Tube amp ohm mismatch with speaker info & question

  • Thread starter Thread starter MadAsAHatter
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psychodave said:
I get that, but there has to be something relative otherwise the fluctuations would be too much?
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Think of an engine. One form of power converted to another with a give and take between the two.

Look at the Celestion curve. It's a guitar speaker so it's normally only seeing full power frequencies in
the 100 to 5K range which is where it's impedance is in the 'comfy zone'.

Like crusing in a car between 20 and 60 MPH.

If you were to run a static signal of 55 or say 15K-16K Hz into that speaker sooner or later the amp is going to say f-ck this!

Same goes for a standard car engine being driven on a highway for hours at either 5 or 120 MPH.

Keep the signal going into the speaker within it's intended operational zone and the amp will never
have to strain. An ohm meter will be jumping around like crazy but all within specifications of both
sides of the engine.
 
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Yeah Donnie has the concept correct - impedances fluctuate drastically when the source is nonlinear and the load is nonlinear. Think of the impedance mismatching as a dampening effect that doesn’t allow for maximum power transfer, and instead the power gets reflected back at the OT and the energy is then reflected back at the plates.
 
glpg80 said:
This is all wrong.

You’re assuming RMS values and that cannot be done if the load is not purely resistive. Because the source is a frequency dependent inductor and the load is a frequency dependent transducer, you have to go much, much deeper into the math with power factor calculations and frequency dependent Z parameter plots if you’re going to prove it mathematically. It would all be done in VA (voltamps).
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It's not wrong, it's simplified to demonstrate the very basics of what's happening where a layperson can grasp the concept.


glpg80 said:
This is sound knowledge.

Just know impedance mismatching will put additional stress on your power tubes, especially if you’re also cranking on them volume wise while mismatched.

Is it ideal? No. Can it be done? Yes.
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Except it goes against several articles on the subject which say that for tube amps the closer you get to infinite ohms the more dangerous it is for the amp and why its advised to never run a tube amp without a load connected. A tube amp has a better chance at surviving a short circuit a 0 ohms than infinite ohms when looking at extremes. The opposite is true for solid state where infinite ohms is okay but 0 ohms will fry it.

Mismatching with the speaker impedance higher is more dangerous because of flashback voltages that put stress on the transformer by arcing through insulation layers and causing a short that fries the OT.
Mismatching with a lower impedance speaker puts stress on the power tubes wearing them out quicker, but doesn't pose much risk to the OT.

Either way you'll be putting stress on one component or the other, just depends on which side the stress goes to. And if something does go wrong, what would you rather replace; tubes or output transformer? Most amps with quality components should be fine at either double or half the ohms it's set at (with maybe the exception of going into 2 ohms). Some manufacturers even state it will be fine a 1/4 the ohms. The risk of mismatching is never zero, but are pretty low as long as you don't go to extremes and stay within tolerances.
 
Does the same rule apply for a 300w tube bass amp? Is it safe to run a 2 ohm out to a 4 or 8 ohm cabinet?
 
MadAsAHatter said:
It's not wrong, it's simplified to demonstrate the very basics of what's happening where a layperson can grasp the concept.
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It’s simplified to be wrong. MSEE here.

MadAsAHatter said:
Except it goes against several articles on the subject which say that for tube amps the closer you get to infinite ohms the more dangerous it is for the amp and why its advised to never run a tube amp without a load connected. A tube amp has a better chance at surviving a short circuit a 0 ohms than infinite ohms when looking at extremes. The opposite is true for solid state where infinite ohms is okay but 0 ohms will fry it.

Mismatching with the speaker impedance higher is more dangerous because of flashback voltages that put stress on the transformer by arcing through insulation layers and causing a short that fries the OT.
Mismatching with a lower impedance speaker puts stress on the power tubes wearing them out quicker, but doesn't pose much risk to the OT.

Either way you'll be putting stress on one component or the other, just depends on which side the stress goes to. And if something does go wrong, what would you rather replace; tubes or output transformer? Most amps with quality components should be fine at either double or half the ohms it's set at (with maybe the exception of going into 2 ohms). Some manufacturers even state it will be fine a 1/4 the ohms. The risk of mismatching is never zero, but are pretty low as long as you don't go to extremes and stay within tolerances.
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All of this is correct whether you’re copy/pasting from another source or not - I don’t disagree with any of it.
 
retracted my post... I don't have the energy to go back and forth with anyone. I try to contribute to the community with some of my experience and get crap for it. I'm done with this thread.
 
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LP Freak said:
Does the same rule apply for a 300w tube bass amp? Is it safe to run a 2 ohm out to a 4 or 8 ohm cabinet?
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Solid state normally can take open circuit just fine as it’s usually fed off the drains/emitter in parallel, what they don’t like is direct shorts to ground on the output. If it’s class A/B then follow the textbook guidelines of matching within the limits of what it says - you won’t get away with mismatching like a tube amp. If it’s any other class you’ll know you’ve hit the rails and get distortion in a non musical way when you’re mismatched and crank on the volume.
 

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