Another 'OHM' question.

[Pauldem]

Okay, I don't think I've ever got to the bottom of this one so I thought I'd ask the good people on here. I am running my 2 heads at 50watts which is half power. This means that I run the heads at 8 ohm rather than 16, which I would do if I was using ful power(100watts).

I have 2 cabs, a 2x12 and a 4x12 that are both 16 ohm. Is it okay to run both cabs from one head using 8 ohm or should I change the amp to 4 ohm? I've never got a clear answer, so any thoughts would be appreciated.

Cheers

Paul

[Twinfan]

I am running my 2 heads at 50watts which is half power. This means that I run the heads at 8 ohm rather than 16, which I would do if I was using ful power(100watts).

That's odd.  Have you removed two power valves to do this?

I have 2 cabs, a 2x12 and a 4x12 that are both 16 ohm. Is it okay to run both cabs from one head using 8 ohm or should I change the amp to 4 ohm?

Using both cabs, the load is 8ohms.  If you've removed two power valves from the amp, you now need to set it to 4ohms to use the 8ohm load.

[Pauldem]

Ah, mine apologies Dave, the Splawns have a half power switch. I guess this disengages two of the power valves and in turn means I have to set the amps to 8 ohm from 16. So do I still need to set the head to 4 ohm to run the 2x16ohm cabs if the head is running at 8 ohm?

[Twinfan]

If in the Splawn manual it says to half the ohm setting on half power, then you'll need to set the head to 4ohms when running both cabs (two 16ohm cabs in parallel, gives an 8ohm load).

Sorted?

[Pauldem]

I think we may have an accord. Just to qualify if I was running the amp at full power would I need to set the output to 8ohm to run the 2 cabs? Does this also apply to running the second cab as a slave?

Dear me! I'm all questions today..

[Twinfan]

Yep!

* Full power, two cabs, 8ohms
* Half power, two cabs, 4 ohms

Not sure about the "second cab as a slave" thing!  Care to elaborate????  :)

[Spitfire]

simple equation.. more ohmz = more tonez....

they should put diagrams in the manuals that explicitly draw out the correct various speaker setups.  It is a massive cause of confusion for many people.

When i was younger i had a lot of arguments with people over correct matching of cabs to amps, i was right but its so easy to get the wrong end of the stick.. i mean, if it says 16 ohms on both sockets why not stick 16ohms into both... just doing what it says... but no.

[hamfist]

Yep!

* Full power, two cabs, 8ohms
* Half power, two cabs, 4 ohms

Not sure about the "second cab as a slave" thing!  Care to elaborate????  :)

+1

[martinw]

Check your manual (as TF said.)

There are various ways of achieving half power; not all of them require impedance adjustment.

RTFM!

[Pauldem]

I'm quite happy to admit that I'm a rather niave in these matters, as I've been running one head and one cab for some time. I will also say that I've read the manual the and the only line regarding my question reads as follows:

7. Half Power Switch - Disconnects the outside power tubes. Always set the amps impedence to half of the speaker cabs impedence when using half power.

That's all she wrote and to me this is a little vague when it comes to 2 cab operation. So at 50 watt I set the 16 ohm cab to 8 ohm that's all good and what I've been doing. I asked on this forum, as everyone has been so genuinely helpful when I've had questions.

Thanks to everyone for their help for this especially Dave, as I now get it. With regards to the slave thing, my legacy cab has an extension jack which I believe can be connected to a second cabinet. Again excuse the naivety but I wasn't sure if this was standard or whether you'd normally just run cables to each cab from the amp itself.

Cheers

Paul

[martinw]

I'm quite happy to admit that I'm a rather niave in these matters, as I've been running one head and one cab for some time. I will also say that I've read the manual the and the only line regarding my question reads as follows:

7. Half Power Switch - Disconnects the outside power tubes. Always set the amps impedence to half of the speaker cabs impedence when using half power.

That's all she wrote and to me this is a little vague when it comes to 2 cab operation. So at 50 watt I set the 16 ohm cab to 8 ohm that's all good

In which case, what Dave said is correct. When you use 2 cabs, you end up with half the impedance of each individual cab.

[Will]

I think the slaving thing is running two cabs in series, which has the opposite effect on impdence?

[Twinfan]

Depends how they're wired - if they're like Matamps then they're in parallel.  Parallel is most usual I would think  :)

[martinw]

Doesn't matter how the cab is wired, if you put two of them into your amp, they're in parallel.

I'm not aware of any cab you could put in series with another; at least one of them would need both an "in" and an "out" socket.

When the OP said slaving, I think he was referring to connecting one cab to another's parallel jack, rather than directly to the amp.
Of course, it doesn't make any difference where you connect the second cab, they're still in parallel.