running two cabs

[CJ]

ok... i think i've had a topic on this before, but i can't even find my own thread.

so i want to run two cabs. problem is they're two different ohms, and they both only have one input jack on them and nothing else. i've got a 4x10 cab at 8 ohms, and a 1x15 cab at 4 ohms.

my amp has two speaker outputs and a switch for 4 or 8 ohms.

[AdamB]

Run your amp at 8ohms. That then splits into 2x4ohms. So you'll be under loading one cab, so it will be a bit quieter, but you're not over loading it, so you won't risk blowing your OT

[Twinfan]

Hmmm.  I'd be wary.  I personally wouldn't want to chance running a mis-matched setup like that.

Might depend on the amp though, what is it?

[jpfamps]

The speaker output will be in parallel.

A 4 ohm cab in parallel with an 8 ohm cab will give a total impedance of 2.67 ohms. The nearest match to this would be 4 ohms (NOT 8 ohms).

An amp with a good quality transformer will probably cope with this OK, however it is always best with valve amps to match the load. Additionally, maximum power is achieved with the load matched.

Just to clarify a point before someone posts otherwise: you are more likely to damage you output transformer by connecting too high a load (the ultimate high load is no speaker!) to your amp rather than too low a load. THIS IS THE OPPOSITE OF TRANSISTOR AMPS, where too low a load will destroy the output transistors (unless they have protection circuitry).

[Catalyst77]

Not ideal but you could change the speaker in your 1*15 cab to an 8ohm speaker, depending on whats in there it might not be that expensive.

[murraymurray]

Otherwise try and swap the whole 1x15 cab for an 8 ohm if you can find someone locally willing to do it.

[CJ]

Not ideal but you could change the speaker in your 1*15 cab to an 8ohm speaker, depending on whats in there it might not be that expensive.

thats actually a pretty good idea that i never thought of. only problem is i have no idea how to take it apart. i remember looking once and there were no screws to be found, and definitely none on the back...

[Twinfan]

Could be a front loader, in which case the grille will come off?

[Catalyst77]

Not ideal but you could change the speaker in your 1*15 cab to an 8ohm speaker, depending on whats in there it might not be that expensive.

thats actually a pretty good idea that i never thought of. only problem is i have no idea how to take it apart. i remember looking once and there were no screws to be found, and definitely none on the back...

Its not as duanting as it appears.  If its a 1x15 the chances are twinfan is right and its a front loader.  So the screws will be on the front.

If you take those out, your grille and baffle should come right out.  Then i believe theres just four screws to remove the speaker, if its quite a new cab- then you should just be able to pull the speaker wire off the terminals on the speaker (i.e. it has those little metal clasp type connections.) Make sure you rember or better still draw a diagram of how its currently wired - and then its just a simple swap job!

[CJ]

Not ideal but you could change the speaker in your 1*15 cab to an 8ohm speaker, depending on whats in there it might not be that expensive.

thats actually a pretty good idea that i never thought of. only problem is i have no idea how to take it apart. i remember looking once and there were no screws to be found, and definitely none on the back...

Its not as duanting as it appears.  If its a 1x15 the chances are twinfan is right and its a front loader.  So the screws will be on the front.

If you take those out, your grille and baffle should come right out.  Then i believe theres just four screws to remove the speaker, if its quite a new cab- then you should just be able to pull the speaker wire off the terminals on the speaker (i.e. it has those little metal clasp type connections.) Make sure you rember or better still draw a diagram of how its currently wired - and then its just a simple swap job!

its pretty old... i'd imagine 70's or 80's. its a Peavey 115E by the way.

[jpfamps]

If you rewire your cab you should also check the phasing,

If your cabs are out of phase you will loose a lot of bass response.