measuring the power of an amplifier

[michael the B]

Hi.
I'm new to this forum and am hoping someone can direct me to some info on a definative method for measuring the output power of an amplifer.
I'm assuming that p=IV or P=I squared x R is the formula but am wondering whether commercial amps are spec'd using a standard input signal as speakers are?
Any help welcomed...thank you and goodbye

[Diablo1]

If we're talking about tube amps, it's a little more complex. You hook up a large wattage resistor of the correct impedance to match your amp ouput power section. Max power is measured when the ouput signal waveform is undistorted. So you need a sine wave generator operating at some standard frequency (1000 Hz), and an oscilloscope to look at the output signal. You turn up the volume control until you can start to see the output distortion on the scope, then back off the volume a little until you are sure the waveform is not distorted.

[Andy Le Blanc]

you need two pieces of information, in addition to the max signal power output as per the available tube data in your receiving tube manuals to extrapolate the amps behavior.

zero-signal plate current

max-signal plate current

max signal is what ever you dictate. max output is measured as voltage over the load.

put a voltmeter over the load and run the amp up until you see it peak before beginning to drop as the rig clips out or depletes the power supply
and use that number to do the math.

dont forget to grab the power tube current and plate voltage during the test.

[VacuumVoodoo]

Who is this joker
http://www.youtube.com/watch?v=1b2jQWK8xlQ

[Structo]

Oh, that's Ol' Gerald Weber.

Some love 'em and some hate 'em.

[michael the B]

Thank you. I appreciate the information.

[Diablo1]

Who is this joker
http://www.youtube.com/watch?v=1b2jQWK8xlQ

His method is quick, dirty and very inaccurate.
Doesn't sound like a clean output to me, and amps are rate at clean output power.
Everyone should know that distorted output can be much higher than clean output.
A speaker rated at 8 ohms isn't really an 8 ohm load at all frequencies, so when he plays his guitar through that speaker he really has no idea what the resistance is, but I'll guarantee it isn't 8 ohms. So his calculation of watts is off again.

He should define his own scale of power: the Gerald Weber Whatt.

[michael the B]

I had thought that the speaker rating, Impedence and sensitivity was at 1kHz. My plan was then to use an input of 1 Khz to drive the amp.
I like the idea of looking at the waveform for signs of assymetry on my CRO and then backing back to the maximum point of nil visable distortion.
I have a commercial amp (laney vc30) which is supposedly rated at 30 watts so I will see if I can come up with a routine that gets me close to that and then try it on my own amp build.
Thanks again for the information.
Cheers
Michael

[Diablo1]

Using a large wattage resistor is a lot more accurate and sounds a lot better than blasting a 1000 Hz sine wave signal at full power through the speaker.