Stereo output to mono?? Will I kill my amp (or myself)?

[Furiousgeorge]

I'm new to the technical workings of amps, but have been wanting to build an amp for awhile now...which is how I found these forums. While browsing, I decided I ought to ask a question:

I have a 100w tube head that I usually stereo out to my 4x12 8ohm cab, but recently snagged a much nicer 2x12 8ohm MONO extension cab (for a combo) which I want to use with the 100w head. I'm trying to figure out if I can use it safely without destroying my nice head, and what is the best way to do so (just a single cable to the mono input, or find a Y cable, etc). I am able to select between 4, 8, and 16 ohms on the back of the head.

I've been reading info on how the speaker/ohm/load deal works, but it seems pretty confusing. I almost didn't post because it seems like this question is way below you folks, but you have to start somewhere and the best way to learn is asking questions, so what the heck. Thanks for the help!

-FG

[Enzo]

No question is below us, it ain't like we will snicker behind your back at work tomorrow or something. We do cover some pretty technical stuff, but we also find ourselves discussing how Ohm's Law works too.

First, what head do you have. Is it an actual stereo power amp? Unless it is specifically a "Stereo" amp, it is more likely mono anyway. Just having two speaker output jacks does not make it stereo. Your 412 cab might be stereo capable. All that means is you can plug into it so half the speakers are separated from the other half. Running two cables from the mono amp to the stereo inputs of the cab doesn't make it stereo.

That is however a way to get a 16 ohm cab down to 4 ohms if it serves our purpose.

If it is a true stereo amp, we can discuss that. If it is just a typical 100 watt like a MArshall, then it is simple. The number of speakers in the cab is irrelevant. What matters is the impedance they add up to. As far as the amp is concerned, a 1x12 8 ohm cab is the same as a 2x12 8 ohm cab. All it sees is 8 ohms.

You want to run an 8 ohm cab? Plug a cord from the am[p to the cab and set the amp output for 8 ohms. If the amp has different jacks for impedances, then plug the cord into the 8 ohm output.

Impedances add up sorta backwards, but it is still pretty simple. When they are the same, two of them makes the total half of what one is. SO two 8 ohm cabs in parallel makes a 4 ohm total. Two 16s make an 8, and like that. DOn't worry about series wiring, since you wil almost never see any cabs wired for it. INSIDE a 4x12 cab the four individual speakers can be wired in series or in parallel or a combination of both. But that is a different matter from connecting cabs to amps.

Parallel just means both wired to the same place. The two outlets in a typical wall outlet are in parallel for example. A lot of amps have two jacks on the back wired in parallel. That means both are wired together, so both are the same. They give you two so you can run separate wires to separate cabs - for example to have a cab on either side of the stage.

SPeaker cabs often have two jacks. In simple cabs, the two jacks are wired in parallel, so you can plug into either one. The extra jack is there so you can run a second cord from there to a second cab. For example, in a stack of two 4x12 cabs, there is no reason to run two speaker cords from the head, you can run one from the head, and then a short one from the top cab to the bottom. We call that daisy chaining the speakers. They are still connected in parallel though.

In a stereo 4x12, one jack can run all four speakers, or you can have one jack for two of them and the other jack for the other two. There are different ways to do it, but if the two sides are 8 ohms each, plugging them both into an amp combines them to make 4 ohms. In mono mode they might be in series making 16 ohms or they might parallel them for 4 ohms. In that case, there is no difference using one cord or two.

[Furiousgeorge]

Great! Exactly what I wanted to know.

It looks like the amp is really mono (Carvin Legacy VL100), with 2 parallel jacks. I think I've got the concept now.

Thanks for the great explanation.

-FG