Amp Protection question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 908SSP, Jul 2, 2004.


  1. 908SSP

    908SSP Member

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    Years ago I tried running a 10ohm 50watt resistor as a extension speaker to quite down my Marshall in a vain attempt to get power tube distortion. Well it didn't work it might have made a very slight difference but I won't swear to it but I did leave it plugged into the extension speaker jack for a long time, I simply forgot it was there and it didn't do any harm. Anyway the question is would that load act as a safety measure for the amp if I were to blow the speaker? The resistor would still be in a parallel circuit acting as a load would that work?
     
  2. VaughnC

    VaughnC Supporting Member

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    With no load on a tube amp, the output transformer could be permanetly damaged. So, even though you're converting some of your output power to heat with the resistor, it should protect the output transformer if your speaker's coil opened.
     
  3. 908SSP

    908SSP Member

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    I just got off the phone with Dave Zimmerman from Maven Peal he assured me that I was in fact onto something. I could in deed put a 20ohm resistor on the 16ohm speaker jack and that it would protect my amp if the speakers blew. In theory it should not have an effect on the sound but that would have to be tested to know for sure. This of course leads to the second part; if the Amp is now protected can I run fuses on the speaker leads to protect the speakers and the answer again is yes a 1 amp fuse for Celestion Alnico Blues would be a good place to start and if they blow too easily go up in steps to maybe 1.5 amp. With my Ganesha amp I can run it into full distortion as low as one watt but the sound really shines at slightly more then that these safety measures will allow me to more comfortably run the Blues whose sound I really like.
     
  4. Old Fuzzface

    Old Fuzzface Member

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    Putting fuses in the signal leads is probably going to degrade your sound noticeably.

    Plus, there's more chance of a nuisance fuse blow (for no real fault) than a driver failing, so by putting fuses in, you're actually increasing the chances of a "no sound" situation.

    I think the only safe option with low power handling drivers is to have enough of them to handle your amps full power. I know this is the boring engineering solution and lots of people will disagree with it, but the numbers are what they are and you ignore them at your peril.

    The other possibility would be to stick a (fixed value) attenuator on the amp that will reduce the max power fed to the driver(s) to an acceptable level. Which is sort of where you're going with the resistor idea.
     
  5. John Phillips

    John Phillips Member

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    What Ian says.

    The power resistor idea will work, but firstly you're throwing away around half the total power of the amp (which may or may not matter, depending on your application) but more importantly it will significantly change the tone.

    A real speaker has an impedance curve dependent on frequency - it's not a fixed resistance. In fact, the curve is quite steep - a typical "16-ohm" speaker may reach as high as 80 ohms at the upper end of the useful frequency range.

    When two loads are connected together in parallel, the power is distributed in inverse proportion to the impedances - so at low frequencies where the speaker impedance and the resistance are about the same, about half the power goes to each. But at high frequencies, the resistance is now far lower than the speaker impedance, so the bulk of the power will go to the resistor. This will make the speaker sound very dull.

    Make sure your speakers will handle at least the maximum distorted power of the amp (typically 150% of the RMS, give or take a bit, and never more than 200%) and you're not in danger of blowing them anyway.
     
  6. Wakarusa

    Wakarusa Member

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