hey guys, is it ok to run a peavey 6505 with the volume knob on 3 (that's like 30% of the maximum volume) thru a single 75 watts speaker? the head is like 120 watts, and I wouldn't even be using half its power.

It would probably be safe, just listen for speaker clipping. I will say that 3 on the knob is probably more than 30% of it's power, the way you're thinking of it isn't really correct. 6 or so on the knob is probably 100% of it's power and anything past that and it's producing more than 120 watts.
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Thank you Kevin, it makes more sense to me now.

I wonder if in all cases checking for clipping is a correct way to check if a speaker can be ok.

For example if I have a 30 watts tube head 16 ohms, will be ok if i run it thru an 8 ohms 150 speaker cab if clipping doesn't occur?
The ohms need to match, otherwise you can do damage.

Most speakers are designed to take 50% more power than they are rated for, so it's ok for it to clip, but if it's excessive you're probably close to it's true limit.
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