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Scumback M75 65w (AGI) tale...

Discussion in 'Cabinets & Speakers' started by Scumback Speakers, Sep 10, 2021.

  1. Scumback Speakers

    Scumback Speakers Well-Known Member

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    So I got a call from a client (JL from So Cal) on Tuesday. JL was disappointed that he blew one of his M75 65w speakers in his 4x12 cab after three years.


    I asked how he determined it was blown, and he told me how he tested to find out, and I agreed he was right.


    Still, this was first time in years I've heard of this, so I asked him a couple of questions.


    Me: Ok, so what were you doing, what amp were you using?


    JL: 100w NMV Marshall.


    Me: Ok, so where did you have it set for volume?


    JL: 8 or 9.


    Me: Alright, but even if you've got one of the old 68 Plexi's with the high plate current I doubt you could blow 260w of speakers.

    Those high plate current models typically are 125/130w clean (4 on the volume knob), so even at 8/9 you shouldn't be past 220/230w.


    JL: Well Jim, it is one of the high plate voltage models (typical Plexi 470v to the plates, his Plexi clone 510v I found out from fusionbear later).


    Me: Ok so you don't use a pedalboard, do you?


    JL: Yes


    Me: Alright but, you weren't doing some dumb ass thing like stepping on a clean boost, then an overdrive pedal plus a fuzz with the amp volume set at 9...were you?


    JL: AS GOD INTENDED! It sounded f**king glorious right before it blew up!


    Me: Ok...well, you know you kinda hit about 300w plus right, so this isn't exactly a warranty issue after three years.


    JL: Right! I got it. I just wanted to call to order four more M75's for my 2nd 4x12 cab.


    Me: Ok...now you're talking.


    I'm going to have T-shirts made up...I'll let you all see the design when they're ready.
     
  2. ricksdisconnected

    ricksdisconnected Well-Known Member

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    mine must have my name on it. :p :fever:

    somewhere on this forum you once told the interesting story of how scumback speakers got its name.
    ive been trying to find that story for a repost.
     
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  3. Scumback Speakers

    Scumback Speakers Well-Known Member

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    http://www.scumbackspeakers.com/name.html
     
    Last edited: Sep 11, 2021
  4. Torren61

    Torren61 Well-Known Member Gold Supporting Member

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    This Scumback sounds great...

    [​IMG]

    ...in this amp.

    [​IMG]

    My buddy, Michael Norton

     
  5. StingRay85

    StingRay85 Well-Known Member

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    300W out of a 100W NMV?
     
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  6. tschrama

    tschrama Well-Known Member

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    Yeh, I understand that the rms value of a sine is sqrt(2) is Vpeak, and since power is V^2 , the maximum output of an amp, completely turning a sine wave into a square wave, is sqrt(2)^2 = 2x its undistorted output.

    How would an 100w amp manage 300W plus?
     
    Last edited: Sep 12, 2021
  7. Scumback Speakers

    Scumback Speakers Well-Known Member

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    @StingRay85 "300W out of a 100W NMV?"

    Well, according to more learned amp techs than me, if you run 40-45 more plate volts than the stock spec, you'll get another 15-20% RMS. The high voltage 68 spec of his 100w clone has some big old transformers. Peak would be 220/230v (as mentioned above). I understand from the client's amp tech that there are other mods so his screen resistors and power tubes are blowing once in awhile. Add on a clean boost (good for up to 30-40%, I know I've done it), OD pedal set high (probably another 20-25%), and then a fuzz box producing more signal and a square wave, and there you go. The amp tech said over 300w is quite possible.

    The 68 high voltage Plexis could run up to almost 600 plate volts if you modded them. JL's was running 505/510v. 140w clean vol @ 4, 230w cranked gain maxed to volume @ 9. All you need is a boost after that, which he certainly had!

    Moral of the story, you can't just step on anything you want, because and when you want and expect your speakers to last forever.

    Frankly, I'm surprised they lasted 3 years with all of those settings, and pedals engaged.

    This is similar to my Corvette LS-1. 350hp stock from the factory, but put a supercharger on it, and it makes 685 hp now. I didn't change anything internally, just the supercharger increasing the intake. I'm still running the stock cam, and exhaust manifolds, too.
     
  8. ibmorjamn

    ibmorjamn Well-Known Member

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    F__ck , has he been to a DR. to check his hearing?
     
    Last edited: Sep 13, 2021
  9. ibmorjamn

    ibmorjamn Well-Known Member

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    100w I have read at about 180w cranked volume to speakers straight in. Pedals increase that , not 100% on numbers myself.
    Probably why you need a full stack to crank it with 25w speakers.
     
  10. StingRay85

    StingRay85 Well-Known Member

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    Highly doubt a Marshall Super Lead, no matter which model, is capable of 300W output. We're talking about Ampeg SVT power output here, a 6x 6550 95 lbs amp, with at least twice (if not more) the amount of iron compared to a Marshall. At some point you can't keep multiplying factors like a boost pedal anymore. If the power amp is done, it's done and no boost pedal can increase power with 30% anymore. There are also fuses inside the amp to protect the transformers...

    Maybe the issue is more that speaker power is determined with a continuous sine wave, while a blocking distortion could potentially hurt the speaker at lower than max. R.M.S. ratings.? I don't want to sound disrespectful, but somehow I think the speaker should have survived the Super Lead? Maybe something else is at play.
     
  11. GIBSON67

    GIBSON67 Well-Known Member

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    I've always heard that square wave Fuzz pedals will destroy speakers in a hurry at full volume.
     
  12. StingRay85

    StingRay85 Well-Known Member

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    I think you have to intigrate the function of the signal to come to the power. With equal V max is does mean a square wave puts out more than a sinus. But the difference between clean V max and blocking distortion like a square wave is just the factor sqrt2
     
  13. tschrama

    tschrama Well-Known Member

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    correct. But that is just for voltage. Since power scales with the square of the voltage, the power becomes sqrt(2)^2, which offcoarse equals 2.

    The square of the integral of a block function is 2x the integral of sine sine function. Hence , a power amp can put out allmost 2x its undistorted rating.

    in practice, because the supply voltage sags a bit and the amp is rated at 10% distortion, it wont manage 2x , but some what less.
     
    Last edited: Sep 14, 2021
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  14. StingRay85

    StingRay85 Well-Known Member

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    That's a solid explanation :yesway:

    Indeed P= U I or P = U²/R

    Let's assume R is a constant and not open the next can of worms
     
  15. Scumback Speakers

    Scumback Speakers Well-Known Member

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    With all due respect to your math, etc...I've put a Zendrive 2 pedal set at unity gain (0 boost) in front of my JTM 45 at the LA Amp Show and it easily doubled the volume by stepping on that pedal. The amp was set at 4, probably below it's rated output of 33/35w.

    The fact that Hendrix used to blow speakers with just a Vox wah, Univibe, and a Fuzz Face when he had two cabs with 30w speakers and a single 100w head just proves they exceeded 240w output.

    As I understand it, pedals have only gotten more powerful, with more signal, gain, and output since 1970. So when two different amp techs tell me he's got 40/45 plate volts over the 68 Plexi spec, and then add pedals to the mix, if the OT can handle it, or if the circuit fails due to DC leakage to the speaker, that still means a signal WAY past what it was designed for got produced.

    And holy crap, three years later, one of his four speakers finally caved in. It would seem to be quite possible given the circumstances of all the factors involved.
     
  16. StingRay85

    StingRay85 Well-Known Member

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    Yes but you mentioned 300W. Thats 75W per EL34. I don't doubt peaks of 200+, maybe 250. Hendrix probably blew a lot of amps too, not just speakers
     
  17. Scumback Speakers

    Scumback Speakers Well-Known Member

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    Well, I have to go with the guy who is working on the amp for the client. He's got the Fender amp repair certificate on his wall, has serviced my amps, and builds them for others. So if he says the client is making 300w with that amp modded, cranked and stepping on pedals, I'd have to think he knows more than I do about it.

    Let me be clear, though. I had a client who had one of my Scumback 9w amps (10.2w clean @ volume of 3, 18w @ 7) and after 14 months he fried a G12-65 with his amp...using a fuzzbox and an attenuator.

    I kinda doubt the amp made it past 25/30w even with the fuzz, but the square wave produced by it, could have killed the speaker by itself. The client wound up saying "F**K IT!" and got an extension cab to go with his combo and he got two T1534 G12H30 55hz pre rola 014 cone speakers installed.

    He's never had a problem again since 2006 with that setup.

    There's probably something to be said for splitting the signal between multiple speakers to handle the fuzz as well. But if there's DC voltage leaking into the speaker, that could help kill it as well.

    Lots of variables once you mod the amp, circuit, etc from stock.
     
  18. anitoli

    anitoli Well-Known Member Gold Supporting Member

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    Once the sine wave becomes a square wave you have dc across the speaker on the clipped part of the waveform.
    I have heard this elsewhere. On a BBS years ago some tech was explaining how he was getting 300w out of a super lead by using a fuzz box.
     
  19. StingRay85

    StingRay85 Well-Known Member

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    It's not really DC, it's still AC but it's a quasi constant current for 0.5 (1/frequency) s. So for 1 kHz it's for 0.5 ms a quasi constant V max, then current flows to the other direction, which makes the cone move in the different direction. This is not really DC, it's a square wave. As pointed out, it can be maximum 2x V max, and in reality lower due to sag and voltage drop. These kind of stories of 300W, I think it's just a bit exaggerating and a way to say "it's really a lot".
     

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