OK, but what happens when you connect an additional extension cab to a tube amp? What if the amp doesn't have discreet jacks or a switch, just a jack for adding another speaker? (like my old Fender Pro Reverb combo). Or another way to ask.. say you had a toggle switch on a head labeled 4 ohms / 8 ohms / 16 ohms. What actually happens if you have a 4 ohm speaker load and you set the switch to 8 or 16? Or vice-versa? What are the reasons to either do that, or not? I've never been clear on this, actually.

Old Fender amps don't give you any options about setting the output impedance. So if you have an 8 ohm internal speaker and you plug in an 8 ohm extension cab the total on the amp is indeed 4 ohms, from two 8 ohm loads in parallel.

If you have a switch and two speaker outs then if you plug in 2 speakers, one to each jack that are both 8 ohms then you again have a 4 ohm total and should set the switch for 4 ohms.

If you have only one jack for 4 ohms, one for 8 and one for 16 then you would have to "daisy chain" the loads, that is you plug the 1st 8 ohm cab into the 4 ohm jack and the second cab into a second jack on the 1st cab, if it's got one.

Now, what if you don't have such choices or simply forget? It's an old Fender or a newer amp and you simply forgot to change the switch?

The answer is, not much! You see, output transformers change the speaker load of 4 or 8 ohms up to several thousand ohms to be suitable as a load to the output tube(s). All transformers work in a fixed ratio. A pair of EL34's usually gets a 3400 ohm plate load. The OT would ratio it down to 4, 8 or 16 ohms by using a tapped secondary winding for the speakers. The higher loads have a higher number of wire turns in the coil. Any of the speaker outs should transform up to 3400 ohms, having different numbers of turns to change to the right ratio. If you put an 8 ohm speaker into the 4 ohm output jack the ratio can't change so double on the 4 ohm side means double on the tube plate side, or 6800 ohms instead of 3400.

Tubes are forgiving creatures and will tolerate a wide range around their "perfect" load impedance. The power output changes a little bit, the distortion might get a little better or worse, and/or the tone also gets tweaked a bit. A worst possible mismatch like a 16 ohm cab into a 4 ohm jack might stress the tubes a bit if you're playing on 11 for an all night gig but it's not likely anything will blow up!

You'll just find that if you keep doing things like that your output tubes will last a few months less than you usually get from them.

Hope that makes things more clear...

:food-smiley-004: