811A Tube History and Construction
AL811 and AL811H Cooling System Mods
Unless you change to 572B tubes, there is absolutely no reason
at all to modify the cooling system. The stock fan and air system in the AL811
and AL811H maintain components well below rated temperatures, so there is
absolutely NO tube or component life improvement with more airflow. The 811 tubes are actually
rated for convection cooling, but the AL811 forces air across the tube envelopes
anyway. The absolute maximum average dissipation in the 811A tubes is 65 watts
per tube, or 260 watts total. This, when added to dissipation of other
components, is well within the air system capacity for a 15C temperature rise of internal
Assuming a 15 minute on and 15 minute off cycle, the air
system was tested at 350 watts average tube dissipation and still remains safe.
If you go much over 350 watts of average tube dissipation for any length of
time, you will ruin the 811A tubes in fairly short order. No amount of air, even
a gale force wind, will extend the life of the 811A tubes. This is because the
anode is NOT exposed to the airflow, it is inside a vacuum, so the extra air
does no good. The most extra air will do is cool things outside the tube, and
that is not an issue until average dissipation gets well beyond the 811A rating.
The amplifier was retested, this time using a 5 minute on and 5 minute off test.
Users should increase airflow if extending duty cycle with 572B tubes
substituting for 811A tube. For higher duty cycle operation with 572B tubes, a
doubling of CFM is required. The replacement’s CFM rating above
.08 inches of backpressure must be compared to the AL811’s rated flow at 18 CFM.
Ideally any new fan would flow 35 CFM at about 0.15 inches water. Some reduction
in backpressure will occur by removing the area where exhaust holes are, and
replacing it with a 50% opening copper mesh screen. DO NOT extend the cabinet
hole area, because the exhaust hole area was chosen carefully to prevent
stagnant internal airflow areas .
The original fan is a Pabst 8800N found at this link:
The AL811 and 811H use a fan that follows curve
5 below at about .07 inches backpressure:
Years ago, when I designed the AL-811 and Al-811H amplifiers, I made a series
of careful temperature measurements. In light of Internet articles and comments
about “improving airflow” that often include adding a larger fan, I decided to re-visit airflow with a current
production amplifier and see if anything changed.
Here are internal air temperature measurements of a current, as of early
2011, AL811H amplifier with absolutely STOCK cooling system:
50% duty cycle carrier, 450 watts output, 5 minutes on, 5 minutes off
||LF Tank area
||Tube compartment area
At that point the tube anodes are red, which is really hammering the tubes.
Lock-down idle, 2 hours, no RF just quiescent current causing 170 watts total
tube dissipation (this result will vary with idle current)
||LF Tank area
||Tube compartment area
Extending Tube and Component Life through Increased Airflow
This sounds nice, but unless we are melting the tube’s glass or ruing seals any additional
airflow is unnecessary. The 811 tube anode or plate, which is the weakest
element in the tube, gets rid of heat mostly through infrared radiation.
Infrared radiation is a form of electromagnetic radiation, but well above radio
frequencies. Very little heat is conducted out of the tube. The anode cooling is
primarily determined by the temperature differential between the tube anodes and
the surrounding objects in the amplifier. it is heat radiated out for a large
distance, not conducted to the glass.
In the 811 tube, normal unforced convection airflow in a large box can
adequately maintain tube
glass and seals far below rated temperatures! With 811A tubes in the AL811H or
AL811, forced airflow is better than already adequate convection in a large box.
Actually, this is one of the reasons the cabinet is so roomy. With a tighter
cabinet, more airflow (and more noise) would be required for the same cooling. Modifications
to the stock air system cannot increase tube or component life. In this case tubes are dissipation-time
limited by characteristics inside the tube. Tube life is not even remotely close to
being influenced by cooling system airflow volume.
Now in fairness there is one thing that might require more airflow.
The AL811H, as well as the three-tube AL811, might need more
airflow with 572B tubes if the amplifier is
operated significantly harder than with stock 811 tubes. This
should not be done anyway, however,
because the power transformer will become unreliable with significantly higher
duty cycles or output power. 572B tubes, substituted into the 811 series, should not be driven
significantly harder than original 811 tubes. At approximately 800 watts PEP
voice IMD products are -32 dB PEP. Beyond 800 watts IMD will be worse than most
radios, and considered excessive. This is because of filament emission limits in
the 811 and 572B tubes. Both tubes share the same filament designs and have the
same peak emission. The 572B primarily buys extra time or extra anode
dissipation margin for mistuning safety or use of heavy speech processing, not
significantly more peak power.
The AL811 amplifier, with the fresh look in early 2011 to look for any
possible improvements, is actually a very good design. All components are sized
conservatively enough to not be a problem except the tubes themselves, but not
wastefully oversized. While it would be nice to boost sales and get more life into the product line (and get paid to do it)
by improving the 811H, there really isn’t anything worthwhile that can be done. Changing any single part or small group of
parts, short of making an entirely new amp, would really be a waste of time and
Where did the idea the cooling system needs improvement come from?
Perhaps a little of it came from another 811 amp. Years ago, someone else marketed a
four-tube 811 amp. They used a larger cooling fan than the AL811, and openly promoted this as a “feature”.
Perhaps Hams just equate life increase to airflow increases, as with hot running
semiconductors or hot running electrolytic capacitors, so they think more noise
and fan size means more life. Unfortunately the amplifier, with much more noise
and fan speed, actually ran much hotter internally than
the 811 series! While a larger fan was used, the fan inlet was jammed up against air restrictions.
Restricting the inlet or
placing things near the blades does two very bad things
to almost any fan or blower. First, air moving through or following the blades
is disrupted. Noise increases from this, and airflow decreases. Any inlet
restriction is much worse than an outlet restriction. Restricting the inlet has a much greater
effect on airflow than any other restriction, and restrictions generally
increase fan speed and fan noise because the fan cannot push air smoothly with
minimal turbulence. The fan really needs to be in a clear area, particularly on the
low pressure inlet side.
The second problem was the manufacturer used a very compact cabinet without much room
around components. While that is great for desktop room, it makes the amplifier much more
dependent on forced-air. The AL811
series was primarily designed to be inexpensive, as reliable as possible for the
tubes used, and quiet.
Perhaps some backyard engineer somewhere decided his melted anodes would be
corrected by blowing more air past the tubes. This would be true if the anodes
touched the airflow, like in external anode tubes, but it is not true in the 811
tube at all. If the glass is maintained significantly below damaging temperatures,
and if cabinet internals are not significantly above inlet air temperatures,
additional cooling is meaningless.
Tube life is almost exclusively determined by duty cycle and tube quality.
Other than keeping tuning times short and avoiding heavy speech processing or
duty cycles that actually melt anodes (overheated anodes appear as a silver
color or shiny area on the anode), there is little that can be done to improve
I am doing a page on new AL 811H changes.
That page will be up soon at this link.