>"I also saw Paul, W9AC had a PA-70V years ago. I would be curious to read
comments about that amp and how well did Alpha do with engineering the
plumbing. I suspect it was a beast."
Mark,
I still own and use an Alpha 70V, S/N 156 and recently added the companion
70A air-cooled version, S/N 002. This 70A was hand built by Dick Ehrhorn
when he started ETO in Brooksville, FL back in 1970.
Both ALPHA SEVENTY amps are a joy to use, but especially the 70V. It uses a
common boxer fan to assist in the heat-exchange process. The plumbing
system is deceptively simple and consists of: a condenser, copper
air-equalizing tube, about 12-inches of polypropylene tubing, three
polypropylene union connectors, and a boiler attached to the anode of a
3CX1000A7, making it a 3CV1500A7. The 3CV1500A7 hasn't been made in 30+
years. Over time, I've been fortunate to have acquired several NOS backups.
The condenser is about the size of a motorcycle radiator used on a 750 cc
class of engine. A bit more than one pint of distilled water fills the
system. Filling is accomplished by removing a bronze plumbing cap at the
back of the amp. I use a small Pyrex measuring cup from the kitchen to help
with the fill.
A pair of front panel lamps indicate any faults with excessive high-voltage
leakage, and water level. I typically drain/purge/fill the water system
every 12 months. The interval could probably be extended, but in Florida,
mold and mildew can quickly form and attach to the inner plumbing surfaces.
Any contaminates will affect HV leakage.
When water does not contain any impurities, it's a very high resistance
insulator. In fact, it's so good that when the amp is purged and filled,
the water level indicator will often show a fault for the first 30 minutes
of operation. The detection circuit sees zero leakage and assumes the
boiler is empty. After operating a bit, a small amount of leakage builds
and the fault clears.
As the tube heats during operation, water in the boiler evaporates and turns
into steam. The steam rises and exits via a polypropylene bellows section
that connects to the top of the condenser. The condenser converts steam to
large water vapor drops that collect at the bottom of the condenser. As
water moves from the condenser's top to the bottom it is cooled and assisted
by airflow through the fins of the condenser. Cooled water at the base of
the condenser then feeds the boiler through a polypropylene tube, about
12-inches long, and the process repeats. It's not much different than a car
or motorcycle's cooling system. The boxer fan does not directly assist in
the water cooling process. Rather, it's mounted at the rear of the power
supply module. As the fan rotates, it's pulling air into the amp's power
supply, then moves into the RF section, then air exits out the condenser
fins. So, there's total cabinet airflow. As one may suspect, it's
important to try and isolate intake and exhaust air as much as possible,
just as it is with any amp.
A big difference between vapor-phase and liquid-water cooling is that vapor
cooling is much more efficient per volume unit of water passing through the
system. From the Dick Ehrhorn's description on cooling efficiency:
"In typical closed-loop liquid-water-cooled systems, the maximum outlet
(hot) water temp must be held well below 100 deg C to avoid hot-spot boiling
on the anode surface, which can and does create steam bubbles, which in
turn "insulate" the hot spot from the water so the hot spot gets even
hotter. This typically creates a temperature runaway and may lead to
destruction of tube and/or cooling components. Typical inlet (cool) water
from the chiller may be specified as </= say, 45 deg C, and maximum outlet
water temp as 80 deg C to avoid spot boiling.
Anyway, in this example each gram of cooling water passing through the
tube's water jacket can absorb not more than (80-45) = 35 calories of heat.
Conversely, each gram of 45 deg C water entering a vapor-cooled tube's
boiler absorbs approximately [540 + (100-45)] = 595 cal/gm while
vaporizing. So, vapor cooling requires passing only about 35/595 = 1/17 as
much water
volume through the system as does water cooling."
Paul, W9AC
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