Measuring power out

[C Moore]

I have never measured the output power of a guitar amp, and I am not sure I even know how. For you guys that are familiar with getting this number, does 235 from a Marshall SL seem possible.?
I am referring to the Gear Page link below. I am not trying to stir up any forum rivalry, or doubt any body's claims. I am just wondering. I am not sure most guys even really know how to obtain the Power Out of a guitar amp.
Anyway.....235 watts. Does that seem relatively legitimate.?
Thank You

http://www.thegearpage.net/board/showth ... p?t=855247

[Masco]

Does it really matter?

[C Moore]

Does it really matter?

That could be the answer to any question. Including any that you may ask....ever.
Thank You

[Alexo]

I'd have to see that to believe it. Not that it's impossible, but theoretically, the difference between full clean power (pure sine wave) and full clipped power (pure square wave) can never be more than double. So that plexi would have to rock 118 watts clean. Which would be mighty impressive!

Think about it:

You have a sine wave, let's say it's peak to peak 100 volts.

You want to measure the average power over its duty cycle. For complex reasons, ignored for brevity's sake, this works out to about .35 x the peak to peak signal, when converted to RMS. Actually, 35.353535...

Take the formula for output power of RMS squared over load, in this case, we have 35.3535 x 35.3535 = 1250, divide by 8 and we have 156 watts.

When you crank that amp up into square wave territory, the peak level *does not change* but the area under that peak is more "filled out" with signal, if that makes sense.

A square wave completely fills the area between the peaks, there is no RMS averaging necessary, so you take the total peak voltage and divide by two (because we are really only looking at one half of the waveform at a time) and we have 50 volts from our example.

50 x 50 = 2500, divide by 8 and we have 312 watts. See anything familiar about that number?

So unless you are underestimating clean output power - i.e. taking a reading before the signal has hit its max, peak to peak (and most people go in the opposite direction and overestimate clean power - i.e. take a reading with whatever percentage of distortion), it is more or less impossible for clipped output to exceed clean output by more than 2X. Add to that all the places a tube amp can compress - power supply, screen voltage, preamp, bla bla, and you find that 1.5X is a pretty normal figure.

So I'd have to see what said mystery-tech did to get those figures - not that it's impossible, but it's incongruent with standard practices.

If you're asking is it possible to get 235 watts from a quad of EL34's at 400-whatever volts, I'd have to say no freakin' way, at least not that I've seen in the real world. Unless I'm wrong about the plate voltages and they were north of 600.

This looks kind of like one of those tall tales that keeps getting taller. Marshall built some loud and powerful amps, but they weren't above the laws of physics. Even in the 60's.

EDIT: Sorry, I have the stomach flu and went in the wrong direction with this post a little. 235 is really dang loud, but it's theoretically possible if you're approaching class B with really high plate voltages and a supersized power supply. Not what I would expect to see from your standard Marshall output power section, but one never knows....

[NickC]

It is fairly complicated to determine an exact measure in watts output, since there are variables. Most amps are rated with a certain RMS wattage output for a certain input (for example: 1khz sine wave) at a certain measure of distortion. Music is much more complex than a single frequency sine wave. Generally speaking, it takes more amp power to amplify low-frequency waveforms than high-frequency.

http://www.teamrocs.com/technical/pages/amppower.htm

Kirchhoff's Law may also shed some light on the matter (it seems like a 1,000 years ago I studied this stuff in an academic setting and have forgotten most of it), as it relates to the circuits that comprise a guitar amp. It gets complicated in a hurry.

http://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws

http://electron9.phys.utk.edu/phys136d/ ... chhoff.htm

[JamesHealey]

My plexi superbass put out about 161 watts totally cranked..

200+ hard to fathom but could be possible.

[C Moore]

Thanks for your replies. It is interesting on a few levels. One reason is that it just is not something we deal with, or measure, on a day to day basis.
Alexo -
I really do appreciate the time it took for you to post that detailed response.
Thank You

[Masco]

Does it really matter?

That could be the answer to any question. Including any that you may ask....ever.
Thank You

Well, it's not an answer, its a question. I was really being sincere (is there an emoticon for that?).
If you've got time to kill, do some calculations. IMO, kind of meaningless

[VacuumVoodoo]

Four EL34s with 500V plate supply into a 2k OT will deliver around 150W out relatively clean at some 5%THD. So it's possible to get 235W out fully cranked as claimed. Under condition that the PT can deliver enough juice because 235W out will eat close to 500W supply power.
BTW there's no such thing as RMS Watts. It's just watts. Please stop using this heresy