The Dyson ad is misleading: it merely has no EXPOSED blades. What it is
is a venturi pressurized by a bladed fan in the base. In this way it is
trading high-pressure low-velocity for low-pressure high-velocity which
is appropriate for a fan in free-air to cool human bodies. Fans
designed to push air through heatsink fins are designed for moderate to
high back-pressure depending on fin density and height.
When we modeled our convective heatsinks for the KX2 & KX3, we also did
designs with fans (and brought some to Dayton). After iterating a
heatsink design in ThermalCAD for hours to get the last percent more
convective cooling, it is sobering and amazing how much heat you can
remove by forcing air to remove the stagnant boundary layer of air from
the heatsink. It makes you realize natural convection is a very weak
motor for air movement.
A tube amp can potentially be quieter than a solid state unit due to the
fact that the total heat removal in BTUs is directly proportional to the
temperature differential between the air and the object being cooled
(Newton's Law of Cooling).
Analyzing the thermal path in each situation; the anode fins of a common
3CX or 4CX series tube are directly cooled by the air and can run at
250°C, whereas the transistor has some additional series thermal
resistances to overcome before meeting the cooling air which can reduce
the effective heatsink temperature at FET max junction temperature and
heat flux to 125°C. Assuming 25°C air, there can be a 225°C
differential between air and tube anode, and a 100°C differential
between the FET heatsink and air. This means for any given volume of
air passing through the two devices, there can be 2.25 times as many
BTUs removed from a tube. The other issue is the fact that the ceramic
tube fins are designed for high pressure low-volume cooling, and most
non-ducted FET heatsinks are designed for lower pressure, higher volume
air flow. High-velocity fans typically create more noise than do higher
pressure fans due to the relatively slower air movement and lower
turbulence past the fins. This being said I have used some really noisy
tube amps.
I opt for an Elecraft KPA amp for many reasons; the fantastic K-Line
integration, included ATU (in the 1500) and protection circuitry they
design in. I use a KPA500 which also can get loud, but I appreciate
what it takes to remove 500 W of heat to keep a 500 W amp cool (assuming
~50% efficiency). Based on the reliability of the KPA500, I also do not
think there is a tube amp on the market which protects it's devices as
well as Elecraft does their FETs. In order to address the noise issue,
perhaps a remote control like the Acom 2000A offers is a good option for
the KPA1500 at this point?
I was not paid to make this endorsement, just a happy customer.
Cheers & 73,
Howie - WA4PSC
www.proaudioeng.com
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