Most of the guides you'll find use the heat-reducing effects of undervolting in combination with overclocking as a means to an end rather than a goal in and of itself, since lower voltage = less heat = higher overclocks. Like you, however, I don't need to run higher than stock clock; personally, I was seeking reduced power consumption to help reduce strain on my power supply until I buy a better one. Ultimately, this changes how best to go about undervolting; when an overclock is the plan, disabling functionality such as Turbo Core and Cool & Quiet is often recommended to keep a rock solid overclock active all the time, but C&Q is actually great when you're specifically trying to save power or reduce voltage.
Phenoms use something called p-states when C&Q is active; the processor multipliers are automatically adjusted based on actual load, running at various fractions of full speed to reduce power and cut fan noise. If you run something like CoreTemp, you can watch this in action. Like I mentioned, most voltage adjustments are done in BIOS, which if I understand correctly, only take effect with C&Q disabled, which really negates most of the power-saving and overall heat reduction.
Luckily, when in Windows, the actual values set in various p-states can be adjusted using a couple of different tools, namely K10stat and PhenomMsrTweaker. You can customize not only the multipliers for each of the four p-states, but also the applied voltage, and this is where it gets useful for people like you and I.
First thing first, before you start messing with voltages, I'd recommend you download the following tools:
PhenomMsrTweaker 1.2 - - The tool you'll need to adjust voltages for our particular need; read over the page for more basic info.
HWMonitor 1.18 - - This will help track system statistics during your undervolting adventures, so you can keep track of how high/low voltages and temps are reaching and monitor voltage ranges at any given setting during stability testing.
CPU-z 1.58 - - This provides detailed CPU info, as well as current actual CPU vcore to a more accurate degree than HWMonitor - optional, but still helpful.
FFT-z 2.0.1 - - While you're welcome to use your own stress tester, the stats this one provides I found to be very helpful and useful in this particular application.
Also, be sure you understand what things like vdroop are; there's a great rundown here on Tom's, although the rest of the article is a bit out of date (such as how C&Q kills the voltage adjustments, which PhenomMsrTweaker fixes):
Tom's Hardware - Undervolting
Now, to get started:
Step 1) Check your BIOS, and make sure Turbo Core is disabled, Cool&Quiet is disabled, and any voltage adjustments to the CPU (vcore) are set to auto or are left at stock. You're going to manually use a custom C&Q driver with the PhenomMsrTweaker, so you don't want it enabled here. Save, and boot into Windows; your temps will run hotter faster than usual if you had C&Q on, as now you're running at full speed all the time.
Step 2) Close all unnecessary services and applications, especially those that might be writing to the hard drive. You WILL crash your PC, and you don't want to mess anything up.
Step 3) Run HWMonitor and resize the windows so all CPU and system stats are visible. It will begin tracking highs/lows, so you can see your actual voltage and temperature ranges. Also, your core temps are more important than your CPU temp, but some (all?) Phenom II X6s have an improperly calibrated core temp by about 10 C, and there is only one sensor, despite six separate temps being shown. If your core temps show up about eight-ten degrees lower than the CPU temp, you know this is the case, and adjust your interpretation accordingly.
Step 4) Run CPU-z, if you'd like, to watch the actual fluctuation of the vcore to more decimal points.
Step 5) Run FFT-z and look around a bit, testing is started from "Test and Results" (set "Stop Tests on Error" to enabled), and status can be monitored from "Status and Health" during a test. Don't start it yet, though.
Step 6) Run PhenomMsrTweaker. Settings adjusted in this app take effect once you hit apply, but disappear once you restart your PC (for now), which is fine during testing; if you crash because you go too low, it's fixed by a simple reboot. Hit "Services". The first checkbox is for once you've found your stable lowest settings, and will make your changes semi-permanent; PhenomMsrTweaker will run at startup, make the adjustment, and then kill the service, without you having to change it every time. If it's not disabled already, uncheck "Enable Hardware Turbo". Lastly, check "Enable custom Cool & Quiet", ignore the settings for now, and hit apply. Your temps should drop immediately, and the little asterisk (*) next to the "P" labels on PhenomMsrTweaker should start moving from tab to tab, indicating changing p-states and letting you know your custom C&Q is active.
Step 7) Get a pen and paper, and keep track of your voltage settings, in case your system crashes. You can ignore P0 (p-state 0), really, as that's the Turbo Core setting, which we disabled. Go to p-state 1 (P1), and note the current voltage (CPU VID), and do the same for 2-4. Leave NB VID alone, for now; that's for the motherboard. CPU-VID should be 1.4000, if I'm not mistaken, for p-1, down to about 1.175v for p-4. To start with, I am almost willing to guarantee p-1 can be dropped to 1.3000 in most cases, but subsequent changes should be done in 0.0125 or 0.0250 increments. If you're more comfortable, 1.3500 is fine to start. Whenever a change is made, you have to also make sure than all higher p-states (lower clock speeds) are set to the same or less, so keep track of them, too, and adjust as necessary. I'd recommend once you start reducing p2-4, set them no lower than you've gone with p-1 if p-1 is now lower than they would default to.
Step 8) Adjust CPU-VID on p-state 1 (and other states, if necessary) as desired, and hit apply. Your voltages are now reduced for that p-state, and CPU-z/HWMonitor should reflect this once that p-state is reached.
Step 9) Run FFT-z test for five minutes, which will cause your CPU to go under load and reach p-state 1 (and your custom voltage). If you adjusted all the windows to show at once, you can monitor actual voltages and temperatures with HWMonitor as the test is performed. If your CPU can at least appear to handle the undervolt in a short term test, the test will pass.
Step 10) Repeat Step 8 and 9 until either the tests fail or your system BSODs/crashes/reboots. Congratulations, you've found your system's minimum voltage! Now, you've got to decide how fine-tuned you want your undervolt to be. If you want to waste less time stress testing, bump the voltages up to 0.0250 higher than what your system failed at, otherwise you can try 0.0125 higher. Again, I would leave all the higher p-states (lower clock speeds) at default or your new selected voltage, whichever is lower. For example, if you get down to 1.2v, then p-states 1-3 would be at 1.2v and p-state 4 would be at it's default 1.175v. You could adjust these lower, but you'd have to stress test each p-state setting individually for stability, and it's really not worth it, as you have to adjust p-state 1 to match the reduced multiplier/voltage so it hits that under load to test them, and it's just more work than it's worth.
Step 11) Set your finalized voltages in PhenomMsrTweaker, hit "Services", and check the box to make your changes semi-permanent. If they didn't update, hit the update box to the right of the now-visable voltage list, then hit "Apply".
Step 12) Stress-test! A five-minute test is a good start, but you need at least a few hours to know for sure whether your new settings are stable long-term. Also, not all stress-tests are equal; a system may run stable on Prime95 for a 24 hour test, yet fail the more vigorous StressCPU v2.0, which simulates a Folding@home workload, after a half hour. That particular test, which you can Google, got my CPU to the highest core-temp under load, maxing out at 44 C. Hopefully, the voltage reduction will result in lower load temps, less noise due to less need for the CPU fan to spin constantly, and a lower draw on your power supply (may be really significant on Phenom II X6s!).
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