Ohms: How important is it that the amp and speakers match?

[Yossi]

I have a 2x12 cab that is set at 4ohms and I have a combo amp that is set at 8.
And I have a Marshall 4x12 that can be switched from stereo to mono and I am not sure off hand what ohms it is at.
Do I have to be careful when hooking extension cabs up to different amps?
Could I cause damage or effect the sound?
I really would appreciate the run down on the ohm subject.
Thank you.

[Old Tele man]

...general "rule-of-thumb" for tube amplifiers using transformers (OT) is: ± 100% from the "optimal" load (where maximum power occurs).

...which means an amp with an "optimal" 4-ohm OT will handle loads between 2-ohm (½-optimal) and 8-ohms (twice-optimal)...at these two extremes, power will only be down by about 12-15% (from optimal)

...however, some amps seem to be more "touchy" with loads than others, notably the original Marshalls.

[meburdick]

MESA recommends that you connect a speaker of equal or HIGHER impedance to an output jack on their amps. Equal is optimal, and a higher impedance will actually cause the tubes to run a little cooler and change the tone slightly.

To be honest, I've tried the internal speaker in my combo (8 ohm) and my external 2x12 cabinet (also 8 ohm) in both the 8 and 4 ohm jacks on my amp.. I can't hear a difference.

[EarlTX]

Here's what I have to say about this subject -

A) follow the manufactures recommendations to protect any warranty.
B) OTM's rule of thumb is a good rule.

C) Does it really matter? Speakers are a lousy representation of a "load", and the nominal value that speakers are rated at are somewhat ambiguous.

Since a speaker is a Impedance, with a reactive component, the actual 'load' value changes over frequency. As well, the reactance of a speaker can change from capacitive to inductive over frequency.
Add to that, the Back EMF, and what the output section actually 'sees' when a guitar is played is a crap shoot.

SS amps typically have almost no output resistance/Impedance, so it doesn't really matter what's hanging off of it, as long as it's not too low to cause too much current draw, and burn the output devices.

I reckon that the +/- 100% 'rule' is probably about as good as any.

[spikeRI]

11 post and yer a expert?
......prolly even own a epi or two

[cameron]

I reckon that the +/- 100% 'rule' is probably about as good as any.

Sorta. The language of 100% is a bit misleading. Taken literally, a load 100% lower than any given load is 0 Ohms, which no amp wants to see.

A more accurate statement of the rule of thumb in question is that tube amps can tolerate a mismatch by a factor of two in either direction. Thus, anywhere from half the optimum load to double the optimum load.

Solid state amps are different of course, they don't have an optimum load. They have a minimum load . . .

[EarlTX]

Thank's for pointing that out, Spike....! :fo

Cameron - You're right in the literal sense.

[spikeRI]

Thank's for pointing that out, Spike....! :fo


I got yer back.......

[EarlTX]

I'm forever grateful, Spike!