Tube amp impedance setting if I "don't know"

[Rex B]

If I just plain "don't know" the impedance of a speaker cabinet, and MUST hook it up to a tube amp, (like to finish a gig) is it best to set the amp impedance setting at the lowest setting or highest? Middle?

[arenavore]

carry a pocket multimeter with you and get an exact reading before plugging in.  They're pretty cheap at any Radio Shack or similar store.

[EdBass]

If I just plain "don't know" the impedance of a speaker cabinet, and MUST hook it up to a tube amp, (like to finish a gig) is it best to set the amp impedance setting at the lowest setting or highest? Middle?

Hmmm... Not exactly a question one would expect from someone whose signature reads “Engineer Sunn 1977-1983, Fender 1984-1990”.
I have an opinion (as usual ), but: What do YOU think the answer is Mr. Engineer?

carry a pocket multimeter with you and get an exact reading before plugging in.  They're pretty cheap at any Radio Shack or similar store.

In actuality a multimeter won't give you a speaker's impedance. It will read DC resistance; you need to use that number to make an educated guesstiment what the actual nominal impedance is based on the theory that the nominal impedance of the majority of drivers is a multiple of 4.
 "Nominal" being the operative word here; as with horseshoes and hand grenades when matching impedances "close" is usually good enough.

[arenavore]

of course, but most people just ignore that fact.  Isn't the actual measurement of impedance much different from the system used in labeling speakers and amps?  Thought I heard that somewhere, but I could be wrong.  As I've stated before, I'm only just starting to learn this stuff myself...

[EdBass]

of course, but most people just ignore that fact.  Isn't the actual measurement of impedance much different from the system used in labeling speakers and amps?  Thought I heard that somewhere, but I could be wrong.  As I've stated before, I'm only just starting to learn this stuff myself...

Well, the actual measurement of nominal impedance is exactly what the manufacturer puts on the label. However, as I mentioned it's not a real exacting science.
The point I was trying to make is that if you hook up a multimeter to, for example, an 8 ohm driver you won't get an 8 ohm reading, and that can be confusing for people at first.

Rex B. has a good explanation of “nominal impedance” in this thread;

http://sunnforum.ampage.org/index.php/topic,5891.msg26048.html#msg26048

[Rex B]

Ed: Yep, you would THINK as an engineer for Sunn and Fender I would know the answer. However, I was not a tube amp designer, I was primarily the Electro-acoustic guy...speakers, cabinets, crossovers etc. I was also involved in some of the solid state stuff, mostly bass amps, but still not the primary circuit designer on those either...Howard Goetz was (a true genius). Heck when I was at Sunn, we didn't even design a tube amp, we were just building previous designs..Model T etc. At Fender: no lack of tube expertise around there. No need for me to muck up a good thing   You guys know infinitely more about tube amps than I do. MY answer was to set the impedance selector at the lowest setting I thought the cab setup might be, in this case 4 ohms. If the cab was actually 8 ohms, then I just wouldn't be delivering as much current. Sound right?

[EdBass]

I think tube amps are pretty forgiving of impedance mismatches, and the rule of thumb seems to be that a 100% mismatch will be tolerated by most amps. Obviously, as this thread has touched on the actual load the output stage at any given time fluctuates significantly as is illustrated by the use of the term “nominal” in front of speaker rating impedance, so that type of tolerance is pretty much required for a tube amp to function.
So I suppose by that 100% logic, while without question a proper impedance match is optimum if forced into a guessing game with an unknown cabinet, going in the middle would seem the best bet.

The big no-no is an open output. Leo Fender’s amps had jacks that defaulted to a dead short if they were empty, so I suppose at least he thought that 0 ohms were safer than infinity ohms.

As arenavore suggests I personally carry a multimeter in my gig bag with my rig cables, along with a flashlight, tool kit, soldering iron, etc. I’ve only had a handful of emergency situations arise in the past decades but when they do it saves a whole lot of frustration and cursing.