Originally Posted by Downunderwonder
I don't understand the confusion.
An SVT runs happily at eiither 4 ohm or 2 ohm.
I could be wrong but as I understand it, running at a higher impedance than the transformer tap rating causes the power tubes to run hotter as transformer runs higher volts. Opposite of solid state.
In first instance this ohm matching consideration is of pure theoretical thinking only. The impedance curve of a given cab looks like a roller coaster which can't give any impression that there would something exist like a steady nominal impedance.
But indeed with a Pink Noise signal or an average audio signal program the resulting rms current consumption Irms (most of the time) measures very close to the equation of
Irms = Vrms/Znominal
By the other hand it is legal to calculate Z if Irms and Vrms are given:
There is a tricky context between different frequency spectrum and current consumption Irms but steady voltage Vrms.
In general the cabs current consumption drops down if the spectrum power distribution is very sharp focused to the lows whereby Vrms remains steady.
That means, holding the amps output voltage Vrms = const. but changing the signal spectrum (i.e. sharp boosting the lows) results in a dropped current consumption Irms.
Not unlikely Irms is nearly halved if the bass boost is +12dB.
Which means that Z would be doubled in this case.
ok, in practice the increase of Z is probably lesser because most players don't boost the lows by +12dB.
Every cab consists of a system resonance where Z is at its maximum. Lets say 4 Ohm nominal cab with resonance at 80Hz and Zmax = 25 Ohm.
Now I connect the 4 Ohm cab into the 4 Ohm output of my svt, I connect a sine wave generator into the svt input, I select 80Hz at the sine generator and gain the output of the svt to 35 Volt rms. 35 Volt rms calculates to 300 Watt power at 4 Ohm nominal. But Z is 25 Ohm at 80Hz. So ouput power calculates to little 48 Watt instead of "theoretical" 300 Watt.
I have no idea what would happen to the svt if the amp is running 35 Volt rms into a 25 Ohm load but, I can't imagine it would be very healthy to the amp.
A very sharp focused frequency spectrum around system resonance is by far as not as critical like a pure sine wave but, even in this case the system resonance with its high Zmax would impact the current consumption and therefore Z.
At last some practical considerations.
Sealed bass cabs like the classic svt 810 tend to have a uncritical Zmax. Different frequency spectrum does impact the current consumption and therefore Z in a slightly manor only.
Some modern neo speaker like the Deltalite II provide a moderate amount of Zmax even in a vented cab.
The resulting Zmax at system resonance is not as low like a sealed cab construction but, nothing to talk home about.
However there are some modern high tech speakers out there like the Kappalite series which leads to a very significant Zmax at system resonance. IMO it is a good idea in this case to spend some considerations regarding to the ohm matching.
It is well known that tube amplifiers sometimes tend to sound not very well on a given cab although the nominal ohm matching is fine.
It seems to be a complete joke that Zmin of a speaker/cab is discussed in forums all around the world all along the line whereas the amount of Zmax at system resonance of a cab seems to be not existent at all in forum discussions everywhere.
It seems to be a complete joke because the amount of Zmax impacts a tube amplifier in a similar manor like Zmin of a given cab impacts a solid state amplifier.
Whereas Zmin is defined to be not lower then appro -20% below Znominal there is in opposite no existing definition for Zmax.
Probably that is the reason in general why most of folks don't spend any consideration at all on the amount of Zmax.