Determining Amps Positive and Negative speaker output

What colour are the wires?
Does it have an extension socket in parallel?. If so, insert a speaker cable ( amp off! ) and meter from the tip of the plug to each wire.
Meter from each wire to chassis ground ( may be a fruitless exercise )
Are the speaker terminals different sizes?
If it won't be used with an additional cab, it doesn't matter. If it is/will be used with another cab in parallel, and you get it wrong, the worse that will happen is the 'fixed' connection will be out of phase with the cab and it'll sound a bit odd.

 

What I would do is take an ohm meter and check if one of the wire has continuity to ground. If so that wire would go to the speaker "-" terminal. Some older speakers have the minus terminal grounded to the basket. If it is an Ampeg where the speaker wires come directly from the output transformer it doesn't matter.

 

Supposing that the manufacturer has actually wired the speaker the right way, the negative output of amp coincides with the negative terminal of speaker.

If the speaker has no indication of polarity, disconnect one of its terminals. Get an 1,5V battery and two pieces of wire. Connect the negative of battery to one of the terminals of speaker (no matter which one) and touch the positive of the battery to the other. The cone will move back or forward. If it move forward, the polarity of speaker and battery are coincident.

 

1. Open up the amplifier.
2. Find which secondary wire goes to the negative feedback resistor.
3. Make THAT wire your '+' lead.

Or another way, if you have an ESR meter;

1. One lead could be connected to the chassis.
   
2. Make THAT wire your '-' lead.

A regular DVM may not work, as measuring the DC resistance of the secondary is always very, very low. Most meters, even that $$$ Fluke can't measure low ohms accurately. To prove this, hold your meter leads together. Did you get exactly 0Ω when you did this? Of course not.
You can try a resistance to ground, but this works if there is a negative feed circuit. You'd get the feedback resistor plus the cathode resistor in series; maybe 1KΩ or 2KΩ or whatever. The other lead should be very low, or even open, especially if it's not connected to the chassis. Then do the battery trick to get the positive and negative terminal right, unless they've grounded that to the speaker frame.

 

Speakers don't really have polarity, so it's not important except one terminal is connected to the basket or you have more than one speaker. In the latter case you must get all speakers in phase.

 

The electrical current to the speaker is AC, not DC. So there isn't a positive and a negative in the DC sense.

If there's only one speaker, it makes no meaningful difference which way you connect it as long as each speaker terminal is connected to one wire. You can test this for yourself.

 

Not a stupid question. Phase relationships depend on multiple factors. The short answer is yes, but the slightly more useful answer is that the out-of-phaseness doesn't really matter. The way you set the tone controls of each amp will have a much larger impact on the overall sound than the relative phase relationship.

 

Respectfully, I disagree partially.

The speaker really works with AC so, theoretically, it doesn't matter what terminal is connected to what polarity of the amp, but that's true when you're using just one speaker (or just one amp with one speaker).

When you're are connecting 2 (or more) speakers, or using two amps reproducing the same signal, they should be all in phase, or you'll have speaker's cones moving in different directions when submitted to the same signal.

That translates into cancellation of the air waves, with reduction of perceived sound pressure (volume).

Maybe if the speakers are used far each other, the effect is negligible and, sometimes, desirable, but it has to be taken into account.

I think the best way to determine if it sounds OK or not is TRY. Nothing bad (electrically speaking) will happen if you use speakers out of phase, only the tone and volume will be affected.

 

Actually, some people argue that even with one speaker in one amp, it can be "out of phase". Apparently, the first hit should make the cone push outwards.
There are threads to be found on different forums about this. Personally I have no opinion in the matter and won't go into the reasoning behind it.

The following example is from a column by Chad Weaver in Premier Guitar, mostly about a multi-amp setup but including this:

When an amp is in phase, the speakers will push—literally. I'm sure you've watched your speakers when you’re playing and seen them move back and forth. The first tone that is emitted through them will make the speaker basket push toward the grill cloth of the speaker cabinet. That is in phase. When the speaker is out of phase, the first tone will make it draw back toward the speaker's magnet.

 

If you can tell the difference in a double blind test, I'll buy you the Telecaster of your choice.

@drob, wire the speaker either way. If you want to use the Ampeg with an identical model side by side, then worry about whether the speakers are in phase. Also remember that phase can be flipped at other points in the signal's journey from vibrating strings to vibrating speakers.

 

Geez, Louise, there was the answer all along.
Usually the green wire is your 8Ω tap. With an American amplifier, I'd say 8Ω. Marshall has been known to mess with your mind and use weird colour schemes, but here I'd go 8Ω from the green, and the black is 'common'. And I'd almost bet my middle nut (I've been trying to get rid of this thing for years) the green wire also goes to a feedback circuit, as I said way back when. Unless there is no negative feedback. The reissue Jet has no negative feedback. The G12 has a 270K feedback resistor, and by golly, on their own schematic it's a green wire to the 8Ω speaker.

Elementary, my dear Watson.

 

Trust me, I've been to lots of gigs with unlistenable guitar tones just because the speaker cone moved inwards on the first transient. Does your offer include Custom Shop?

 

Are you serious? What about strumming upward on the guitar? The cone would move inward as the string is pushed upward.

Two guitar cabs placed perfectly side by side and in phase. Move one of them an inch to the rear and the pressure waves from the speakers are no longer in phase.

When speakers share a cab they need to be in phase because they share the cab interior space. Two separate cabs, then it doesn't really matter.

 

I had a friend who was working with a great singer, who was a terrible guitar player. He said it wasn't a problem with a little combo, but he traded for a 2-12 Showman, and there was a problem. Without the guys knowledge, he swapped the speaker leads where one was out of phase. He said the singer couldn't tell from the stage, but the output to the floor was greatly diminished!

 

There are different degrees of ‘ out of phase’. IMHO. Moving one of two cabs which are in phase with each other a bit forward or backward will not put the two cabs 180 degrees OOP. This might yield some interesting phase phenomena that might be discernible, but I am going to think it is a minor effect. If you turn one of the cabs around 180 degrees, then the cabs will be 180 degrees OOP.
Now, IF you have a 2 x cab with one side closed and one side open with inphase wiring. There is a fantastic phasing result...big multidimensional soundstage with brilliant and pronounced harmonics flying all over the place. This would also happen if the cabs were 1 x cabs, in phase, and one was open while one was closed back.