two amps one quad, is this ok?

dr.colossus

dr.colossus

New member
my guitarist want to switch between his jcm800 and mosfet100 during songs, this is simple enoug with the use of an a/b box... question is his cab has only one input, can we just use a Y connector here and have both amps running through the cabinet without blowing each other up??? the speakers will disipate the power and it won't travel "up" the speaker cable of the idle amp and blow it up will it? just want to be on the safe side here :D
 
Don't do it, at least not to the best of my knowledge. I'm not sure about the signal traveling up the other cable, but there is another concern.

The problem is that the amps will see each other as another impedance in the circuit when both are connected to the cab at the same time. When you add the impedance of the cab and the impedance of the other amp, the total impedance will be lower than the cab by itself. The mosfet may be able to handle the odd load, but tube amps and Marshalls in particular are very impedance-picky. The Marshall could suffer.

Rane Notes: Why not wye?
The combiner boxes there won't do the trick, as they are for line-level devices which do not need to pass current.

If buying another cab isn't an option, I would get a plastic jack panel with another input, and rewire the thing so each amp can be plugged into two of the speakers. You could even wire it so that one of the jacks serves two purposes, using a switch. One of the jacks will be left/mon, the other will be right.

There is a diagram in this thread, along with the article it was with.

I suppose you could build some kind of speaker switcher box, but it would be a pain, and I think you would run the risk of damaging the Marshall during switchovers.

There may be other options, but this is what I've got for you. :)
 
boingoman said:
The problem is that the amps will see each other as another impedance in the circuit when both are connected to the cab at the same time.
Click to expand...

excellent point!!!

could you place a high power didode on each speaker cable prior to the "wye" to perhaps stop signal flow in the wrong direction

spliting the cab in two is a pretty good solution, however it would be nice to mic the setup with just one mic...
 
dr.colossus said:
excellent point!!!

could you place a high power didode on each speaker cable prior to the "wye" to perhaps stop signal flow in the wrong direction

spliting the cab in two is a pretty good solution, however it would be nice to mic the setup with just one mic...
Click to expand...

Interesting. I suppose you could maybe build something like a full wave rectifier, one each for the positive and negative wires. The problem to get around is that though each speaker wire has a + and a -, current flows in both directions through each. The designations of + and - are really to help keep you polarity straight. You have to design your network so current can flow both ways through each + and - but they still stay separate.

Hmmm...interesting. :) Solve that and you might be jake. (All set)
 
The only other thing I could think of was some kind of battery-powered relay switcher that operated by footswitch. A double-pole single-throw battery powered switch-type thing. Don't know if it even exists.

Yes it does, ready-made for guitar.
Found some links. Google search "amp switcher."http://www.skrydstrup.com/ampswitcher.htm

Apparently Mesa Boogie makes one too. There are some nice A/B boxes out there that they call ampswitchers, but aren't. These two might be worth looking at. :)
 
You cannot ever connect two power-amp out's together, you will damage one or both. All the diode circuit stuff discussed will not work, or was that a joke?

You need to physically switch them, but a relay would have to be pretty big, were talking lots of watts here. But, you cannot leave the JCM800 without a load, tube power amps don't like that. So a dummy load would need to be switched in at minimum.

My recommendation is to use the power amp in the JCM800 only and switch the pre-amps into that. It's a lot easier to switch pre-amp signals. The JCM800 probably does not have a line out and power amp in, but that's not to hard to add.

You circuit would look like this:

Guitar --> JCM800 pre --> A
|-> Mosfet100 pre --> B --> C --> PowerAmp --> Speaker


The AB box should be a passive type (just a switch). There will be some small switch noise, but it won’t be a lot because the switch is post pre amp, its bounce will only be amplified by the power-amp which does not have a lot of voltage gain.
 
That is assuming either amp has a preamp out, and the tone from that set-up is acceptable. :)

He was not going to run them at the same time. :D I think between us we knew that wasn't OK without even talking about it.

Although I guarantee 100% that someone, somewhere, has done it, and payed the price. Too many guys and too many amps and to many beers in the world for it not to have been tried.

Thanks for chiming in- I kinda thought that diode thing wouldn't work.

That switcher I linked to looks cool, though, if you need to get the job done.
 
Those amp switchers look like an expensive solution to small problem! unfortunatly i wasn't joking about the didode thing :D my electronics theory bathed in misconceptions as i am self taught and have had a terrible teacher!
 
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