Barry K. asked, Ian supplied a reasonable short answer. The long
answer is: At my place of employment, we keep the water clean, using
deionizer bottles (resin exchange?) from Culligan (US television ad
says Hey Culligan man) or other vendors. There is a closed loop, and
a 'slipstream' of about 10% of the total capacity is flowing through
those bottles. The most recent upgrade we did added ability to remove
oxygen. Keep it below 5 ppb. OK, so what I am about to say is
probably overkill for an occasional HF contact using a 3CWxxx fed
with a funnel from the top. I am assuming a reliable system that you
can leave running CW for 24/7.
Eimac, Burle, and Thales all published notes that recomment a certain
level of ionic resistivity (inverse conductivity). There are two ways
to deal with it. 1) If under 1 Meg-Ohm - cm then use special
electrodes in each end of the main hoses to the tube, which are
sacrificial. Check them occasionally, these electrolytic elements
will slowly be eaten as leakage current flows between them. They
should stick out further than the water fittings, to make them the
preferred points for the current. 2) Try and keep a few MegOhm - cm.
or greater and keep leakage under fractional mA DC. Then the hoses
can be inspected every few years, if significant metal migration is
seen, such as a copper-ish deposit in one of the hoses, then its too
much leakage. This second approach requires more length in the hoses
and also to design to the diameter needed for the anode flow without
excess pressure drop, with no larger than this size.
Use Barnstead or Foxboro or other conductivity gauges, with either
closed loop control points to control the slip stream flow, or with
just an interlock which trips off the HV when the resistivity is too
low. I run them at 2.5 Meg Ohm - cm for one sytem and about 4 for
another. This is done to keep the current flow in the hoses down at
about 100 uA or less. The length of the hose, and the cross sectional
area of the water determine what resistance it will have, then
calculate using the B+ DC Voltage . Stay away from black rubber hose
or hose with carbon in the jacket, as leakage will also happen there.
You have a gradient down the hose, so you have to place it away from
the walls of chassis, on standoffs, until it comes to ground at one
fitting. Some times for larger hose, you coil it around a G10 form or
pipe. I saw one system using PVC hard pipe, with elbow fittings, in a
spiral, to get adequate length. In 1997 we discovered that the PEX
tubing used for underfloor radiant heat (hydronic heating) is perfect
for making the anode hoses, it is virtually leak proof when applied
per manufacturer recommendations, and it can be formed in a shape
using a heat gun, and will hold this set forever. Its also
semitransparent, so that we can observe if any metal is being
deposited near the fittings. Usually copper from inside the anode is
first to go.
One more thing, use only stainless steel and brass/copper fittings in
such a system, as regular steel will destroy the effectiveness of
your water purification system. Tap water is useless for water
cooling at higher than a few kV, as the conductivity cannot be
controlled adequately. Not to mention that it leaves a lot of
deposits inside your anode cooler.
Vapor phase cooling is rarely used in new designs now; except in
older rigs such as some of the Harris SW100 transmitters and others
using the 4CVxxxxxx tetrodes. Ian pointed out that you are operating
a distillery inside your tube. Multiphase or Hypervaporphase cooling
is the best method now for larger tubes, where the boiling water is
at contact with the anode cooler inside the tube, and it condenses
back into water before leaving. You essentially deal with water
cooling system, but the purity and processing is quite stringent. The
4CM400,000KG is an example of a tube like this. Also the TH558
tetrode.
Some simple water cooled tubes from Eimac appear to be nothing more
than the air cooled tubes minus the fins, with copper tubing brazed
around the anode. One company in the microwave heating field used to
buy Chinese magnetrons slightly larger than home microwave ovens use,
braze tubing around them, and raise the power to 2-3 kW.
The conclusion to all this, use water if you have to, but be aware of
all the precautions and chemistry requirements. Air cooling is much
easier to deal with, if you have the blower, filter, flow switch,
etc. But, water cooled amplifiers are very quiet! Its not as simple
as hooking your water hose to a faucet and going for the PTT or key
down. Unless you know the impurities and conductivity along the hoses
into your tube, expect to have long term problems with corrosion and
even scale buildup inside the tube.
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