what is the default voltage on core duo 6600 please ,

mines running at 2.4ghz - 1.296v  

hum i just o/ced mine to 2.8 at 1.224v, can the cpu be destroyed with not enough voltage or is it safe ?

As long as you have enough cooling, Intel says up to 1.55v is safe.

Too little voltage just makes it unstable, and causes it to crash more. It wont have any permanent effects.

1.325v is default voltage for 6600. It doesn't need all that voltage for the stock clock though, so you should be able to get a decent OC before raising it (I'm 3.0 @ 1.325)

Most E6600's I've seen never use all 1.3V at stock speeds - mine averages about 1.26 even under load.

1.325v is the BIOS stock, but many boards (specially Asus) run actual vCOREs lower than this.

Whoops lol. I set my voltage to 1.6 volts when I took my clock speed up to 3.65 Ghz.

If you have a retail box, check the product sticker on the side. Mine shows 1.35v max. But as has been mentioned, a much lower voltage typically works well at stock clock. At 3.7Ghz I'm stable with a BIOS setting of 1.5125, which yields 1.480 ~ 1.504 (24mV fluctuation) as per SpeedFan 4.31.

Hope this helps.

Default cpu voltage for the 6600 is 1.2 volts.