How do I find out the wattage ouput of my homebuild?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JubileeMan 2555, Aug 11, 2008.

  1. JubileeMan 2555

    JubileeMan 2555 Member

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    Some of you may know my current project is an odd blend of Tweed Pro and Blackface Vibroverb circuits. (tweed pre, vibro power).

    I am looking to measure how much my amp is putting out, to help find a decent lower wattage 15" speaker that will work.

    Currently, with my 7581a tubes, 450v plates, and Mercury Magnetics upgrade Vibroverb Output transformer, I measure 30v out at the speaker with the amp on 10 and me hitting the guitar as hard as I can.

    Can anyone help? I'm not looking for a precise wattage number, Just a narrow range to know what I'm looking at for wattage. I'm going to be hopefully switching to 5881 tubes soon to drop the wattage even further, but this way I know the MAX my amp is sending to the speaker.
     
  2. hasserl

    hasserl Member

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    Power = volts squared divided by the load, or 30 x 30 / 8 = 112.5

    You realize this is a very inaccurate measurement, power rating is typically measured at the max clean undistorted level, which you'll reach with the volume at around 2; and that against a resistive load. Also, you'd need a good true RMS meter to get an accurate measurement the way your doing it, an inexpensive meter will not be very accurate. 100+ watts seems way high for this amp. The data sheet for those tubes show a power of 55 watts for a pair operating in push pull, AB1 w/ 450 volts on the plates.
     
  3. JubileeMan 2555

    JubileeMan 2555 Member

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    Thanks! Yeah, I used a cheapy meter to get the 30v. I read that same equation before and figured 112watts is unrealistic.

    Now, that data sheet you read, is that 55watts absolute max? Or the Max clean undistorted level?
     
  4. rooster

    rooster Member

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    Try about 56W. That 30V you're reading with the volume on 10 is probably a square-ish wave at 30V peak. The RMS of a 30V peak reading would be about 21.2V, which you need to square, giving you about 450V^2, divided by an 8 ohm load, which would be about 56W. That's assuming an 8 ohm speaker. Plus, your meter, if it doesn't have a frequency option, probably can't measure voltage accurately at any AC voltage other than about 60Hz or so.

    HTH.

    rooster.
     
  5. JubileeMan 2555

    JubileeMan 2555 Member

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    Interesting....

    What your saying makes sense because I wanted to get the absolute HIGHEST value out of my meter, so I kept SLAMMING the guitar with all the controls dimed until I got it to peak out at 30. I think around 22 is where it wanted to be most of the time.

    well, that tells me if I don't crank my amp, I might be able to get away with a 50watt speaker (especially if I get 5881s in there).

    Thanks guys. (yes, I know I'm risking it at 50watts, but I don't have too many options and this amp isn't dear to my heart or anything)
     
  6. hasserl

    hasserl Member

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    Clean undistorted.

    I think a 50 watt speaker is risky with the amp, considering you only have the one speaker. If you burn up the voice coil while cranking the amp up and digging in to the stings, the failure will occur under the worst conditions, which could cause several other problems, such as failed tubes, sockets, OT, etc. I'd go with a little more margin if I were you.
     
  7. JubileeMan 2555

    JubileeMan 2555 Member

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    hmmm....

    well, again, i'll be switching to 5881 tubes which, from what I can read, puts out about 1/3 less then 7581a tubes put out.

    Secondly, This whole project is with new parts. There is no vintage parts I need to worry about.

    I want to find a higher wattage speaker, but anything over 50watts in the 15" catagory I've already tried and don't want (EV or JBL). Besides, I'm hoping to get a more vintage tone out of the amp and want a speaker that will give the sound correct for the 50s era tweed.
     
  8. phsyconoodler

    phsyconoodler Member

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    Here's an easier formula to figure out wattage.
    Plate voltage squared divided by the primary impedance = watts output.
    So say 450x 450 = 202500 divided by 4.2k = 48.2 watts.
    This works except it doesn't take into account plate current and some losses in the circuit and screen current and some other lossy items in most tube amps.
    So if you are lucky it makes 40 watts or thereabouts.If you aren't playing on 10 all the time a 50 watt Jensen will work great.Alnico if possible to get that woody old school tone with lots of compression when overdriven.
    Weber also makes some great speakers,but I think they are all big wattage in the 15" categories.
    I have a 50 watt 15" RSC Canada speaker that sounds divine with a 45 watt blackface amp.Check E-Bay for deals on older Jensen's or RSC or Even Marseland.If you can get a good one you will be in old-school grainy,sweet tone heaven.
    They ain't loud but the TONE!
     
  9. JubileeMan 2555

    JubileeMan 2555 Member

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    Yeah, I'm eyeballing a reconed C15N Jensen speaker.

    This amp will probably never be used live, just recording. I'll let my super reverb take that spot.
     

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