How do you measure wattage?

Discussion in 'Amps and Cabs' started by skydog, Sep 19, 2016.

  1. skydog

    skydog Supporting Member

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    say if you have an amp and you try different output tubes, how do you properly measure output?
     
  2. tele_player

    tele_player Member

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    I do it with a signal generator, dummy load and oscilloscope.
     
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  3. skydog

    skydog Supporting Member

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    Out of the three, I've only got the dummy load covered!
     
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  4. Papanate

    Papanate Gold Supporting Member

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    And with a lot of caution.
     
  5. tele_player

    tele_player Member

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    Not especially, this is done without opening the amp, takes a few seconds once amp is warmed up.
     
  6. tmac

    tmac Goldmember Gold Supporting Member

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    here ya go
    if you have a meter: quick - and if you guess the right voltage & do the math, it could be right around the actual WRMS of your amp.....or not. :bonk
     
  7. Karl Houseknecht

    Karl Houseknecht Supporting Member

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    I'm guessing 8 ohm load on that speaker so that's what? 3 watts? What's he playing there, a Pro Jr.? That's rated at 15 watts. Is the true RMS of that amp 3 watts? Seems a little low.

    The accurate answer would be obtained with the signal generator, dummy load, and scope. I had a local tech do that with my Mark V to verify power output once. Simple procedure with a tiny bit of math.
     
  8. tmac

    tmac Goldmember Gold Supporting Member

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    that's a 4 ohm speaker in a old tweed Champ (or else it won't sound good, it wants a 4 ohm load). He's pretty close on the measurement actually.
     
  9. Karl Houseknecht

    Karl Houseknecht Supporting Member

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    Pfffft! Fenders! They all look the same! :D
     
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  10. skydog

    skydog Supporting Member

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    So the ohm component in the equation would just be the factory ohm rating of the speaker? The video seemed to run out about 10 seconds early!
     
  11. Karl Houseknecht

    Karl Houseknecht Supporting Member

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    Yes, that's right. Original Champ was rated at 3 watts. Interesting. But with a 4 ohm speaker the math works out to 6 watts.
     
  12. skydog

    skydog Supporting Member

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    The "5" on the voltage seemed rather arbitrary, too. If he'd used "4", that would drop the watts down to "4".
     
  13. support

    support Member

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    Well, the OP asked how to "properly" measure output power. That does require a fixed, non frequency dependent load (ie, NOT a speaker), a stable, fixed test frequency (ie, NOT a guitar), and a way to tell exactly when the amplifier reaches maximum undistorted power, which is typically an o-scope. Testing for output power also includes knowing what percentage of distortion is being generated. To obtain the most useful and accurate result, you also need a distirtion analyzer. Most of the time though, you can just set for onset of clipping and assume 5% distortion.

    The video posted above should be titled "least acurate way to overstate your amps output power". :)
     
  14. tele_player

    tele_player Member

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    Correct, but for our purposes, the distortion analyzer is overkill (though I want one!), since the amplitude of the sine wave doesn't change significantly as it starts clipping. Just use the scope to measure the P-P voltage when clipping just becomes visible on the top or bottom of the sine wave.
     
  15. support

    support Member

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    Yes, a distortion analyzer is overkill...until you need one...then you really need one...which is why I qualified that you can eyeball it and call it 5% for all intents :)
     
  16. DeaconBlues

    DeaconBlues Member

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    Yeah, had that been a PJ it would measure quite a bit more than 3 watts. I ran a wattage test on a PJ with the Allen TO20 OT installed. Using the signal generator, dummy load and scope method, I measured 22 watts before the amp went into to OD
     
  17. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    That Weber video is a hod of buffalo excreta, and is of zero technical value. You would gain greater insight watching a video of a cat unrolling toilet paper.
     
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  18. guitar007

    guitar007 Member

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    I have all three devices. If it's no trouble...can you please outline your procedure?
     
  19. tele_player

    tele_player Member

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    Typically, 1khz sine wave is used. All volumes and tones at 10, adjust signal until waveform across load just starts to flatten at top or bottom. Measure peak-to-peak voltage (Vp-p).

    R is load resistance in ohms
    RMS voltage is .35 x Vp-p. (Vrms)
    RMS power in watts is (Vrms ^ 2) / R
     
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  20. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    It's not difficult, actually I like to use a true-RMS meter in addition to the scope.
    • Connect the amp to a dummy load of the proper verified resistance to match the amp's design load. The load should be resistive, not a speaker or speaker motor.
    • Set your signal generator to produce a sine wave of an amplitude consistent with what a guitar produces, I use 100mV RMS. 1000 Hz is a typical test frequency, some use 400 Hz.
    • Plug the sig gen into the amp's input.
    • Connect the oscilloscope and the true-RMS voltmeter to measure AC voltage across the dummy load.
    • Power the amp up.
    • While observing the oscilloscope presentation, increase the volume pot on the amp to the point at which you see the peaks of the output sine wave just begin to clip or flatten. Back the volume down a hair to where the clipping just goes away.
    • Note the AC voltage on the meter and write it down.
    • Square that voltage and divide by the load resistance to get power in watts.
    A distortion meter is nice, as it allows you to determine power at a given % of distortion, but not everyone is going to have one.
     
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