Impedance dumb question

[mtb777]

What happens when you run an 8 ohm speaker through a 4 ohm head?

[perogato02]

If it is a solid state: nothing.

If it is a tube amp, you are shortening the life of the tubes

[RickenBoogie]

Nothing. It's fine.

[bass_lord_mutha]

Originally Posted by mtb777

What happens when you run an 8 ohm speaker through a 4 ohm head?

I don't have much experience with tube heads so I can't speak on that, but here's what I know on solid-state heads:

A solid-state head doesn't have an impedance designation other than it's minimum impedance rating. The amp 'sees' the total impedance load given by the speakers and adheres to it as long as it doesn't go below it's MIR. So the 4 ohms you're seeing is probably it's MIR, meaning you can do any combination of speakers as longs as the total impedance doesn't go lower than 4 ohms, so one 8 ohm cab is fine. You could even run two 8 ohm cabs or one 4 ohm cab, which would bring the impedance down to 4 ohms and you'd be using the amp to its full potential.

[billfitzmaurice]

Originally Posted by bass_lord_mutha

I don't have much experience with tube heads so I can't speak on that, but here's what I know on solid-state heads:

A solid-state head doesn't have an impedance designation other than it's minimum impedance rating.

Tubes are the other way around, the tap rating is the maximum impedance that should be used. SS amps can be run with no load but a short will kill them, tubes aren't bothered much by a shorted output but no load will kill them.

[mtb777]

Thank you, thank you!!! I knew about the 8 + 8 = 4 ohms and all that for the most part as far as cabs go, but not about the amp end of it. Thank you all again for the wisdom!!

[Stiggs]

Since we are on the subject. I have a late 80's GK 400rb with two 4 ohm outputs. Do these two outputs work separately from each other. Where as one output with two 8 ohm cabs is be 4 ohms. Would that be the same for the second output.

[Sartori]

Originally Posted by mtb777

What happens when you run an 8 ohm speaker through a 4 ohm head?

From your profile, it seems that your amp is solid state. Nothing bad will happen, the 4 ohm rating is for the minimum impedance you can safely run it with. You'll just have a bit less power. No big deal.

If you had a tube amp, it's the other way around, sort of. Running an 8 ohm cab from an amp only rated for 4 ohms (most that are not rated for 8 ohms are rated for 2 ohms and 4 ohms) is bad for the amp. Not as bad as running it with no cab at all, however.

[lbwdog]

Originally Posted by Stiggs

Do these two outputs work separately from each other. Where as one output with two 8 ohm cabs is be 4 ohms. Would that be the same for the second output.

NO!
The amp will run at a 4 Ohm minimum....TOTAL
The two outputs are parallel, which means they are tied together.
So, the amp can handle....(regardless of which output jacks are used)
1-8 Ohm cab
2-8 Ohm cabs (4 Ohm total)
1-4 Ohm cab

Don't try any other configuration.

[Sartori]

Well, you could also use a maximum of four 16 ohm cabs (4 ohm total), but hardly anybody makes bass cabs that come stock at 16 ohms.

[JTE]

Basics are that for a solid-state power amp (tubes in the pre-amp don't affect this at all) you can run the amp into a HIGHER impedance number without damage, but at the cost of less power. So, as has been said, an amp rated at 400 WRMS @ 4Ω can safely power an 8Ω cabinet, but it'll put out around ±200 WRMS. No load is not 0 (zero) ohms, but infinite impedance, so it will safely operate with no load.

Most amps have the jacks wired in parallel (the only exception I know of is the Music Man amps from the '70s where the jacks are in series), as do most cabinets. Again, for a solid-state power amp, it generally doesn't matter which jack you use (or whether you run two cabinets off the back of the amp or if you daisy-chain cabinets together) it's still ELECTRICALLY in parallel.

Tube amp's have an output transformer that needs to have a load on it. Whether a specific tube amp is safe to operate at a different impedance than its transformer tap is designed for depends entirely upon the amp. The safe thing is to run a tube amp with the correct load. However, there was at least one guitar amp company (Buddah? I don't recall) that purposely didn't tell you what the impedance for the speaker jacks were. They wanted you to try them and decide for yourself which one sounded best to you. But that was an amp designed for guitarists who were after a specific sort of power-amp distortion, and designed for professionals who could a.) LISTEN the amp and back off if it sounded bad and b.) could afford and would ensure regular maintenance, not just fix it when it broke.

John

[Stiggs]

Originally Posted by lbwdog

NO!
The amp will run at a 4 Ohm minimum....TOTAL
The two outputs are parallel, which means they are tied together.
So, the amp can handle....(regardless of which output jacks are used)
1-8 Ohm cab
2-8 Ohm cabs (4 Ohm total)
1-4 Ohm cab

Don't try any other configuration.

Thanks thats what I figured.