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SET tube amp and blowout?

Discussion in 'Electronic Basics' started by Paul Moore, Nov 18, 2005.

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  1. Paul Moore

    Paul Moore Guest

    Hey guys,

    I'm a noob to the world of DIY home audio, so please forgive any
    ignorance on my part.

    From what I understand the SET amps are popular because the distortion
    adds to the pleasant sound, even though they are low wattage devices.

    Knowing that most speaker blowout is caused from low wattage amps
    distorting the signal, what is the likelihood of speaker blowout on SET
    amps? And is there a way to determine a "red zone" for speakers when it
    comes to harmonic distortion and wattage?

    Thanks again
    Paul Moore
  2. Tim Williams

    Tim Williams Guest

    Huh? No, speakers blow from exceeding ratings (note that most speakers
    rated "RMS" are damned if they can handle even half that, in real watts!).
    You could add a circuit to detect when the output tube's cathode current (or
    grid voltage) starts drifting, coincident with drive signal peak coming near
    0V (when the grid goes positive, it pulls current from the cathode and a DC
    voltage appears on the grid leak resistor, shifting bias). Or a peak
    detector when grid voltage peak approaches say, -5 to -1V with respect to

    But that's useless, because it'll sound like shit by the time you reach that
    anyway. Consider music that has a dynamic range of 20dB: that means, from
    an average low level of say 0.1W, you have peaks up to 10W. This might
    barely be acceptable on a 5W amp, since the peaks are getting clipped only
    3dB, for a dynamic range (at the output) of 17dB. If you want to play it
    twice as loud, you just crunched your headroom to 7dB and it *will* sound
    nasty (especially on bass, which needs a lot of power to register the same
    intensity as midrange, and especially on peaks, which need a lot of power
    for only a short duration).

  3. Well, sometimes.

    The "RMS" rating, which is wrongly named, usually refers to the correct
    average heating power rating of the speaker. Most "Pro Audio"
    speaker/cabinets, i.e. the ones used by live bands, from reputable
    manufactures, will handle their "rms" rating with continuous white noise
    and most music signals. If the amp does not clip, then the average
    average power outputed by the amp will actually be less than its rated

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  4. Bob Masta

    Bob Masta Guest

    The speaker blowout you are referring to from low wattage amps
    is where a clipped signal has so much harmonic content
    that it overdrives the tweeters. This is (or used to be) a problem
    in disco houses that tried to get away with using consumer speakers.
    (Consumer grade speakers typically have a tweeter power rating
    that is much lower than the woofer rating, since most music has
    much less energy at the high frequencies.)

    But the "coloration" distortion from a single-ended triode
    (when not driven into clipping) is typically much milder,
    no danger to tweeters under anything remotely like normal
    listening.. Added to that, most of these amps are only capable
    of a few watts output anyway. You'd have a hard time blowing
    anything but the cheesiest speakers!

    Best regards,

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    Home of DaqGen, the FREEWARE signal generator
  5. Guest

    This is very good information, but as to the question of blowing
    speakers, the amps the original poster is referring to are transformer
    coupled. Therefore any DC on the grid resistor, plate circuit or any
    other element of the output stage would be isolated, and not present at
    the speakers.
  6. Tim Williams

    Tim Williams Guest

    Um? I see no reference to DC on the output, and obviously, bias current
    changes (including bias itself) are not going to be present, since SET OTL
    is well...yeah...

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