ohms questions

miller · 10-07-2003, 09:27 AM

basically i want to know what you can and cant do as far as hooking up your head and cab

ex. if your cab is 16ohms and your head is 8ohms or something like that.
i just need some info

Major Tom · 10-07-2003, 07:44 PM

OK, here's an answer:
If its a TUBE amp you should always connect the exact load that its output is set for, i.e., 8 ohm output to an 8 ohm load. This is pretty much conventional wisdom and accepted practice with amp manufacturers, tube amp experts and users, with very few exceptions. A few maverick users will purposely hook up a mismatched load (higher or lower) and report that they have had no problems. Do this at your own risk. I do have a Fender blackface Champ rated at 4 ohm out that has an 8 ohm speaker, its still working, although I will get a proper 4 ohm speaker soon...If you are going to hook up a mismatch, i would recommend only going with a higher load, never a lower, and never run it that way with the amp running full-bore for extended periods. Worse than a mismatch is to run a tube amp with no load at all, no speaker connected. This applies to amps with a tube output, also known as an "all-tube amp", not amps like the Marshall valvestate hybrid with just a preamp tube.

If it is a solid state amp you can use the exact load its rated for or higher - example, 8 ohm amp output into an 8 or 16 ohm cabinet is ok, but not a 4 ohm. This would also include the hybrid amps like the Valvestate with a solid state output. Its ok to run a solid state with no speaker load.
What amp do you have?

**Rainer.** · 10-07-2003, 08:29 PM

I believe it is OK to run an speaker with a higher impedance than its amp; however, it is not a good idea to run the amp with no speaker or with a speaker with a lower impedance. For tube amps at least. Solid-state amps can handle being used with a lower impedance speaker, or with no speaker at all.

Brent · 10-07-2003, 09:16 PM

It's also worth noting (since no one mentioned it) that if you connect a higher impedance cabinet to your head then you will get less power from your head because the resistance his higher at your cabinet. Vice versa, you'll get more power if you connect a lower impedance cabinet, but you also run the high risk of burning something up.

Moral of the story is it's best to match the impedances of your amp and cabinet(s).

Now if you're going to connect more than one cabinet to an amp then that's a whole other story. It involves lowest common denominators, division, addition, and other fun things like that. Not too hard, though.

**Rainer.** · 10-07-2003, 09:36 PM

Read this if you need more info: http://www.soundme.com/eng/ampli/amp08.htm