Help with Marshall 4x12

[Ant]

[color=#141414][font=verdana, geneva, lucida,]Just picked up a Marshall 412 bass cab and just trying to dig up some history on it, I'm not sure it's all original as the circular badge on the back says its 100w but it's loaded with 4 eminence drivers which don't look too old, but then i know these cabs are notorious for being mislabelled.[/font][/color]

[color=#141414][font=verdana, geneva, lucida,]I just got it to beat around on tour so I'm not bothered about it being original, but I have no idea about ohmage etc or what kind of power I can push through it safely, looking up the drivers online seems to suggest 250w each.[/font][/color]

[hubrad]

That rear view pic looks they're wired as 2 pairs in parallel, each pair being in series. Basically that means that if the individual drivers ar, say 8Ohms, each pair will show as 16 but then the two pairs in parallel will make 8 again. So whatever the individual impedance (assuming all the same), that's the imp. of the cab.
Power handling is double that of one speaker, as a series pair can only handle the single amount whereas a parallel pair will split it between them. Again assuming all the same impedance drivers.
Eminence usually have a little white label on with the impedance, usualy on the side face of the magnet.. owt like that?

[Ant]

[quote name='hubrad' timestamp='1460631682' post='3027382']
That rear view pic looks they're wired as 2 pairs in parallel, each pair being in series. Basically that means that if the individual drivers ar, say 8Ohms, each pair will show as 16 but then the two pairs in parallel will make 8 again. So whatever the individual impedance (assuming all the same), that's the imp. of the cab.
Power handling is double that of one speaker, as a series pair can only handle the single amount whereas a parallel pair will split it between them. Again assuming all the same impedance drivers.
Eminence usually have a little white label on with the impedance, usualy on the side face of the magnet.. owt like that?
[/quote]

im not with the cab right now but will check for the little label later on!

thanks a lot

[Dad3353]

If you're going to tour with that, I think I'd recommend having the wiring re-done, maybe with slightly thicker gauge wire..? Those solder joints look very flimsy to me; the copper should be wrapped around the lug, through the hole, before soldering. One should ideally have firstly a good mechanical joint, which the solder then reinforces. Those present joints look like candidates for failure by vibration in the quite short term. Whilst it's open, it's the right occasion to get that beefed up.
Just sayin'.

[hubrad]

This kind of thing.. this is a chinese made Eminence

[barkin]

[quote name='hubrad' timestamp='1460631682' post='3027382']
Power handling is double that of one speaker, as a series pair can only handle the single amount whereas a parallel pair will split it between them. Again assuming all the same impedance drivers.
[/quote]

Is this right?

[Bill Fitzmaurice]

Those are 8 ohm Eminence Beta 12s, with a thermal rating of 250w. In that small sealed cab their real world power handling before farting out is more like 125w each, and being sealed there's not going to be much happening below 70Hz. It really should be lined with damping material as well.
[quote]Power handling is double that of one speaker, as a series pair can only handle the single amount whereas a parallel pair will split it between them.[/quote]You're confusing what happens with power versus what happens with voltage. On that matter with parallel connections each driver sees the same voltage, with series the voltage is split amongst each driver.

[Ant]

Thanks bill, will dampening the cab myself be an easy job? What kind of material is best / what kind of tonal changes can I expect?

[taunton-hobbit]

If you can get hold of some old-fashioned carpet felt, you can't go wrong.....

[Bill Fitzmaurice]

[quote name='taunton-hobbit' timestamp='1460661201' post='3027742']
If you can get hold of some old-fashioned carpet felt, you can't go wrong.....
[/quote]That will do, as will 5mm polyester upholstery batting or 5mm open cell foam. It will tighten the mids quite a bit by eliminating internal reflections.

[Ant]

thanks guys! and would i just simply cut it out and stick it (staple or adhesive i imagine?) to the inside walls and back?

[Bill Fitzmaurice]

[quote name='Ant' timestamp='1460666595' post='3027798']
thanks guys! and would i just simply cut it out and stick it (staple or adhesive i imagine?) to the inside walls and back?
[/quote]+1.

[hubrad]

[quote name='Bill Fitzmaurice' timestamp='1460655540' post='3027676']

You're confusing what happens with power versus what happens with voltage. On that matter with parallel connections each driver sees the same voltage, with series the voltage is split amongst each driver.
[/quote]
Hehe, quite probably; that was the simplistic 'everyman' description of what goes on that I was given decades ago. It was much later that the idea of the 'rating' being quite inaccurate came up. Still, with say 2 x 250W speakers in series, you'd not hit the front one with 500W, would you? And with the same in parallel they'd get an equal share of the power? (again, simplistic approach..)

[Bill Fitzmaurice]

[quote name='hubrad' timestamp='1460710459' post='3028004']
Still, with say 2 x 250W speakers in series, you'd not hit the front one with 500W, would you? [/quote]You would not. Half the voltage would be dropped in each, therefore half the power would be dropped in each. [quote]And with the same in parallel they'd get an equal share of the power? (again, simplistic approach..)
[/quote]Yes, because half the current would be dropped in each.

[hubrad]

[quote name='Bill Fitzmaurice' timestamp='1460720411' post='3028146']
You would not. Half the voltage would be dropped in each, therefore half the power would be dropped in each. Yes, because half the current would be dropped in each.
[/quote]
Grand, thanks.. at least I was told it on the safe side!

[Guest bassman7755]

[quote name='hubrad' timestamp='1460710459' post='3028004']
Hehe, quite probably; that was the simplistic 'everyman' description of what goes on that I was given decades ago. It was much later that the idea of the 'rating' being quite inaccurate came up. Still, with say 2 x 250W speakers in series, you'd not hit the front one with 500W, would you? And with the same in parallel they'd get an equal share of the power? (again, simplistic approach..)
[/quote]

Power is shared equally between identical speakers whether they are wired in series or parallel. Identical series chains of speakers act like identical speakers for the purposes of power distribution when those chains are wired in parallel (ditto for vice versa), hence in the 2-chains-of-2-series-wired-in-parallel setup of that cab, all speakers share the load equally and hence the full combined capacity of all speakers can be utilized provided you can drive enough power into the overall impedance of the cab.

In short given an even number of identical drivers, you can use use any "symmetric" series and parallel wiring combination to evenly distribute power.

Given say 6 drivers you could wire them all series, all parallel, 2 parallel chains of 3 series or 3 parallel chains of 2 series, 2 parallel sets of 3 wired in a series chain or 3 parallel sets of 2 wired in a series chain (I think thats all of them ...).

Edited April 16, 2016 by bassman7755