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Discussion in 'Amps and Cabs [BG]' started by Bloc Riff Nut, Mar 23, 2011.
I don't think we are. I think we're getting a feel for the real problem.
I've been running my all tube Ampeg SVT 2PRO (rated at 300 watts into 4 ohms) into an 8 ohm cabinet for maybe 5 years playing 130 to 150 gigs a year and guess what...NO PROBLEMS!!
Well, you've also got electronic phase reversals on either side of the cab resonances. The reactance component of the impedance is going to flip from capacitive to inductive at a resonance, the quicker the higher the Q. I'm looking at the resonance Qs as f0/delta-f where f0 is the resonant frequency and delta-f is the width in Hz of the -6 dB points relative to the resonance peak of impedance, since it is analogous to a parallel resonant electronic circuit. So, without phase information, you can't completely describe the load the amp sees.
Man, what's worse is that a ported cab has two resonances, so you go from inductive (below 1st resonance) to capacitive (above 1st resonance) to inductive (below 2nd resonance) to capacitive (above 2nd resonance). And, you're pure resistive at both resonances. It's a fine mess for an amp designer.
Just to add to the experiences posted here, I had an SVT-CL that I often ran into an 8 ohm cab from the 4 ohm tap. No issues, ever, though I suspected (but could not prove) that there was some tonal difference.
The sticky (and my amp tech) advises that, on a tube amp, you should be safe to go 100% either way, so putting an 8 ohm load on the 4 ohm tap should be safe. So far, it seems nobody is able to offer evidence to the contrary.
Did you not read Bill's post?
I could also relate the experience I had with an amp that was set on 2 ohm accidentially, when a 4 ohm cab was plugged in, and died within an hour of use.
In response to a PM from greenboy (couldn't reply privately)
No offense taken, man. If one can't take the cut and thrust of a TB conversation, then one shouldn't be swordfighting in the kitchen eyebrow. I guess I was waxing philosophical on a point that really has a fairly clear answer in "real" terms. Acoustics is not my field at all beyond being an interested amateur, and I enjoy the threads that wander into the esoteric -- it's where I learn the the most. So keep dumping! Same goes for Bill and the other engineers/designers who weigh in.
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