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Measuring Amp Power

7/14/1999 1:53 PM
Ron Orlando
Measuring Amp Power
I'm sure this has been touched on before but can anyone tell me how to measure the output of an amplifier.
7/14/1999 2:03 PM

Connect a dummy load to your amp and inject a 100Hz signal(recommended to me by Bruce of Mission Amps). Adjust the amp for the max signal out before clipping occurs. Now measure the rms voltage across the dummy load. Square that voltage and divide it by your dummy load value.  
hope that helps,  
7/14/1999 2:54 PM
Ron Orlando

Thanks Jason...what level should the 100Hz be at, coming from the signal generator?? I'll be testing a 5E7 Bandmaster and a 65 Deluxe Reverb.
7/14/1999 2:49 PM

Substitute a large (about 50w rating) resistor in place of the speaker load, and measure the rms AC signal voltage across it. Using Ohm's Law, take the square of the signal voltage and divide it by the resistance value. (It's hard to get an accurate measurement with simple tools when there's an inductive load, so the resistor must be used.)  
In order to get an output signal, you'll need to input a signal somewhere in an earlier stage of the amplifier. You can use any kind of a tone generator, and decide where you want to inject the signal. Which stages do you want to include as contributors to the amplification?  
That brings us to distortion figures. Amps will generally produce fairly clean power up to the point where the output stage becomes saturated or overloaded, whether it be the output tubes or the output transformer, where the input signal can no longer be followed accurately. The output power can still go up, but distortion level increases rapidly also. In order to tell how high of a percentage of distortion products is contained in the total power you measured across your load resistor, you'll need an instrument called a distortion analyzer. Most people don't have this luxury, so the next best thing is to rely on your ears. You decide where the sound quality is taking a dive. You can look at the waveform across the resistor on a scope, to get an idea where heavy distortion sets in, which is usually noticeable to the eye at around 5%.  
Anyway, power output figures should be referenced to a particular level of distortion to be meaningful, and comparable. Tube manual charts and guitar amps are (I believe) referenced to 5% total harmonic distortion. If you just want to see how much power your amp is putting out at a particular subjective loudness level, the straight E**2/R measurement is good enough.
7/14/1999 3:06 PM
Ron Orlando

I built a 5E7 bandmaster recently. What I would like to do is to determine if this little guy is putting out what it should. The amp sounds great but I've always played thru 40w Fenders (BF ProReverb and Bassman RI) to my ears it is a little weak and has a difficult time keeping up with a 7 pc R&B band (horns and B-3. Clean seems to drop off quickly. Anyway, what I was trying to do is determine if the output is within spec, 26 to 30 watts @ around 5% distortion. If the output is correct then it is what it is.
7/14/1999 3:54 PM
ken g
You can also do the old empirical method.  
Measure the voltages at idle--i.e. B+ at various stages. Now load the amp down, and remeasure. If the supplies sag too much, that would be the loss of headroom right there.  
If you've got an AC voltmeter, measure the signal voltages at various points. If something seems low, check that stage out--replace the tubes, first. Then check resistor values with the ohm meter... and so on.  
I dunno... 25-30W may have a hard time with a 7 piece.  
7/14/1999 4:42 PM
Benjamin Fargen

Do you have a friend that could loan you a comparable amp to use as a control in your testing? That way you could do the tests that Doc and Ken suggested with the two amps side by side on the bench. This would be a great way to determine how your 5E7 compares in output.  

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