2:90 and two outs to one Cab

[Boris]

Hi!
I have a question: I'm just waiting for the delivery of my new Triaxis, 2:90 and Orange 2X12 closed-back (120W) cab. My question is:
Since the 2:90 is stereo can I run 2 outputs into this cab? Will it act as a stereo/will it affect the sound or is it better to use one side of the 2:90 only and have other one muted? (It's such a horrible vision to me to have 2x90W channels but use only one :shock: - the other one could get jealous !!! Why they even do them stereo as a default set up? To have 2 Triaxises??? If someone could explain this silly question as well please... :wink: )
I am also really confused about the Ohms on the cabs. I was always thought that the ohms and impedances must match and when I was checking out forums these last couple of days I found out that people set their amps in a way that if they have 16Ohm cab they use the 8ohm out on their head....is this correct? And why?

Thanks a lot for your replies...

Boris

 

[Jackie]

You can use your amp with a 1-step mismatch (8ohm tap into 16 ohm cab, or 4 ohm tap into 8 ohm cab for instance) safely - even the 2:90 manual says so (that's how you connect a single 16 ohm cab in mono).

The TriAxis has 2 outputs, and the 2:90 has two inputs. Then the 2:90 has two final outputs, so you can run it into two cabinets. It's a really versatile piece of gear, especially if you have another amp that has Poweramp in/Preamp out (I have a Marshall JVM and I combine the Mesa rig and Marshall for some awesome tones).

If your Orange is wired for stereo (like my Marshall 1960A for example), you can run each half of the poweramp into half of the cabinet (each side to one speaker). I have it set up like this and it sounds great.

If it isn't, just run one side (turn the other one's Level to 0 and Presence to max) into the cab's input.

I checked out your cabinet (PPC212), and it does have two inputs. I think that it has two 8 ohm speakers wired in series, so the cab in mono has 16 ohms, and in stereo it has 2x8ohms...

 

[lento]

I don't know if this is a good idea to run just one cab from one side of the 2:90, unless you have a load box for the unused channel.

Whilst a solid state amp wouldn't complain, a valve amp needs to see some sort of load for both channels.
The manual states " Don't forget: Duplicate the proper load for both channels"

I always run a (thiele) cab for each channel to be on the safe side (also sounds better to have your fx return in stereo 8) )

For using 16 ohm speakers you should be alright, but see my post here:
http://forum.grailtone.com/viewtopic.php?f=5&t=46952&p=337648#p337648

 

[crane]

You can use your amp with a 1-step mismatch (8ohm tap into 16 ohm cab, or 4 ohm tap into 8 ohm cab for instance) safely

Right. But never make a mismatch in the other way : never plug an output tap into a lower ohms cab, like a 4 ohms cab in the 8 ohms tap, or 2 ohms cab in a 4 ohms output tap.

 

[Jackie]

Right. But never make a mismatch in the other way : never plug an output tap into a lower ohms cab, like a 4 ohms cab in the 8 ohms tap, or 2 ohms cab in a 4 ohms output tap.

+1, I forgot to mention that.

I don't know if this is a good idea to run just one cab from one side of the 2:90, unless you have a load box for the unused channel.

You don't have to have a load box if there's nothing connected to the input of the unused channel. There's nothing "going on" for it to need a load.

 

[Boris]

Thanks a lot guys....very helpfull. (I still don't understand the cause for this "OHM law"-but at least I know the rule (and will research it))
I will try to compensate the unused channel by using one side of the 2:90 for a week and then use the other one for next
Jackie...one more question:

I checked out your cabinet (PPC212), and it does have two inputs. I think that it has two 8 ohm speakers wired in series, so the cab in mono has 16 ohms, and in stereo it has 2x8ohms...

So is it (the cab) stereo or not? I cannot find this anywhere (maybe it will be in the manual once I have it...) But if it is then it would mean that I can feed it a stereo signal from 2:90 - correct?
Thanks again.

 

[rocknroll9225]

I don't know if this is a good idea to run just one cab from one side of the 2:90, unless you have a load box for the unused channel.

You don't have to have a load box if there's nothing connected to the input of the unused channel. There's nothing "going on" for it to need a load.

slow down... thats like saying you can run an amp head without a speaker connected because the guitar isn't plugged into it. the guitar signal's presence isnt why power amps need loads attached, its because the power amp is generating a **** ton of power to move the speakers, and all that power needs to go somewhere. the only way what youre saying could be correct is if mesa designed the 2:90 to turn off a side when nothing is connected to its input. ive never used a 2:90 so thats just my speculation. ive heard from other 2:90 users that you can indeed set the unused side's level to zero and run it safely.

It's such a horrible vision to me to have 2x90W channels but use only one :shock: - the other one could get jealous !!!

dont be afraid to only use one side. that way you can have an immediate backup if you blow a tube at a gig.

 

[Jackie]

the only way what youre saying could be correct is if mesa designed the 2:90 to turn off a side when nothing is connected to its input. ive never used a 2:90 so thats just my speculation. ive heard from other 2:90 users that you can indeed set the unused side's level to zero and run it safely.

You're probably right, it was a bit quick of me to generalize.

Anyway, the manual tells you this:

LEVEL A/B
These two pots control the volume level of each side or channel of the 2:90. Since there's
only one Standby Switch on this unit they may also be useful as channel muting controls.
NOTE: If only one speaker is to be used on one channel and no speaker load on the other,
always turn the unused channel’s Level Control to zero.To prevent oscillation (motorboating)
turn the Presence Control of that channel all the way up.

However, I don't understand how an amplifier can "amplify" a signal if there's no signal going into the amp. Could you please explain this?

(sorry for the potential thread jack)

Boris, I think your cab is indeed a stereo cab. Since it has two speakers and the final impedance is 16 ohms, it probably has two 8 ohm speakers wired in series (8+8=16) and when you plug in two jacks it's 8 ohms per speaker.
So you would plug the "A" side of the 2:90 in one jack and the B into the other, putting 8ohms per side into 8 ohms per speaker.

I'm not 100% sure if that's correct, but it's the only way I see that a cab can have 2 speakers and a 16 ohm rating (with two jacks on the back). Check your manual anyway.

 

[crane]

However, I don't understand how an amplifier can "amplify" a signal if there's no signal going into the amp. Could you please explain this?

A signal can occur without anything pluged in :
- input tube could be microphonic
- bad/dry solder
- signal path isn't totally isolated (true for most amps...) and then :
- neon or 12V lights could generate a signal.

The best way to protect your amp is to remove the power tubes from the unused channel. BUT this will increase voltage on the other tube's plate and heaters, so depending on how much it could not be a solution.
You can also remove the PI from the unused channel. Voltage increase is ridiculous and signal to the power tubes should be cut (again depending on the amp's design, bad solder, ... anything that could bring some noise to the power section).

As a personal experience, I've run my 50/50 more than 10 years without a load on channel B, simply turning volume to "0". Not any pb...

 

[rocknroll9225]

However, I don't understand how an amplifier can "amplify" a signal if there's no signal going into the amp. Could you please explain this?

its not so much about the amp amplifying a signal as it is about the fact that its literally sending power (90 watts of it) through the speaker cable to your cab, so the signal will be strong enough to move the speakers. thats why you don't need to plug your cab into the wall and turn it on. regardless of whether or not its amplifying anything, the power amp will always be sending power to the cab (or dummy load) when its on and not in standby. if that power doesn't have anywhere to go, it'll stick around in the amp and literally cook the power tubes and output transformer.

 

[Jackie]

Thanks for that, I understand now.

Though in the 2:90s case you can just turn down the Level and max the Presence of the unused channel, according to the manual. Thanks for saving me a cable, Mesa! :mrgreen:

 

[msi]

When there is no signal going into the poweramp the poweramp is producing very little current. When there is some signal going into the amp then the power tubes start producing current, and that current needs to go somewhere, usually the speakers. The amp does produce a little idle current when nothing is happening so you should have something plugged in usually. Or, in mesa's opinion not having anything plugged into the input, turning the master volume down, and turning the presence up (so the idle current has some place to go) is enough. If you plug something into the input I would always plug a speaker load in, just in case.

 

[VS]

To answer on of you first questions: the main benefit of the stereo design - besides running 2 cabs - is, that you can use stere effects like stere echo,...

I checked out your cabinet (PPC212), and it does have two inputs. I think that it has two 8 ohm speakers wired in series, so the cab in mono has 16 ohms, and in stereo it has 2x8ohms...

I think that's wrong.
I went to the Orange site... it has two connectors but looks like it's only mono. The other connector is for another cab, which runs parallel to the first cab when connected. So one is input, one goes to another cab.

 

[Jackie]

I checked out your cabinet (PPC212), and it does have two inputs. I think that it has two 8 ohm speakers wired in series, so the cab in mono has 16 ohms, and in stereo it has 2x8ohms...

I think that's wrong.
I went to the Orange site... it has two connectors but looks like it's only mono. The other connector is for another cab, which runs parallel to the first cab when connected. So one is input, one goes to another cab.

Indeed it is. I checked out a couple of other forums and people modded theirs to be stereo, the other input is for daisy chaining cabs.
There is a 412 that has the stereo option though, and the graphic's pretty obvious on that one. So no, the PPC212 isn't stereo.

 

[VS]

Yeah.. the 2x12 look like this:

 

[shatteredsquare]

You can use your amp with a 1-step mismatch (8ohm tap into 16 ohm cab, or 4 ohm tap into 8 ohm cab for instance) safely

Right. But never make a mismatch in the other way : never plug an output tap into a lower ohms cab, like a 4 ohms cab in the 8 ohms tap, or 2 ohms cab in a 4 ohms output tap.

um, what happens if you do that?

 

[Jackie]

You'd probably blow the speakers and could also damage the output transformer.

If you mismatch "up" you're sending out more than the speaker can take. If you mismatch down you're sending less, that's no problem - BUT, if you have a 2 step mismatch (4ohms into 16ohms) that will probably have a negative impact on your tone, whereas some folks prefer to run their rig at a 1 step mismatch (4ohms into 8ohms or 8ohms into 16ohms) for tone/feel reasons.

 

[crane]

You can use your amp with a 1-step mismatch (8ohm tap into 16 ohm cab, or 4 ohm tap into 8 ohm cab for instance) safely

Right. But never make a mismatch in the other way : never plug an output tap into a lower ohms cab, like a 4 ohms cab in the 8 ohms tap, or 2 ohms cab in a 4 ohms output tap.

um, what happens if you do that?

Could blow one or more tubes, but could also damage the output tranny.