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Two Amps into One Marshall 1960 cab

mcdyas

Gold Supporting Member
Messages
1,210
Is this safe? If I set the switch to stereo and use two different amps at the same time each into one side of the cab, will the signals remain totally separate inside the cab? The Marshall handbook only talks about doing this with a stereo power amp, not two entirely separate amps.

My second noob question of the day is this: why does just running into one side when set to stereo give a louder and clearer sound? I've got a 1960A with Greenbacks on top and Mesa V30s on bottom, not in a X pattern. When I use either side by itself it is louder and clearer, but when I set it to mono and run both pair it loses clarity/definition and volume. I know I have it wired correctly internally and the impedance set properly in each case.
 

Fulldrive-1

Senior Member
Messages
5,926
As to the latter problem, my guess is that you do indeed have them wired backwards. Like when you set up your stereo system with one speaker reversed and the sound is weird. Check it out thoroughly.
 
Messages
5,083
1) Yes, the 2 pairs of speakers *should* be totally separate. That being said, it will only be that way if the cab is wired correctly, which is doesn't sound like it is based on your description. Also, you are trusting both of your amps to a $0.75 switch. There's no reason to think it would fail, but there's some level of risk there, however minimal. If it were me, I'd hard wire them separately to negate the risk.

2) I agree with the above, it sounds like it's wired out of phase. Something isn't right.
 

handtrix

Member
Messages
2,360
Unless you are purposely going for an out-of-phase tone, and with the exception of push/pull Bass & P.A. enclosures; Guitar speakers (cab's) were meant for the cone to push out. When the +/- are attached properly the signal should be projected. If the terminals we reverse the the signal bottoms out the voice coil if enough signal passes through.
In an enclosure, "compression" becomes an issue with breathing room. Bass & P.A cab's tend to be ported on average for the most part and frequencies are delivered differently.
 

mcdyas

Gold Supporting Member
Messages
1,210
As to the latter problem, my guess is that you do indeed have them wired backwards. Like when you set up your stereo system with one speaker reversed and the sound is weird. Check it out thoroughly.
I don't know, I just opened it up again and looked at every connection. It is all red to positive and black to negative at every connection. These are 16 ohm speakers wired in parallel on each side. I guess I could break it all down and re-connect every wire but it definitely seems correct as is.
 

Geetarpicker

Member
Messages
3,042
Plug in a speaker cable to the cab and quickly tap a 9v battery onto the tip and sleeve of the other end of the cable. First identify which terminal on the battery is + and touch this to the cord tip, the battery - to the cord plug barrel. Don't hold the voltage, just quickly tap the battery and remove. You will hear a pop/thump from the cab. If you can also use a flashlight or other means to see the cones you can verify they all move forward with this test. Even if they seems correctly wired it's worth s test. I once did a gig with a rental 4x12 Marshall with the stereo back plate. There were phase issues and I had to play the gig with just half the cab as it had the same issue you describe. A properly wired stereo cab will naturally sound a little weaker and smaller running just half the speakers, not the opposite!
 
Messages
7,230
Geetarpicker is right.... Physically test everything is in phase. All the a speakers need to go the same direction out or in when doing the test.
 

mcdyas

Gold Supporting Member
Messages
1,210
I did the battery test- thanks very much for the suggestion. They all move in unison and the wiring is correct. So, I took a closer listen. One side is definitely brighter or has more presence than the other side. I was hearing them combined as softened up due to this brightness on one side being mixed out a little bit when combined with the other side. The speakers are 2 UK Greenbacks and 2 Mesa V30s. I thought they were all fully broken in, but the one side has that V30 sound of a speaker not fully broken in.

My main question got lost here so let me re-state it. Has anyone used a single 1960 cab with two separate amps in the stereo jacks simultaneously without any issues?
 
Messages
7,230
Yes you can use any two guitar amps and your cab.

Remember each side with those two speakers is rated at 50 watts.

You might find a ABY with phase switch useful. I use a Fulltone. The amps are often out of phase and if you change channels the phase often changes. I also prefer each amp has it's own cab. Just personal taste...
 

mcdyas

Gold Supporting Member
Messages
1,210
I was thinking of using one 1987x set clean and another 1987x dirty (both have MVs) with the one cab to see how it works, but I don't want to blow anything up in the process.
 

Stormin

Tele's and Plexis
Messages
4,116
I did the battery test- thanks very much for the suggestion. They all move in unison and the wiring is correct. So, I took a closer listen. One side is definitely brighter or has more presence than the other side. I was hearing them combined as softened up due to this brightness on one side being mixed out a little bit when combined with the other side. The speakers are 2 UK Greenbacks and 2 Mesa V30s. I thought they were all fully broken in, but the one side has that V30 sound of a speaker not fully broken in.

My main question got lost here so let me re-state it. Has anyone used a single 1960 cab with two separate amps in the stereo jacks simultaneously without any issues?
Are you 100% sure that the output jack polarity from each of your amps is phased the same? I bet if you take either amp and plug it into both sides of the stereo cab all will be fine. I'm also willing to bet that both amps running simultaneously are pushing one stereo pair with opposite polarity than the other - meaning, you'll only see it with both amps running.

If you have a few minutes, reverse the wires of 1 speaker cable at the plug and see if it's noticeably better. If it is, then figure out which amp was out of phase.
 






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