Help with power handling question...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Meeotch, Jun 23, 2019.

  1. Meeotch

    Meeotch Supporting Member

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    So I've been running my Diezel Herbert (180w) through a 4x12 with Greenbacks (100w) and everything seems fine so far. I have read unofficially that unless you are cranking the amp, it's not producing anywhere near it's rated power spec.

    However, a fairly credible source recently told me that even with the amp's volume on minimum, the load is still seeing at least 60% of the amp's power rating. That would mean the Herbert is putting out 108w minimum...but again everything seems fine so far. Am I just getting lucky?
     
  2. ironman28

    ironman28 Member

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    You mean with the volume on 0 as “minimum “?
    No power is output with no signal applied.
     
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  3. Tron Pesto

    Tron Pesto Member

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    Maybe we're confusing plate dissipation with speaker power handling? Perhaps when you say that when the amp is at "minimum", they are talking about the plate dissipation of the output tubes.

    With no signal, push-pull output tubes in an AB1 configuration are typically biased to dissipate up to 70% of maximum output. So figuring on a somewhat conservative bias of 60%, yes, the tubes are dissipating that much power with no signal applied.

    The goal with biasing is such is the assumption that upon maximum signal swing of about 30%, the output tubes will not be forced beyond their maximum plate dissipation.

    While this certainly is correlated to the actual wattage driven by the amp, plate dissipation is not the same thing as power output. It seems like they are comparing apples to, well, different apples. If the amp is designed to deliver 180W at maximum signal, then yeah, that would be more than the 100W speaker cabinet you have. Can you play the amp at a volume that is somewhere less that maximum power? Sure. Can you do that with an under-rated speaker cabinet? I guess - it depends. However, you're leaving a lot on the table - tone-wise.

    IMHO, tube amps sound best when they are pushing the output tubes and speakers "well". If you are living in a lower volume world, you might be better served with a lower wattage rig so you can get into that "sweet spot" without peeling paint off yer arse.
     
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  4. SecondFloorTones

    SecondFloorTones Member

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    If you don't hear sound, there's no power being fed to the speakers. If it's plate dissipation as mentioned above, I'd think it would dissipate as heat - but I'm no expert when it comes to the inner workings of tube amps.

    The Herbert is designed to have a clean power section, so I don't think I'd think of it as a good candidate for power tube distortion. This is perfectly OK, lots of cool tones have been produced by preamp overdrive alone. As for using an underrated cabinet - when you're actually dumping 100 watts of power into the cab, you probably won't be able to (or want to) be anywhere near it. You're probably looking at around 120 dB SPL, and that spells out permanent hearing damage very quickly. For fun I turned my TRRI up to nine in a rehearsal studio once. Our drummer - who also played with a death metal band - was completely inaudible at that volume, and the TRRI is rated at 85 watts. It probably outputs more than that when cranked, but still. When you're at a level where your cab can't keep up, you'll be loud.
     
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  5. Meeotch

    Meeotch Supporting Member

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    Thank you all for the info. I think I'm following most of this, but still hoping for some clarification on some basics of the topic:

    1) is it true that as you turn up an amp's volume, the load will see more watts? If so is there any reasonable way to estimate how much the load is seeing in respect to the amp's volume knob? Perhaps watts have a correlation with decibel levels?

    2) Even though manufacturers have different standards for testing parameters like speaker sensitivity (and perhaps power rating), it seems like most people still adhere to the protocol of never overpowering your load (as I am doing, I just thought I would be safe as long as I'm not cranking the amp)?

    I'm assuming that playing a 180w head through a 100w cabinet is a bit of a gamble, but at the same time I don't intend to do this at band volumes (staying at 90db and under). I have a 4x12 rated at 260w for this.
     
  6. BluntForceTrauma

    BluntForceTrauma Member

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    Buy some new speakers soon. You will need them. I suggest 4-100 watters.
     
  7. pdf64

    pdf64 Member

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    G12M25 generate ~97dB with 1 watt. So when closely mutually coupled, 4 of them acting in unison will generate >100dB at 1 watt.
    Each time the power doubles, the dB generated increases 3dB. Hence 2 watts gives 103dB, 4 watts 106dB, 8 watts 109dB, 16 watts 112dB, 32 watts 115dB, 64 watts 118dB ...
    So domestic friendly SPLs will be achieved with less than 1 watt from the amp, the sound levels that would result from pushing anywhere near its 100 watt rating into it would be devastating.
     
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  8. HotBluePlates

    HotBluePlates Member

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    Yes. As the output tubes receive a larger driving signal, they will output more power to the speaker. Power output is not constant.

    Yes in theory, as @pdf64 showed.

    Realistically, no, which is why you use enough rated speaker power-handling to be sure you don't blow anything up. Or take your chances, don't get carried away with cranking it up, and take your lumps if you do wind up blowing a speaker.

    90dB is "fairly quiet" with a 4x12 cab. pdf64 showed you will be staying well under the cab's rated power handling if you truly are staying this quiet.

    (Which might still be "loud" in an apartment, late night...)
     
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  9. Meeotch

    Meeotch Supporting Member

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    This is exactly the info I needed, thanks a ton!
     

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