechanical stress.
Fact: There is no such thing as, "too much cooling air flow" over the tube. Too
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Power tubes in amateur service.
little cooling air can be a big problem, and lead to seal over-heating.
Fact: Broadcast radio/TV stations keep all their final amp tubes running with
filament current, at all times, for two reasons.
1. To keep the tube ready for instant use, should the other tube fail.
2. To keep internal contamination to a minimum.
It would be impractical for amateur radio stations to keep our tubes on at all
times, an alternative is to "rotate" the final amp tubes on an annual basis.
Many hams have a "spare set" of tubes. I have heard of several cases where a ham
had a spare set of tubes stored "safely" away in the closet for years. One day, they
decide to place the spares into operation in his aging amp. Little does he know
that over the years these tubes sat in the closet, molecules of air have been
sneaking into the tube, gathering in a "pile" near the bottom of the tube, which is
usually the negative cathode. The ham puts the tubes into the amp, turns it on, and
begins to tune up as normal, suddenly, BANG!, the tubes flash-over internally and
"self-destruct".
Here is what happened inside, the "pile" of air inside the tube, was ignited into a
"plasma" (a super hot ionized gas) by the combination of high voltage, heat, and
the igniter RF, the plasma is negatively charged, which then travels toward the
positively charged anode of the tube. Most of the time, between the anode and
cathode, lays the delicate grid, which has a large hole burned through it by the
traveling plasma cloud, which is extremely hot (up to 30k degrees). This rarely
happens to small receiver tubes, but it is not completely unknown. I have had a
number of 12BY7A RF driver tubes that over time became "gassy". The lower
plate voltages in these tubes, was not enough to "ignite" the gas, but strange
behavior of the tube made its replacement necessary. A good indication that a
tube has become "gassy" is a "blue/purple" color near the top of the tube, or
around the plate. A tube that is free of contaminant gas has no color, aside of the
filament glow.
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Solution: Keep your tubes "de-gas-ed", by "rotating" the "spares" with the regular
tubes. A good rule of thumb; is to swap out the tubes annually, while removing
dust, lubing the fan, etc. This may not be the "best" way, but likely the most
practical one.
You may ask, "how long should I allow the tube to burn-in, prior to operation?"
On a tube that has an unknown amount of shelf time, 10-12 hrs would do well.
For a tube that has been in your closest for known amount of time, I have
developed a rule; 2 hrs per year of shelf-time. When you "rotate" your tubes,
allow them 2 hrs of burn-in time prior to applying RF drive. With a new tube
fresh from the factory, 6-8 hrs for large glass envelope tubes, 2-6 hrs for
metal/ceramic tubes, burn-in time. A little patience, and conservative use, your
tubes should see full life.
Another reason to "burn-in" a new tube is that it will finish the vacuum, removing
the last remaining gasses and brings the filament/cathode up to full electron
emission.
Other contaminations
There is another source of internal tube contamination other than the outside
atmosphere. When a vacuum tube is driven by excessive RF levels, particles of
oxide and metal gases are released into the tube. These metal gases and oxides
can cause atmosphere contamination of the tube, which can lead to HV "flash-
over" inside the tube. When a flash-over happens it can cause extreme levels of
current to flow which cause further damage to grids, cathode, and tube seals,
which then causes further flash-over, you can see where this is going. A tube that
has had a history of flash-over can be opened and inspected, metallic powder and
or tiny bits of gold or oxide coating will likely be found.
This is just one more reason NOT to over drive your tube(s).
Filament voltage?
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EIMAC states that; " A 3% increase in filament voltage above the maximum
rating will result in a 50% decrease in tube life." For example: a filament with a
max rating of 5.00Vac, increased to 5.15Vac (3% above max) can expect a 50%
decrease in life span. However, decreasing the filament voltage to a low level can
cause a radical drop in electron emissions. Check your filament voltage at the tube
socket, using a VOM that reads "True RMS". If the voltage is to high (many
times it is), and your amp has a separate filament transformer, install an adjustable
power resistor in the transformer primary, and carefully adjust the secondary
voltage till it is with-in the specified range, preferable slightly lower. If your amp
uses an "all-in-one" power transformer, you can use a length of small wire to
lower the filament voltage, but this will take a great deal of "trial & error" to find
the right size and length or wire.
Plate voltage?
Exceeding the manufacturers maximum plate/anode voltage can lead to an HV
flashover, and possibly destruction of the tube and or damage to the PSU or other
parts.
A good rule of thumb; is to not exceed the maximum voltage by more than a 2%
margin. Meaning that if your tube has a max plate voltage rating of 3000vdc, the
max safe voltage would be 3060vdc. So, if you are using 3200vdc, you are taking
a gamble. The key would be to closely match the plate voltage to the tube in use.
For maximum gain, and best safety margin. This rule is most important when
installing a new tube, which has a large gain level when compared to the old weak
tube. Also, the new tube may have a high level of gas remaining in it, and
operating the tube over its max plate voltage rating may well push it "over the
edge".
A tube operating at maximum plate voltage, will reach its maximum RF gain
level, and max power output level too. Anything over that max level becomes
wasteful, dangerous, and sometimes unstable. Sure, it looks great on the watt
meter, but it might come at a high price.
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Emission
Every vacuum tube has an element that is known as the electron emission source
usually it is either an oxide coating as in indirectly heated cathode tubes, or a
treated filament as in directly heated cathodes. When the element is heated, either
directly or indirectly, it emits "extra" electrons that are driven by the electro-
motive force & current flow within the tube, to the anode, this is how a tube
amplifies. This process is continuous, as long as the tube heater/filament is active,
electrons flow, even when the tube is in standby/cut-off mode. The supply of
electrons is NOT endless, every tube that is active, is on the way to the end of its
life span. However, turning the tube "off" and "on" a number times unnecessarily
would cause more damage than to leave the tube on for a few hours between uses.
Thermal cycling is more harmful than continuous operation, but a balance of the
two should be had.
For example; If you use you amplifier for a morning schedule, and then again for
a noon sked, it would be better to leave the unit on and in standby mode during
the interim time. If you operate several times during the day with your amp, it is
far better to leave the amp on all day than to turn it on and off several times each
day. This minimizes the thermal cycling of the filament/heater elements. Thermal-
cycling is far more damaging then loss of emission levels.
Emission levels remain nearly constant all the way from new tube, to 10,000 hrs
or more of continuous filament operation. When a tube is brand new its emission
will be lower than after it has been in operation for a few dozen or more hours.
This interval in known as the "burn-in" time, where-in the tubes vacuum is
"finished" by the heat applied to the "getter", also the emission element is coming
up to full electron flow during this time. After a few hundred hours the tube
emission will reach a peak, after this the tube will have a very long slow glide to
the end of its life span which is the point where the emission falls too low to be
usable, or the filament/heater fails. Usually the end of the tube life is somewhere
between 12,000 and 24,000 hours of filament/heater on-time. By amateur radio
usage standards this is a very, very long time. I would not worry too much about
your tubes failing in this mode, 90% of all tubes used in ham radio fail due to
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chronic over-drive conditions, which damages the oxide coatings of indirectly
heated tubes, and or the grid structures of directly heated filaments. This is NOT
my opinion alone, but info shared with me by the engineers at EIMAC. They have
seen it far too many times, for too many years for it to be an accident.
Above is a graphic representing the emission level verses hours, of a typical
power tube. The graph is not to scale, the hours side is 12K hours.
Keep the thermal cycles of the tube to a minimum, and RF drive levels to the
proper settings as prescribed by the amplifier manufacturer.
Keep your tune-up times down, and extend the life of your tubes.
To read much, much more about RF power tubes, stop by the EIMAC web site,
and read the technical article titled "Care and feeding of power grid tubes."
Mind you, this booklet is fairly technical and might be above the level of the
average ham today, but it is very informative. Please download the PDF, you will
learn much more than I can go into here.
How does a vacuum tube amplify RF power?
The EIMAC booklet goes into much greater detail, but here is a quick and dirty
explanation. The cathode surface emits free electrons, which are propelled by the
flow of DC current and RF, driven by the RF voltage and DC voltage combined.
These free electrons account for a greater number of electrons arriving at the plate
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than are accounted for by the DC current alone. Thus we have amplification of
any signal, audio or RF, that is imposed on the DC current stream.
Tube conditioning:
New tubes should be carefully "burned-in", some will have "flash-over" problems
in the very beginning of their life due to residual gases and metal vapors
remaining inside the tube. I have found that this process can take some time, and
RF drive should be very carefully applied to the tube during this time. If plate
current levels become unstable, the RF drive and or the plate voltage should be
reduced. Sometimes the process of "conditioning" the tube could take all day (8-
24 hrs). During the process, the tube is run with heater/filament only (no RF
drive) for at least the first 4 hrs of operation. Then, a small amount of RF drive is
applied, if abnormal operation is noticed, the drive should be removed and the
tube should be "burned-in" for a few more hours. Gradually, during the
conditioning period the RF drive signal is increased, operating conditions are
closely monitored during this time. When full power out is reached, and normal
operating conditions are observed, the tube can be considered fully "conditioned"
and ready for regular use in the amp.
Used tubes that have been sitting in storage for a number of years should be
treated in the same way, but the time of "burn-in" is shorter since the tube has
already been conditioned when it was new. The main reason for repeating this
process is to "burn-off" any contaminating gases that have reached the inside of
the tube. The general "rule of thumb" for the number of hours of conditioning
required for a used tube is simple. If the number of years is known that the tube
was out of service, add one hour of burn-in for each year. If the number of years
is unknown, use 4 hours to begin with, if the tube does not behave well after this,
repeat the burn-in period.
More difficult cases of HV flash-over can require that the HV be removed from
the tube for a number of hours until residual gases are absorbed. A tube that
refuses to settle down after more than 24 hours of conditioning may be beyond
help. If they do not settle after 48 hours of burn-in, they should be removed and
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discarded.
Remember; have a spare tube on the self, and swap them out every year. The set
will likely out last you, even with daily use.
End of useable life:
Every tube will have an "end of life" point. Directly heated cathode tubes usually
drop off emission until they are no longer usable. Some say that the tube has gone
"soft" at this point. When the filament opens is an obvious sign, but this can
happen at nearly any time during its life span.
Indirectly heated cathode tubes end their lives a little more abruptly with a
sustained HV flashover that damages the grids and or cathode surface to the point
where the tube is no longer stable at any voltage or drive level, or the heater
opens. Emission will have dropped off a great deal by this point. The cause for
the HV flash-over is usually barium contamination of the atmosphere in the tube
making the vacuum conductive at all plate voltage levels. The oxide coating of the
cathode will have been reduced to the point where areas will super-heat, causing
ionization and then HV flash-over and or sparking to the plate. This is not a
reversible condition and indicates end-of-life for the tube. The HV arc in this
condition can cause enough magnetic field that the heater can be distorted to the
point which it opens under such mechanical stress. Either way, it is end of the line
for the tube.
With careful attention to these details, you can get a full life span from your tubes.
Copyright © 2010 M.A. Erickson, KK5DR. All rights reserved.
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