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Tube Amp Peak Ratings?

Discussion in 'Amps and Cabs [BG]' started by anderbass, Oct 22, 2006.


  1. anderbass

    anderbass

    Dec 20, 2005
    Phoenix. Az.
    Every time I hear about testing a tube amp for max output,
    the readings are taken when distortion, just starts to set in.
    (the oscilloscope peaks just start to round off)

    Don't we all on occasion, crank our tube amps up past this point of audible
    power tube saturation?

    Just out of curiosity, has anybody ever benched a 300w tube amp
    to find the peak readings? How much higher would they be?

    For some reason, I'm picturing resistive load boxes a-flame, withered wires
    and mushroom clouds emanating over amp heads right now...;)
     
  2. seamonkey

    seamonkey

    Aug 6, 2004
    "peaks just start to round off" or at the point of squaring/clipping and beyond? I'd like to see the measurements.
    I imagine for a sine wave you could theoretically see 300 * 1.414 = 424 W if an amp could sustain a clipped square wave.

    A sensible test may be with a complex waveform like what a bass does, and a spectrum analyzer to see the added/deleted frequencies at max output. I've seen what simulations do and the frequencies response does limit itself. I suspect it has to do with the resistance of the transformer rising with the higher output and interacting more with the speaker.
     
  3. BbbyBld

    BbbyBld

    Oct 13, 2005
    Meridian, MS
    That's how I would test a solid state amp, not a tube amp.
     
  4. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Now you've got my curiosity piqued. How would you test a tube amp?
     
  5. I'm about to build cabs for my trace quatravalve and also curious about this. Since everybody says tube amps go louder...
     
  6. BbbyBld

    BbbyBld

    Oct 13, 2005
    Meridian, MS
    I'd drive it to 10% THD with a sine wave a measure it there. Right when it starts to round off may only be like 1% THD. 10% would be audible clipping.
     
  7. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    In the old days before manufacturers were required by law to state RMS wattage they used to commonly list "music power" (roughly 50% more than RMS), "peak music power" (double the music power rating) and on some real cheap stuff "instantaneous peak power" (IPP, roughly TEN TIMES the RMS rating!!!).

    My first bass amp was a "60 watt" Univox which turned out to be 20 watts RMS. I also owned a cheap stereo receiver rated at 100 watts IPP which was really a whopping 5 watts RMS per channel.

    Some of the "peak" and "instantaneous peak" measurements were made by inputting a single, short waveform (like one cycle of a 1 KHz sine wave) and if the amp could sustain the output that fraction of a second then the manufacturer would use that number.
     
  8. seamonkey

    seamonkey

    Aug 6, 2004
    Have you done any measurements you can share?

    Any frequency
    spectrum's
    at this level?

    Why not use a complex waveform? So you have a spectrum to measure. i.e. In real life the low frequency of the waveform clips while the high frequencies don't.
     
  9. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Actually, not so.

    If any part of the wave clips, any lower amplitude higher frequencies are also clipped, since they are "riding" on the LF ..... Effectively they are "shut off" for the time of clipping, which can generate some "interesting" effects..... That is one type of "intermodulation".

    However, it is possible to clip a lower amplitude HF signal, while the higher amplitude LF signal is NOT clipped.

    That is because the sum of the peak voltages can be larger than clipping, but the peak voltage of the LF itself is not (yet).

    As far as checking the power of tube amp..... I have indeed seen the 10% THD "system". Some other things are checked that way also..... some car stereo systems, etc. You get a little higher number that way, and the argument for a tube amp is that that is a "realistic" number.......

    But I would prefer to check any instrument amp at the "onset of clipping" which is at no more than 1% THD....

    Theoretically, since the peak and RMS of a square wave are equal, if you measured the power at "hard clipping" you could get "up to" a number of double the power at onset of clipping.

    For various reasons you won't quite get double...... and does it really matter? Save it for the lead guitar..... bass usually sounds kinda bad if severely clipped..... unless you are a Pink Floyd / Soft Machine tribute band.......in the studio with lots-o-effects to add.
     
  10. seamonkey

    seamonkey

    Aug 6, 2004
    Thanks, I was thinking along the lines of this Rane article:
    http://www.rane.com/pdf/old/note128.pdf
    where are you point out between the low level clipping the high-frequency doesn't clip.

    Just in software, I took a sine wave and clipping it did not sound harsh. Distortion added from the amp may make it sound harsh, but ...

    I took a typical bass recording and looking at the waveform. It was asymmetrical. Clipping the peaks did clip some of the HF. The clipping itself added a little HF but not as much as you'd think.

    Increasing the gain did what the Rane article said. And with the increase HF it sounded harsh. This harshness came from the input signal and not a lot from the clipping.

    In this state simply turning down the HF on the input lowered the HF on the output and "harness" was lowered.

    So I'd be curious to see a spectrum at normal power, and one at high power of a real amp to see how the amp treats a complex waveform. Is the frequency response reduced in a real amp?
     
  11. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    I don't have any handy postable curves for you.

    BUT, I have one thing to complain about in the article you linked..... even though it isn't quite on-topic in this thread.....

    What it says is true UNDER THOSE CONDITIONS...... And everything said about the existence of power compression is also true.

    But note that it is a 100W amp, and a 10W tweeter, to start with.

    Also, it is 100Hz, a fairly low frequency.

    Assume a different scenario.... 1000W amp..... Now take everything in his table and multiply by 10.

    And, take frequency up an octave or two..... say 400Hz..... so the power at 1.1 kHz is actually occurring at 4400Hz...

    So now it looks different, with 8+ watts at 4400 Hz, which is getting into the area of small tweeters used in bass cabs.

    With 8W you are already at 80% of the "power handling" rating of the tweeter..... Now, add that 8 W to the naturally occurring harmonics in the range of the tweeter, AND to the effect of power compression, and it becomes clear that it is possible to do some frying.

    Anyhow, I have seen enough exploded tweeters come in to repair shops "after the party"..... when you KNOW the customer cranked it to max, and it probably sounded like a boombox on 12. :rolleyes:

    The moral of that story is clearly not to underestimate the needed power handling REGARDLESS of the source..... Pretty much all of us manufacturers are guilty of ignoring that in at least some models......
     
  12. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    No, unless the amp is clipping. Modern amps have sufficient slew rate to have frequency response independent of amplitude.
     
  13. seamonkey

    seamonkey

    Aug 6, 2004
    Tube = Modern?
    Impedance of the speaker is known to interact. So it stays the same at all power levels?
    10% THD distortion is equally distributed?
     
  14. Yep. ( I know that just galls you to hear :) )

    Research a little item known as negative feedback.

    I don't use or support a 10% THD for power rating and the answer to your question is that it depends on what KIND of distortion is occuring to cause the 10% THD, just as Jerrold alluded to earlier.
     
  15. BbbyBld

    BbbyBld

    Oct 13, 2005
    Meridian, MS
    I design my amps so that when they ship, they will do max rated power at about 10% THD. That doesn't mean they won't do better, but when you have to sell something at a reasonable price, and have it be reliable, sometimes you have to bias it a little on the cold side.

    Another problem I've run into is that if you design a tube amp to do rated power at 1% THD, when a customer tries to overdrive the power amp, the power tubes will be running out of spec. For a hifi design, I'd say go for the 1% rating, but for MI where people want to overdrive the power amp and especially where people plug in basses that are very dynamic and can slam the crap out of power tubes, I stand by 10%.

    One design I'm working on will do rated power at about 6% with hardly any crossover distortion, but the difference between 1% and 6% is about 15 watts or so. You probably couldn't even hear that difference. Of course, the bias could be set hotter.

    Even with feedback the cabinet will have a dramatic effect on the output response. Add a ton of feedback, and compare it to an amp with a reasonable amount and see which one people will pick in listening tests.
     
  16. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I assume a ton of feedback means a ton of open loop gain. The rule of thumb I remember is that higher open loop gain means better linearity, but can eventually compromise slew rate. So I would agree that on a tube bass amp, too much open loop gain sort of defeats the whole purpose.
     
  17. BbbyBld

    BbbyBld

    Oct 13, 2005
    Meridian, MS
    Open loop means no feedback. If you have no feedback, the amp will sound really boomy and/or farty. I meant the opposite.
     
  18. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    I meant the "open loop gain" parameter as it is defined for solid state amps and IC op amps, i.e., the gain of the amplifier before closing the feedback loop. For instance, the LM3886 power op amp chip is specified with an open loop gain of 115 dB. The overall gain of the amp when the loop is closed is roughly the inverse of the feedback gain.

    So I just assumed the same thing applies to tube amps. I did not mean actually running an amp in open loop.
     
  19. BbbyBld

    BbbyBld

    Oct 13, 2005
    Meridian, MS
    I'm sorry...guess I didn't quite follow what you were saying.

    It's not quite the same thing as with op amps. With tube amps, the feedback is for controlling the damping instead of controlling gain. I guess as a physicist you could write a book on harmonic oscillation (I'm not worthy!)!

    I was just saying that what works to improve the damping of a hi-fi tube amp doesn't always sound good in an MI application.
     
  20. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    Nah, it's you guys who know the real world stuff, who impress me the most.

    Definitely true.
     

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