A valve amp impedance question for you Gurus out there...

[Noisyjon]

Hello Folks,
A question for you those of you with a wealth of valve amp knowledge:

On my Mesa Boogie Bass 400+ I have 3 pairs of speaker outputs - 2x 2 Ohm, 2 x 4 Ohm and 2 x 8 Ohm.
All fair and good.
I was looking at Alex's 'Barefaced - The Big One' speaker box and noticed that it is a 6 Ohm rated unit.

Now I've read the pinned 'Impedance etc, copied from basstalk' thread and looked at the Wiki but am under the impression that Ohm mis-matches on a valve amp ain't good for it....
So...
What speaker output would I need to use for a 6 Ohm cab or is it not worth entertaining such an idea with a valve amp?

All and any help much appreciated,
Cheers,
Jon.

[alexclaber]

With a solidstate amp the key issue is that the impedance does not drop too low for the amp to handle, thus causing overcurrent problems in the output transistors - hence the nominal impedance just provides a guide to the actual impedance by showing an indication to how low the impedance actually goes.

On the other hand with a valve amp the key issue is that the impedance does not go too high for the amp to handle, which I believe would thus end up causing a lack of current flow out from the output transformer and thus problems in that part of the amp. In that case the height of the resonant impedance peaks are more of a problem than the lower wide troughs. I haven't measured the final version of the Big One but I'm pretty certain there would be no problems running the Big One on an 8 ohm tap.

Alex

[escholl]

Valve amps shouldn't really be run at too high or too low of an impedance -- for maximum power transfer, and to minimize other problems, the impedance should match exactly.

Well, ideally. The tolerance is pretty generous -- amps with 8 ohm outputs, i've run 4 ohm and 16 ohms cabs on, without issues.

And as Alex mentions, it's not like the impedance of the drivers remains constant anyways, and the 8 ohm out should be fine.

[Bill Fitzmaurice]

[quote name='escholl' post='453625' date='Apr 3 2009, 02:41 PM']Valve amps shouldn't really be run at too high or too low of an impedance -- for maximum power transfer, and to minimize other problems, the impedance should match exactly[/quote]Alex is closer to the fact of the matter. In reality there's no such thing as an 8 ohm or 4 ohm or 16 ohm cab, as every cab's impedance varies with frequency. An average 8 ohm cab actually measures anywhere from 5 to 50 ohms, depending on frequency. SS Amps are designed to operate into the minimum load that they might see, valves into the maximum load that they might see. So a 6 ohm rated cab would pose no problems for an 8 ohm transformer tap. It probably wouldn't bother a 4 ohm tap either, though.

[escholl]

[quote name='Bill Fitzmaurice' post='453696' date='Apr 3 2009, 09:26 PM']Alex is closer to the fact of the matter. In reality there's no such thing as an 8 ohm or 4 ohm or 16 ohm cab, as every cab's impedance varies with frequency. An average 8 ohm cab actually measures anywhere from 5 to 50 ohms, depending on frequency. SS Amps are designed to operate into the minimum load that they might see, valves into the maximum load that they might see. So a 6 ohm rated cab would pose no problems for an 8 ohm transformer tap. It probably wouldn't bother a 4 ohm tap either, though.[/quote]

See the next line of my post where it says "well, ideally."

And where i mention that very fact about speaker impedance ^_^


To be honest, odds are the impedance of the transformer isn't perfectly uniform across the frequency range either.

[Noisyjon]

Thanks for the replies Gents and all duly noted.

Have a good weekend,
Jon.