As well as an overclocking card
like the FreeSpeed, the intrepid Athlon hot-rodder also needs a big fat
cooler. And there is no cooler bigger and fatter than the Alpha
P7125.
The P7125 has a huge block of
super-thin fins, two fat 60mm fans (in the "P7125M60" version - you can
also get a P7125 to which you can add your own fans), and a shroud to make
sure the fans pull plenty of air past the metal. It's also got a cutout
in one corner, which perfectly matches the somewhat unfortunately located
power connector on the K7M.
The P7125 boasts a frankly extraordinary
0.215 degree Centigrade per watt thermal resistance. This means a steaming
overclocked Athlon pumping out, say, 90 watts, should only heat the P7125
up by 19.35 degrees Centigrade above ambient.
In practice, though, this isn't
likely to happen. This is because the P7125, like pretty much every other
Athlon and P-III cooler, is a "thermal plate" unit.
One side of modern Slot 1 and
Slot A cartridges is the easy to remove clipped on plastic part; the other
side is an aluminium plate, held to the processor circuit board with some
rather serious clips. This thermal plate makes contact with the CPU and,
hopefully, also the separate cache memory chips of P-II, pre-Coppermine
P-III and all current Athlon processors. Heat travels - or is meant to travel
- from the chips, to the plate, and then into whatever cooling gizmo you've
attached to the plate.
Like several other recent Alpha
CPU coolers, the P7125 has a copper inset in its base, to help spread the
heat from the contact point in the middle.
The P7125 attaches more solidly
than most thermal plate coolers. It uses self-tapping screws, which you
screw in as you build the cooler onto the CPU. Like all Alpha coolers, the
P7125 comes as a box of bits, but is easy to assemble if you have a screwdriver
and can read. The kit form helps Alpha keep their prices down; this may
be an exquisitely engineered extremist Japanese techno-toy, but it sells
for only $US48 before postage and handling.
But the trouble with thermal
plate coolers is that, with the best will in the world, they can only dissipate
what heat gets to the thermal plate in the first place. In the Athlon cartridge,
the central plate-to-CPU contact point is nice and solid, but the cache
RAM just has blobs of thermal transfer compound on it, bridging a gap of
a few millimetres to matching insets on the plate. Thermal transfer compound
(also known as "heatsink grease") is a lot more thermally conductive than
air, but no substitute for direct contact. You're meant to have a little
smear of it to fill the irregularities of two things you've clamped together;
a blob of the stuff is less than ideal.
If you reduce the number of
contact areas heat has to cross, you increase the efficiency of your cooling
system. And if you're overclocking Athlons, it's a very good idea to do
so.
Old-model 0.25 micron core Athlons
like the one I played with run hotter than the new 0.18 micron core ones
- but they're all toasty processors by anyone's standards. Running at 500MHz,
the 0.25 micron Athlon has a Thermal Design Power (TDP) heat output rating
of 42 watts. Wind that core up to 750MHz without increasing the core voltage
- if you can - and it's a 63 watt unit. If you have to boost the FSB by
0.2 volts to get 750MHz stability you're talking something like 80 watts
for 750MHz.
No desktop processor Intel's
ever made runs this hot. Some of the old P-IIs might, if you could overclock
them that much, but very few of them make it to a 50% overclock without
preposterous cooling. The big fat Pentium II Xeon server processors are,
at 450MHz, a bit hotter than the 500 and 550MHz 0.25 micron Athlons. But
you won't see many Xeons in desktop machines; they were horrendously expensive,
and no faster for desktop tasks than plain P-IIs.
I wanted to wring everything
I could out of my Athlon, so the thermal plate had to come off, and a seriously
cool cooler had to go on.
